MOLLUSC SURVEY OF THE MINIDOKA DAM AREA, UPPER SNAKE RIVER, IDAHO

FINAL REPORT Contract # 1425-2-PG-10-16780

pared for

U S Department of the Interior Bureau of Reclamation Pacific Northwest Region Boise, Idaho

TERRENCE J FREST EDWARD J. JOHANNES February 16, 1993

251 7 NE 65th Street Seattle, WA 981 15-71 25 (206) 527-6764

MOLLUSC SURVEY OFTHE MINIDOKA DAM AREA, UPPER SNAKE RIVER, IDAHO

Terrence J. Frest and Edward J. Johannes Deixis Consultants 2517 NE 65th Street Seattle. WA 981 15-7125

February 16, 1993

TABLE OF CONTENTS

INTRODUCTION ....................................................................................................... 1 1 3 6

Shortface lanx 6 6 6 7

Snake River physa 8 Columbia pebblesn . ...... ........................... 8

PROJECT DESCRIPTION 9 9 1 4 1 4 15

1 6 . . . . . , . . . . , . . , , . . . . , . . . , . , , . . , . . . . , , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

17 18 1 9

21

ACKNOWLEDGEMENTS .......................................................................................... 23

SITE DESCRIPTIONS ................................................................................................ 24

REFERENCES ......................................................................................................... 28

FIGURE 1. Map of sites 1 to 16 in the vicinity of Minidoka Dam ...................................... 10

FIGURE 2. Map of site 17 in Massacre Rocks State Park ............................................... 1 1

FIGURE 3. Aerial view of Minidoka Dam showing survey sites ....................................... 12

TABLES 1. 2. Mollusc Fauna at Collection Sites 3. Comparison of Historic and Current Mollusc Fauna .................................... . . 36

INTRODUCTION

Backaround

The Minidoka Powerplant has the distinction of being the oldest federal powerplant in

the Pacific Northwest. Its historic role and continued operation in much the original

state have been recognized by its inclusion on the National Register of Historic Places.

Recently, remodeling of a significant portion of the powerplant has been proposed, and a

Finding of No Significant Impact (FONSI) prepared and published to comply with

provisions of the National Environmental Policy Act (Bureau of Reclamation, 1991). As

part of the FONSI, a routine query as to federal listed and candidate species in the area

likely to be affected by the project was made to USFWS. Initially, the only species of

special concern mentioned were the bald eagle and peregrine falcon (Bureau of

Reclamation, 1991, p. 21). Late in the process, however, concerns about certain

freshwater mollusc species were raised by USFWS.

Some eight freshwater mollusc species found in the middle Snake River or its

tributaries have in the past ten years become either federally listed or listing candidates

(see, e.g., USFWS, 1990; USFWS, 1991). Five were formally listed as of January 13,

1993 (USFWS, 1992). These taxa and their current status are: 1) the Endangered

Snake River physa, Physa (Haitia) natricina Taylor, 1988; 2) the Endangered Desert

valvata, Valvata utahensis Call, 1884; 3) the Threatened Snake River endemic Bliss

Rapids snail (genus and species undescribed); 4) the Endangered middle Snake River

endemic Banbury Springs lanx (Lanx n. sp.); and 5) the Endangered middle Snake River

endemic Idaho springsnail Pyrgulopsis idahoensis (Pilsbry, 1933). Another taxon, the

Bruneau Hot springsnail Pyrgulopsis bruneauensis, will be listed as of February 24,

1993 (USFWS, 1993); but this warm spring taxon is not known outside of the Bruneau

River area.

Not all of the listed species were thought from previous investigations to occur in the

upper Snake River generally or the Minidoka Dam area in particular. Those most likely

to occur in the study reach, as discussed below, were the Snake River physa (Physa

nafficina), the Desert valvata (Valvata utahensis), and the Bliss Rapids snail (genus and

species undescribed). The Banbury Springs lanx (Lanx n. sp.) is known historically

only from three large springs in the Hagerman Reach and lacks a fossil record; hence.

occurrence in the study area was unlikely. The Idaho springsnail (Pyrgulopsis

idahoensis) is known historically from the mainstem Snake River, but only in the reach

from Weiser to Gienns Ferry. The fossil record does indicate occurrence in the Pliocene

Glenns Ferry Formation, but there are no outcrops of this unit in the study area, which

has mostly Pleistocene units. The soon to be listed Endangered Bruneau Hot springsnail

(Pyrgulopsis bruneauensis) occurs only in essentially contiguous warm springs in H O ~

Creek and vicinity, and hence would not be expected here. A superficially similar taxon

does occur in at least one warm spring in SE Idaho; but it is clearly a distinct new

species; and so far only a single site is known. Thus, of the federally listed species, three

(Snake River physa, Bliss Rapids snail, Desert valvata) could reasonably be expected to

still occur or to have occurred in the recent past in the study area.

Aside from the listed species, three other candidate mollusc taxa could occur in the

Minidoka Dam area. The Columbia Basin (including the Snake River) species Shortface

lanx (Fisherola nuttalli), has recently (USFWS, 1991) been downgraded to Category

3C. Subsequently, however, a comprehensive study of the species concluded that it would

eventually be listed federally as Threatened (Neitzel & Frest, 1992; in press). The

Category 2 Columbia pebblesnail (Fluminicola columblana), another Columbia Basin

endemic species, was suggested as deserving federal Endangered listing in the same study

(Neitzel & Frest, 1992; in press). Historic occurrence of this species in the middle

Snake River was noted by Neitzei & Frest. Beetle [Pilmore] (1989) has reported the

species from the upper Snake River in western Wyoming (specimens examined by D. W.

Taylor); so there is some possibility of local occurrence. The Category 2 California

floater (Anodonta californiensis) (USFWS, 1991) has also long been known to occur in

the middle and upper Snake River (Henderson, 1924, 1936), as well as barely into

western Wyoming (Beetle [Pilmore], 1989).

Much ecological and distributional information for the listed and candidate species is

summarized in USFWS (1992); critical habitat was not defined therein for any of them.

Information about the historic and fossil ranges and ecology of the listed and candidate

taxa is extensive [for summary see Frest & Johannes (1992b) and USFWS (1992); for

discussion of listing issues, see Falter (1992)I. Pioneer studies throughout the western

states of Idaho, Oregon, Washington, California, Arizona, New Mexico, Utah, Wyoming,

and Nevada were carried out by W.O. Gregg, S.S. Berry, J. Henderson, and many others

and provide a regional perspective. The efforts of D.W. Taylor require special mention.

Taylor collected Idaho mollusc sites, both modern and fossil, from the 1950s through

1987. USFWS-sponsored status surveys for four of the listed taxa were completed by

him in 1982 (Taylor, 1982a, b, C, d). Several studies of the listed taxa have been

undertaken in the Snake River during the past few years (Beak, 1987: Beak, 1989;

Pentec, 1991a, b; Frest & Johannes, 1991: Frest & Johannes, 1992a, b, c; Konopacky

Environmental, 1992; Frest & Johannes, 1993). Extensive research on western North

American Hydrobiidae has been conducted in the last five years by Taylor and especially

by R. Hershler, NMNH. Also, there has been one other study with regional implications

(Neitzel & Frest, 1989, 1992; in press). As a part of both sponsored and personal

research, Deixis Consultants personnel have collected many freshwater mollusc sites in

Washington, Oregon, California, Idaho, and adjoining states. We are currently conducting

a several-year project (initiated in 1989) on Idaho spring snails. In general, the

middle Snake River listed species are now among the best documented freshwater

molluscs nationally, and listing was clearly well justified. Nevertheless, distributional

knowledge is incomplete in certain areas, notably upstream from Milner Dam (upper

Snake River).

Since the passage of the Endangered Species Act in 1973, only eight U.S. freshwater

snail species (including the five Idaho taxa) have been listed. Listing of freshwater

snails has generally lagged behind that of other groups. To date, approximately two

hundred U.S. freshwater snail taxa have been proposed for listing (USFWS, 1991), but

none were actually listed until 1991. As of October, 1992 the USFWS had a backlog of

over six hundred Category 1 candidates alone. At current listing rates, the Service

cannot evaluate even these critical species before 2006 (GAO, 1992). The slow pace of

listing led to a recent law suit settlement, under the terms of which the Service must

resolve the status of these species by 1996.

Uooer Snake River Malacofauna

Distribution of certain other Snake River mollusc species (aside from the listed and

candidate species) could become relevant to this project. A few taxa thought to occur in

the Minidoka Dam area are known to be restricted in distribution, and hence may become

candidates in the near future, given recent water quality trends in the Snake River. For

example, the Snake River endemic Rustic pondsnail, Stagnicola hinkleyi (Baker, 1906)

was noted as rare and declining by Frest & Johannes (1992b). It was collected live at

only a few middle Snake River sites by Neitzel & Frest (1992; in press) and Frest &

Johannes (1991). No additional live sites were found by Frest & Johannes (1993).

Frest & Johannes (1992b) mention several other unusually sensitive middle Snake

River mollusc taxa likely to occur in the Minidoka Dam area. All are native species

regarded as part of the cold water biota, and maintenance of cold water biota is one of the

designated uses recognized by Idaho Water Quality Standards. The Idaho DEQ declared

portions of the middle Snake River to be water quality limited in 1990; subsequently,

the federal EPA recognized and approved the listing of the middle Snake River as water

quality limited under $30313 of the Clean Water Act. Idaho DEQ has recently denied a

$401 Certification because of water quality concerns, including cold water biota (IDHW,

1993). Possible damage to the species of concern here was an explicit and major reason

for that action.

The native middle Snake River freshwater mollusc component of the cold water biota

was listed by Frest & Johannes (1991, 1992b): most of the same species historically

inhabited the upper Snake River. Frest & Bowler (in press) compiled a similar list.

Species relevant to this study are indicated in boldface on the Tables. Aside from the taxa

mentioned above, these are the Glossy valvata Valvata humeralis Say, 1829, the Vagrant

pebblesnail Fluminicola hindsi (Baird, 1863); the Artemesian rams-horn Vorticifex

effusa (Lea, 1856), the Shiny peaclam Pisidium (Cyclocalyx) nitidum Jenyns, 1832,

and the Perforated peaclam Pisidium (Cyclocalyx) punctatum Sterki, 1895. One other

cold-water taxon not found in the lower portions of the middle Snake River, the Mossy

valvata Valvata sincera sincera Say, 1824, should be included in the Minidoka Dam list.

This species was previously known to occur in the Wyoming headwaters of the Snake

system (Beetle [Pilmore], 1989). There are no live Idaho sites reported in the

published literature, but Frest & Johannes (1993) reported long-dead specimens from

the Auger Falls Reach.

In addition, nonnative molluscs have also become a matter of concern on the Snake

River (for discussion see Bowler & Frest, 1991). The introduced New Zealand mudsnail

Potamopyrgus antipodarum (Gray, 1843) has become a dominant at many middle Snake

River sites: it was first noticed near Hagerman in 1985 (Bowler, 1990). The species

has rapidly spread throughout the middle Snake, and is likely to become a pest species in

the Minidoka Dam area in the near future. Considerations such as outlined above

necessitated collection of all mollusc species for this project. Previous papers

(Henderson, 1924, 1936; Taylor, 1985b, 1987) provided some very useful historic

data. Frest & Johannes (1991; 1992a, b, c; 1993) and Neitzel & Frest (1992; in

press) collected and reported the whole malacofauna of their sites; some other middle

Snake River studies do not. Frest & Johannes (1992b, Table 5) and Frest & Bowler (in

press) assembled a summary list of the middle Snake River subbasin malacofauna. As in

these studies, land snails found in our samples are listed also. However, this information

is not strictly relevant to the study's scope: it is segregated as part of Table 2, and will

not be discussed further herein.

Some consideration of the history of the Snake system is helpful in explaining our

results. The system as a whole has a complex history. As seen now, it is composite, with

various portions having independent geologic histories and faunal influences (for

summary discussion, see Frest & Johannes, 1992b). The Washington Snake River, for

example, can be regarded as an extension of the Clearwater River, and the Hells Canyon

stretch was until relatively recently a Clearwater tributary. Much of the upper Snake

River system shows indications of former connection to Lake Bonneville (Great Basin in

the broad sense) drainages. This history is reflected in its mollusc fauna. The middle

Snake River is of considerable antiquity, and once flowed through southwestern Idaho and

Oregon, reaching the Pacific via portions of the present Klamath and Sacramento systems

(Taylor, 1985a). Some listed species (the Bliss Rapids snail, Idaho springsnail, and

likely the Banbury Springs lanx) are endemics from the ancestral Snake system and

Pliocene Lake Idaho: all three are certainly in historic times local endemics. The Desert

vaivata and Snake River physa lived in Lake Idaho, but also inhabited parts of the Great

Basin drainage and have a broad Pleistocene distribution, including sites in the upper

Snake River. Historic sites for the Snake River physa are limited to the Snake River. The

Shortface lanx and Columbia pebblesnail are Columbia Basin lotic (more specifically,

rapids edge) taxa. The California floater occurs in rivers and river-influenced lakes

(mud-gravel substrate), mostly in the Sacramento River system, part of the Columbia

Basin, and a few Great Basin streams.

The Minidoka Dam area lies essentially east of Pliocene Lake Idaho, in a region with

common Pleistocene lake units. Hence, it would be expected to have both some middle

Snake River and more upstream taxa as part of the historic fauna. The upper Snake

River system is near the eastern border of the Columbia system. Certain basically

eastern and northern North American species are thought to have crossed the Continental

Divide during the Pleistocene. Some or ail of these are presently spottily distributed as

relicts in a few north and east border Columbia Basin streams. The same species become

common only E. of the crest of the Rocky Mountains. This group includes the Mossy

valvata (Valvata sincera sincera), the Threeridge valvata (Valvata tricarinata), the

Sharp sprite (Promenetus exacuous), the Mud amnicola (Amnicola limosus), the Delta

hydrobe (Probythinella lacustris), and the Ornamented peaclam (Pisidium cruciatum)

(Taylor, 1985a; pers. obs.). Potentially, all Could survive in SE Idaho. This study

confirms live occurrences Of the Mossy valvata and Sharp sprite in the Minidoka Dam

area (Table 2).

Previous Records

Old records from the general area of Minidoka Dam have been encountered from time

to time in the literature and in museum collections, e.g. those of the NMNH. Most

relevant to the current study are records for listed species or current listing candidates.

These are discussed on a species by species basis below.

Shortface lanx Fisherola nuttall;

There is a UCMZ record of Fisheroia nuttalli from "near Rupert" (Henderson &

Daniels, 1917; Taylor, 1982; 1985b; Neitzel & Frest, 1992). This is believed from

the original label information to refer to a collection by F. Kenagy in 1913 made near

the old Jackson Bridge. It is of note primarily in that it indicates former occurrence of

this candidate taxon in this reach of the river. We saw no specimens during this survey;

degraded (nutrient-enriched) water, cemented hard substrate characterized by

extensive overgrowths of epiphytic animals and algae, and unstable fine sediment influx

in many areas have likely precluded survival of this species locally for some time.

California floater A. californiensis

This candidate species has long been known to occur in the middle Snake River. One

old record, e.g., is for the Snake River at Firth, above American Falls Reservoir.

(Henderson, 1924): another is for "Rupert, Idaho" (as above: Henderson & Daniels,

1917). This species has been noted live recently in the Kanaka-Boulder Rapids Reach

and the Auger Falls area (Frest & Johannes, 1991, 1992a, 1993). It also occurs rarely

in the lower Snake River (Frest & Johannes, 1992~).

Desert valvata Valvata utahensis

The Endangered Desert valvata has been reported live previously both from Lake

Walcott and from the Snake River itself downstream and upstream from the lake. Figures

of such sites have been published in Taylor & Smith (1981); Taylor (1982b); and Taylor (1985a). Live-collected specimens in the UMMZ collections are from N E ~ sec.

12, T9S R29E, Power Co., ID, DWT! 9/3/61 T61-7405 [Massacre Rocks boat ramp];

center sec. 19, T9S R29E, Power Co., ID, DWT! 9/3/61 T61-7503 [near Cold Water

Camp]; and NW corner sec. 22, T10S R23E, Minidoka Co., ID, DWT! 10/18/59 T59-

14008. These sites probably correspond to T61-74, T61-75, and T59-140 as

recorded in Taylor's notes on the USFWS copy of Beak (1987). The former two are on

Lake Walcott; the latter site is on the Snake River below Minidoka Dam. In Beak (1987)

the corresponding sites and river mileages are: Beak 44A, RM 658.2 [Burley Bridge, US

30? note: Beak RM figures may require correction] (dead only) = T59-140; Beak 48A,

RM 698.3 (dead only) = T61-75; and Beak 49A, RM 703.5 (dead only) = T61-74.

Beak (1987) did collect live specimens in the Minidoka Dam area at Beak 47A, RM

679.3, presumably in Lake Walcott just above Minidoka Dam, apparently on the S. side

of the lake. Beak (1987) also noted dead specimens at Beak 45A, RM 675.0 [Jackson

Bridge], and at Beak 50A, RM 706.5 [E. of Massacre Rocks boat ramp? This may be a

duplicate of 4 9 4 as we found no other boat ramp within 3 miles of either site]. In

1988, the senior author recollected the Massacre Rocks boat ramp site (site 17 herein;

Taylor T61-74; Beak 49A, ?50A).

Some caution must be used in interpreting records of dead shells, particularly of this

species. In this area, we have collected fossil shells (Pleistocene) from a site at Burley,

and from sites near American Falls Dam. Pleistocene specimens, identified by the senior

author, from the American Falls area are also in the ~ 0 l l e ~ t i 0 n ~ of the Idaho Museum of

Natural History. Some of the river drift shells from site 17 are likely fossils (the

majority are clearly recent), and this possibility should be borne in mind for specimens

from other sites in this area.

Bliss Rapids snail

Well-documented live specimens of the Bliss Rapids snail have not been reported

above Twin Falls. For discussion of citations from the vicinity of Auger Falls, see Frest

& Johannes (1993). However, Pentec (1991) did report a site from a spring near

Ferry Bune on the Fort Hall Indian Reservation (RM 749.8). We have not revisited this

site as yet: but we have collected hydrobiids from some 40 sites in SE Idaho E. of Twin

Falls. While numerous undescribed species of Pyrgulopsis and other genera were found,

we have so far not encountered the Bliss Rapids snail. Its occurrence above American

Falls and in the Minidoka Dam area should not, however, be discounted out of hand. Both

Pliocene Lake Idaho-related and Pleistocene deposits contain this taxon, and some of

these deposits occur this far upstream. Also, as none of the Pyrgulopsis species

encountered thus far in SE Idaho are the same as any species described from the western

Snake River Plain, there is some possibility that the Ferry Butte specimens could be

another species in the same new genus as the Bliss Rapids snail. A distinct new

Pyrgulopsis related to P. idahoensis, for example, has been known since about 1960 to

inhabit one of the major river tributaries of the upper Snake in Idaho. The candidate

(Category 2) P. robusta (Jackson Lake springsnail), found in Jackson Lake and

headwater portions of the upper Snake River in Wyoming, is an analogous situation.

Relaxed specimens from the Ferry Bune site need to be examined.

Snake River physa

Taylor (1988) mentions well-documented historical occurrences of the Snake River

physa (i.e., as live-collected specimens) in Owyhee, Elmore, and Gooding Cos., Idaho.

There are a few supposed records upriver from these counties. Some (e.g., Beak, 1987:

RM 675) are plausible but need reconfirmation, and may now represent extinct

populations. Others (Pentec, 1991b) may be misunderstandings (Frest, pers. comm. to

J. Gore, June 2, 1991). The study reach is within the known historical distribution,

which includes Great Basin-influenced parts of the Snake system as well as the middle

Snake: see also P. Bowler (pers. comm. to C. Lobdell, February 15, 1991). The known

Pliocene and Pleistocene fossil distribution includes both Great Basin and Lake Idaho

sites (Taylor, 1988), so occurrence here is possible.

Columbia pebblesnail

This Columbia Basin species has been reported in the middle Snake River, as

mentioned previously. Specimens have also been reported from the upper Snake system

as far upstream as the vicinity of Jackson, Wyoming (Beetle [Pilmore], 1989). We

have been unable to recollect this species in Wyoming recently (see also Neitzel & Frest,

1992; in press); but it may still survive. The study reach is within the historical and

fossil ranges.

In summary, historic records indicated that as many as six listed or candidate

freshwater mollusc taxa could live, or formerly may have lived, in the Minidoka Dam

area. These are the Shortface lanx (Fisherola nuttalli), the California floater (Anodonta

californiensis), the Desert valvata (Valvata utahensis), the Bliss Rapids snail, the

Snake River physa (Physa nafficina), and the Columbia pebblesnail (Fluminicola

columbianaL

PROJECT DESCRIPTION

This survey was conducted under the provisions of Idaho Fish and Game Permit No.

F-76-91, issued to Terrence J. Frest, Seattle, WA. Scope, methods, and collection

protocol were worked out with USFWS Boise Field Office prior t o the survey's

completion. As part of this protocol, all specimens of listed taxa were to be returned

alive to the river after discovery. This was done (e.g., at site 16), and we are using

recently dead material for voucher purposes. Incidental take, unnoticed during the

survey fieldwork, occurred in two of our bulk samples. The field portion of the study

was carried out between September 24-27, 1992. All samples were collected by a two-

person team (Terrence J. Frest and Edward J. Johannes, Deixis Consultants). Many were

collected from 6' or less depths. We collected 17 sites altogether. Most of these (14)

were on the main river and spillway channel and located within one river mile (RM)

below Minidoka Dam (considered to be at RM 575.0). Main channel sites were 1-11

(Figure 1). Sites 12-14 were located in the periodically partially dewatered spillway.

Two sites were in the lower reaches of Lake Walcott (sites 15 & 16). One additional site

was collected at the upstream end of Lake Walcott (site 17: Figure 2). Details are noted

below under SITE DESCRIPTIONS. To establish the regional fauna, a few samples of

recent drift were also collected to supplement the river run localities. Site 12 was

located in the spillway. Site 15 was in Lake Walcott: sites 16 and 17 were along its

shoreline. We were unable to locate sites with undisturbed 1984-85 drift, as was

possible in the Auger Falls study (Frest & Johannes, 1993).

Habitats

Habitats in the Minidoka Dam area were somewhat limited, and rather different from

those typical of the middle Snake River. The Snake here flows through a comparatively

shallow basalt bedrock canyon with a relatively broad and level floodplain. On the whole,

little bedrock is typically exposed in the river proper, although the Dam itself is

CONTOUR INTERVAL 10 FEET

' PLANT

FIGURE 1. MAP OF SITES 1 TO 16 IN THE VICINITY OF MINIDOKA DAM. Base map derived from Lake Walcott West 7.5' quadrangle.

FIGURE 2. MAP OF SITE 17 IN MASSACRE ROCKS STATE PARK. Base map derived from Register Rock 7.5' quadrangle, Power Co., ID.

situated in a naturally resistant basalt unit, and the spillway is rock floored. Much of the

main river channel below the Dam is veneered with reworked Pleistocene alluvium,

with particles ranging in size from silt and sand to boulders. Very near the Dam, large

basalt clasts are the rule, and the river bed is not extensively potholed. The central

incised channel is also relatively shallow and broad. Most Of the cobble-boulder

substrate is now cemented by organic material (often bacteria, protozoans, fungi and

freshwater sponges) to form a rather resistant surface. At low water stages, some

boulders are partially exposed. A short distance below the boulder-cobble areas, soft

sediment deposition is extensive, particularly in a deep pool below the natural spillway

at RM 574.6. The seasonally active spillway on the S. side of the river is rock floored,

and rapid flow through it has enhanced deposition of fine sediment on the S side of the

river just upstream. The area immediately in front of and downstream from the spillway

is periodically scoured; the substrate here is mostly cemented large bedrock clasts.

Downstream from the study area (below RM 574.6), Soft sediment deposition is

extensive (Bureau of Reclamation, 1991, Figure 1: Figure 3 herein).

Soft substrate habitat above RM 574.6 is patchy and essentially confined to shallow

depressions in the bedrock surface and downstream sides of larger boulders. Most of this

material is relatively fine, and likely relatively mobile, being subject to scour during

high flow periods. A few mud-gravel areas on both sides of the main channel near shore

are likely more stable. Most persistent is an area to the E. of the spillway on the S. side

of the river (referred to as the wetland area in the FONSI (Bureau of Reclamation,

1991, p. 22). Sites 10 and 11 are situated in this area (Figures 1, 3). Gravel-mud

substrate is extensive below the natural spillway at 574.6. However, substrate in this

area also appears to be seasonally mobile. Much of this area was dry during the survey:

and extensive areas in the pool had no aquatic macrophytes, epiphytic algae, or live

snails (e.g. site 8). Scour from the combined effects of main channel and spillway flow is

likely severe here during high water periods. During our survey, the spillway was

mostly dry, facilitating access to this portion of the Dam area.

No true springs exist in the Minidoka Dam area. The basalt flooring the Dam and

spillway is heavily fractured, and some cold outflow persists even at very low flow

stages. Much of the outflow is likely from Lake Walcott, but some could be groundwater

flow from more distant sources. Near the E. end of the spillway, groundwater flow and

leakage from Lake Walcott are perennial at the low water spillway base and just

upstream. Similar flow also occurs in the few permanent lakes and ponds on the spillway

proper (most dry completely when the spillway is dewatered). Elsewhere in the Snake

Basin, springs may outflow at any level in the canyon and river bottom (though mostly

at distinct horizons whose location is determined by bedding and other characteristics of

the geologic units present). Here, springs in the river itself appear rare or absent,

again because of physical characteristics of the basalt bedrock. Such springs are common

in the Hagerman Reach, and become common again some distance upstream, e.g. along the

Deep Creek Mountains front near the eastern terminus of Lake Walcott.

Field Collections

Standard methods in malacology were used to implement the study. An initial

(baseline) survey of the study area was conducted to evaluate habitat types, possible

collection sites, and access. Collection methods varied according to substrate type and

degree of aquatic macrophyte or plant and animal epiphytic cover. In general, all areas

were visually inspected first and then spot sampled to insure completeness of coverage

and size and extent of major subhabitats prior to comprehensive collection. More

systematic methods were used for formally defined sites. In coarse substrate areas such

as cobble-boulder bars, a random sample of Stones was removed within quarter meter

squares and the molluscs were either hand collected or brushed from them into a 7.5" X

1 3 tray. Areas with mud, sand, or silt substrate were sampled by excavating small

areas of bottom sediment to a depth of about 3 crn using a dip net with an 8" diameter and

effective mesh size of 40 [Tyler equivalent 35 mesh: openings 0.425 mm]. Areas with

rooted aquatic rnacrophyte vegetation (shallow portions of deep pools and channel edges)

were also sampled using the same size dip net. Vegetation was retrieved with the net and

then placed in 7.5" x 13" trays and vigorously shaken to dislodge all molluscs. In areas

with bedrock or cemented cobble-boulder substrate (most of the study area), the

bedrock or liths were scrubbed underwater with a scrub brush. Dislodged material was

caught and retained in a submerged 7.5" X 13" tray positioned downstream from the

scraped surface. We took at least 10 subsamples from each sample site: the surface area

represented at each was generally about 1 m2. Most of our samples were collected within

a 3 m radius of our site measurement point. Regardless of origin, the collected material

from each subsample was decanted into a labeled 16 oz. container for further treatment.

The subsamples were run through a standard sieve series (to 40 mesh) in the field to

ensure collection of all molluscs and to eliminate very coarse and very fine organic

debris, mud, and silt. For samples expected or known to contain difficult to identify

species, we routinely employ relaxation, fixation, and preservation using a succession of

menthol, dilute formalin, and either isopropyl or ethyl alcohol (Frest & Johannes,

1992b). While we were equipped for such techniques, they were not necessary for all

samples. Field-observed live specimens of listed species were counted and returned to

the river. As Physella was common in many samples, those collected the first day were

relaxed as described above and identified using a binocular microscope. As no live

specimens of difficult to identify taxa were observed in the first day in the field,

subsequently collected samples were generally preserved immediately in alcohol after

inspection for and removal of live specimens of listed taxa. Snails were typically not

common in this area. Samples, however, frequently contained large volumes of organic

material. It was necessary to sieve them upon collection to ensure relaxation and proper

preservation. Sieved samples, generally a concentrate with a volume of 9-16 fluid oz.,

were placed in labeled jars. Each site required an average of 1 hour to collect. Drift

samples were collected near or below the high water mark. These recent drift samples

generally had their bulk consisting of dried aquatic macrophytes and other organic

detritus. The material was run through a sieve series (to 40 mesh) to remove larger

plants and fine sediment, labeled, and bottled. Generally, a 16 oz. volume was sufficient.

The surrounding shoreline was also searched for unionacean mussels; species and

abundances were noted, and a representative sample retained. Notes on collection

conditions, substrate, habitat, and associated flora and fauna were made at each site (see

SITE DESCRIPTIONS for details).

Laboratory Procedures

Preserved samples were resieved in the laboratory to remove fine sediment and

plant and animal detritus, and the full volume was examined. I f candidate or listed

species were present, the whole sample was picked under a low-power binocular

microscope. With many mollusc taxa (especially certain Physidae and Hydrobiidae),

dissection, particularly of relaxed specimens, is necessary for proper identification. Of

the species of special interest to this study, this can apply to the Bliss Rapids snail,

Idaho springsnail, and Snake River physa; but no live specimens resembling these taxa

were noted. No substantive identification problems were encountered with our material,

except with Physella. An odd feature of sites 16 and 17 was that many of the Physidae

were extremely distorted, especially in the columellar area. Such specimens were

sometimes difficult to assign to species. The cause of the teratology is uncertain. Picked

molluscs and the remainder of the picked samples have been retained for further study.

The need for species-level identifications precluded the use of standard textbooks

(e.g., Pennak, 1989; Thorp & Covitch, 1991). Very few of the common species found

here are mentioned in Pennak (1989), and none of the listed or candidate taxa. However,

species-level manuals have long been available for many North American freshwater

forms. Where possible, the standard references (e.g., Burch, 1989 or its two

predecessors Burch & Tottenham, 1980-Burch, 1982b-Burch, 1983 and Burch,

1982a for gastropods; Burch, 1972 and Clarke, 1973, 1981 for sphaeriids: Burch,

1973 for unionacean bivalves) were used. For undescribed taxa and recent changes in

nomenclature, reference was made to the periodical and grey literature (e.g. Taylor,

l982a, b c d). We also made use of our own rather extensive reference collections. We

have also examined large numbers of specimens of some taxa in the major U.S. museums

(e.g. ANSP, UMMZ, UCM, DMNH, NMNH, CAS). Common names, and species endings, are

generally those of Turgeon et a/. (1988) where possible. Higher taxonomic arrangement

is that of Vaught (1989), except for that of the Sphaeriidae, which follows McMahon (in

Thorp & Covich, 1991). We use the species definitions and ranges of Taylor (e.g., as in

Taylor, 1981) in preference to other sources for certain western North American

forms.

RESULTS

Listed and Candidate Taxa

No specimens in any condition of the listed species Snake River physa, Idaho

springsnail, Bliss Rapids snail, or Banbury Springs lanx were noted in this survey. We

also did not find any specimens in any condition of the candidates Columbia pebblesnail

and Shortface lanx. Absence of the latter was somewhat surprising, as there are

relatively recent records below the study area. This species was not found at Auger Falls

either (Frest & Johannes, 1993), but does survive in the Kanaka-Boulder Rapids area

and farther downstream. Dead specimens of other listed and candidate taxa were

recovered. These are discussed separately below.

Desert valvata Valvata utahensis

We found specimens of this Endangered taxon at six of our sites (10, 12, 13, 15,

16, 17) (Tables 1, 2). Of these, half (12, 16, 17) are drift sites. Long- and recently-

dead specimens of the Desert valvata were noted in recent drift from the Minidoka Dam

spillway (site 12) and both ends of Lake Walcon (sites 16, 17). Live specimens of the

Desert valvata (Valvata utahensis ) were found at site 10. A marked aerial photo of the

vicinity was faxed to the Bureau of Reclamation September 30, 1992. The site is located

just E. (upstream) of the E. junction of the spillway with the mainstem Snake River, on

the S. side of the river, approximately RM 674.5 (Figures 2, 3: SITE DESCRIPTIONS).

V. utahensis was found in a T-shaped area at the base of a short basalt cliff. Three

Redcedars (Juniperus) overhang the site. The area with live and recently dead V.

utahensis extends for approximately 28' W.-E. and averages 12' in width. Toward the

center, the width extends to 18'. A total of 8 live specimens were noted in this area

during sample collection. The live individuals were returned to the site. The collected

sample was found to contain an additional 4 live individuals and a total of 81 dead

specimens when processed and picked in the lab. The area is sheltered from direct effects

of the spillway, and flow though it is currently E.-W. and Slow. Maximum depth in the

colony area was 2 8 ; to the E., depth shallows to 6" or less in the vicinity of a small

wetland (Typha marsh). Substrate in the colony is predominantly oxygenated mud over

basalt bedrock. Groundwater flow or leakage from Minidoka Dam through the colony area

probably maintains oxygen levels sufficient to sustain this species. The substrate

becomes mixed gravel-mud to the E., and is predominantly deoxygenated mud in the

vicinity of the Typha marsh. The most common aquatic macrophyte is Potamogeton

pectinatus, present in small patches. Population density of V. utahensis is difficult to

estimate from the small number of live specimens seen, but appears to be in the range of

1 adult/0.25m2.

Presence of live, recently dead, and long dead individuals at site 10 may indicate

long-term occupation. However, the most likely explanation of the colony's presence is

transport from Lake Walcott during the last high Water period, late 1984-early 1985,

and survival due to recent low flows. Dead specimens of this Species were found in the

spillway drift site (12) and a small colony was also found in one of the more permanent

spillway lakes (site 13). The live specimens at site 13 were not noted in the field, but

found during sample processing. The species was quite common as both dead and live

specimens at the one live and two drift sites in Lake Walcon (sites 15-17: Table I). The

live Lake Walcott specimens seen in the field were counted and returned to their habitat.

Both the river colony and the spillway colony are likely ephemeral. In both areas,

groundwater flow maintains conditions during the current drought period that are

atypical of the bulk of the surrounding habitat. The faunal lists for both sites are

atypically large, and contain species not found alive at comparable sites, but still live in

Lake Walcott (Tables 2, 3). It is of some interest that not even long-dead specimens of

the Desert valvata were found at our river sites. This may Suggest that drift specimens

of this species do not travel far; or at least are relatively perishable unless present in

large numbers or replenished from very local sources.

We believe that the live finds at sites 10, and 13 are wash-in dating to the last

normal flow period, probably placed in their present location around late 1984-early

1985. The volume of water typically flowing though the spillway (FONSI, 1991) should

preclude the establishment of permanent mollusc colonies of any kind. The colony below

the spillway is probably insulated from all but the larger flows: however, large flooding

events, such as that in 1984-85, probably scour this area, as well as the whole of the

spillway

The sizeable number of specimens recovered from Lake Walcott (at least 946)

suggests that the population there could be quite extensive. With only two sites, it is

unwise to speculate on its extent: but as the two are widely separated, the species could

occur through the length of the lake. Detailed surveying will be necessary to determine

the species population size, distribution, and ecology; but given the problems with

middle Snake River populations (see, e.g., Frest & Johannes, 1992b), the significance

of the Lake Walcott population should not be underestimated.

California floater Anodonta californiensis

Live or recently dead specimens of the candidate California floater (Anodonta

californiensis) were noted at a total of 3 sites (13, 15, 17: Tables 1, 2). One of these

sites (13) is in a spillway lake, and probably represents an ephemeral population, as

argued above. The other two sites are in or on the shore of Lake Walcott at opposite ends

(Figures 1, 2). It is probable that the species survives near or at both of them, and the

population in Lake Walcott may be substantial. As with the Desert valvata, no specimens

in any condition were noted below Minidoka Dam. This Species was found to be uncommon

to rare in the Auger Falls area (Frest & Johannes, 1993, and is perhaps more abundant

in the Kanaka-Boulder Rapids Reach (Frest & Johannes, 1991, 1992c) Both of these

areas are rapidly becoming poor habitat for this species due to dissolved O2 problems,

sedimentation, and overgrowth of gravel/soft substrate by aquatic macrophytes. The

species could easily become locally extinct if conditions deteriorate much further.

Malacofauna of the Survev Area

Most of the study area appears to have been good mollusc habitat in the recent past.

Cold water native species are common as long-dead specimens in some samples from

below Minidoka Dam, but seldom survive now. A particularly good example is the

Vagrant pebblesnail, Fluminicola hindsi. This species is quite rare in Lake Walcott but

probably formerly a dominant or subdominant in the rocky areas below the dam. As

long-dead specimens, it was noted from a total of ten sites, half of which were river

sites. It was common only at these sites. Only eight cold water species were found in any

condition (as contrasted with 11 or more in the Auger Falls area). Certain species, such

as the Bliss Rapids snail and Snake River physa) may well have formerly lived in this

area, at least below Minidoka Dam: for habitat reasons, survival in Lake Walcott is

unlikely. However, no specimens were encountered in our study, even in the rocky areas

below Minidoka Dam. In part, this may be due to the physical features and flow regime of

the Powerplant. We were unable to locate 1984-85 drift samples in the area, and old

specimens in the river were comparatively rare (contrast Frest & Johannes, 1991,

1993). Ignoring probable ephemeral sites (10, 13), only one cold water species was

found live below Minidoka Dam, the Artemesian rams-horn Vorticifex effusa (two live

sites). This species is somewhat more tolerant than most other cold water forms, and

shows a similar survival pattern at Thousand Springs, Auger Falls, and in the Kanaka-

Boulder Rapids Reach (Frest & Johannes, 1991, 1992b, 1993). In the Auger Falls

study, only one cold water form was at all common, the Perforated peaclam Pisidium

punctatum. In this study, this sphaeriid survived only in Lake Walcott and at an

ephemeral site most likely replenished periodically from Lake Walcott.

The majority of the cold water forms known to survive in the area (at least at long-

term sites) are located in Lake Walcott. These include the California Floater Anodonta

californiensis, the Desert valvata V. ufahensis, the Glossy valvata V. humeralis, and the

Mossy valvata V. sincera sincera (Tables 2, 3). The Desert valvata has been discussed

above. This taxon was historically one of the most abundant in the middle Snake River

(Taylor, in Malde & Powers, 1962), and is an abundant Pliocene-Pleistocene fossil in

(Taylor, in Maide & Powers, 1962), and is an abundant Pliocene-Pleistocene fossil in

the same area. Only a few sites are extant in the middle Snake River (USFWS, 1992).

The Glossy valvata once was widespread in the middle Snake River but is now effectively

extirpated from it. in Idaho, most surviving populations are found in major Snake River

tributaries in the southeastern part of the state. There are two middle Snake drainage

sites, both in large spring-fed streams. V. humeralis is also an abundant Pliocene-

Pleistocene fossil in the middle and upper Snake River area. The Mossy valvata (Valvata

sincera sincera) is a common fossil in Pliocene and Pleistocene middle-upper Snake

River deposits. It had not been previously reported live in Idaho; but the species still

occurs in the upper Snake River in Teton Co., Wyoming. It is relatively common in the

northern states E. of the Mississippi River and in adjacent parts of Canada. The only

specimens seen of another cold water Snake River endemic, the Rustic pondsnail

Stagnicola hinkleyi, were also from one of the Lake Walcott sites. This species is now

undergoing rapid range contraction due to pollution: see Frest & Johannes (1992b,

1993) for discussion.

As compared to the middle Snake River (especially the Hagerman-Bliss Reaches),

the local malacofauna is relatively small. With the exception of Vorticifex effusa, it is

composed only of pollution-tolerant species. Total diversity at the sites below Minidoka

Dam (excluding site 10, which is likely temporary) is only 10 species (Tables 2, 3).

The most diverse site (1) had only 8 species; average diversity is 5. In contrast, Lake

Walcott diversity is about 24 species, and average diversity is 13. For the area as a

whole, diversity is 26 species (17 gastropods; 9 bivalves: Table 3), less than (but

roughly comparable to) that at Auger Falls: the missing species are mostly cold water

forms. One introduced snail, the Big-ear radix Radix auricularia, occurs uncommonly in

Lake Walcott; the New Zealand mudsnail has yet to make its appearance. At most sites

below Minidoka Dam, snails are uncommon. Most of the Minidoka Dam area has a low

diversity fauna dominated in nearly all habitats by either the Ash gyro Gyraulus pawus

or by the Tadpole physa Physella gyrina. Generally, only one or two species (Physella

gyrina, Gyraulus parvus, and Physella integra in order of abundance) are present in any

numbers at any single site. This is also reminiscent of the situation at Auger Falls, and

contrasts with the higher diversity lower dominance faunas at less disturbed middle

Snake River sites. Relatively unimpacted Snake drainage sites, both middle or upper, are

most likely to be dominated by the Vagrant pebbiesnail Fluminicola hinds;.

At site 17, a single recently dead specimen of a new Pyrguiopsis species was noted.

This taxon has not been seen previously in Idaho. We have recently collected a number of

undescribed forms in springs in the Great Basin portion of Idaho; but none appears

identical to this species. Judging from the shell morphology and its rarity, it is likely

that the specimen represents drift from a lake-side spring on the E. end of Lake Walcott

that we have not yet collected. The shell morphology is most similar to that of two other

spring-restricted forms from the Deep Creek Mts. and not closely similar to that of the

known river-dwelling forms. Shell morphology also suggests that this species is a cold

water form; but we prefer to confirm this from live specimens of certain provenance

before adding it to the cold water list.

DISCUSSION

Our survey involved a total of 17 sampled sites. No live specimens of four of the five

listed mainstem Snake River snail taxa were found. The Endangered Desert valvata was

found at six sites. The live sites in the mainstem river and in the spillway (one each)

are believed to be temporary and founded by waifs from Lake Walcott; this lake may have

a large live population, which should be carefully protected. The candidate California

floater was collected at three sites. One of these is likely not a live or persistent

population, but there may be a sizeable live population in Lake Walcott. Both species

probably inhabited the area below Minidoka Dam historically, and it is quite possible

that other listed or candidate species (namely the Snake River physa, the Bliss Rapids

snail, and the Shortface lanx) formerly did so also. Given current water quality

conditions, it is unlikely that any of the listed taxa currently maintain long-term

populations below Minidoka Dam. The proposed project is unlikely to impact any other

than ephemeral populations of the listed or candidate species. Were water quality to

improve, it is quite possible that some or all of these species would be able to reoccupy

the area, as long as basic physical features remain unaltered.

Cold water species are now rare and local below the Dam; they were likely dominant

historically. Local extinction of many of the cold Water native species has already

occurred. Aside from the listed and candidate taxa, these are the Mossy valvata Vaivata

sincera sincera, the Glossy valvata Valvata humeralis, and the Rustic pondsnail

Stagnicoia hinkleyi. A drastic reduction in total freshwater mollusc diversity is noted if

the present fauna is compared to that of Lake Walcott, or if long-dead species are

compared to extant diversity in the river sites (Tables 2, 3). The modern fauna is very

low in diversity, averaging 5 species per site. Only 3 species are at all common. M O S ~ of

the reduction stems from loss of the listed taxa and other cold water species, many of

which are also regional endemics. The common surviving native forms are all

widespread pollution-tolerant taxa. Lake Walcott is fairly eutropified at present, but

remains a reservoir of native diversity. Spring influx into the eastern end of the lake is

likely responsible; irrigation return into the lake may also be comparatively smaller

than at some downstream sites. Macrophyte beds in the lake are dense, and it appeared

likely from our very limited sampling that the central areas of the lake were not

suitable mollusc habitat due to possible hypoxic or anoxic substrate conditions. Further

eutropification of the lake could easily lead to loss of the cold water fauna, including

listed taxa. Species preferring hard substrate may be rare in the lake at present. The

cemented substrate and rapid and variable flows typical of the area below Minidoka Dam

are partially responsible for the current low diversity. High nutrient levels are likely

also proximate causes both of substrate cementation and low mollusc diversity. In this

light the following statement from the FONSI (Bureau of Reclamation, 1991, p. 20) is

questionable, to say the least: " Nutrient rich waters from the reservoir provide a

productive medium for vigorous growth of algae and aquatic invertebrates. The abundant

food source of aquatic insects enhances the area's fish populations and sustains a valuable

fishery (U.S. Fish and Wildlife Service, 1988)". The current invertebrate fauna is best

described as low diversity and low abundance: but epiphytic algae and other organisms

(which may be termed "visible slime growths or other nuisance aquatic growths" or

"oxygen-demanding materials in concentrations that would result in an anaerobic water

condition" under Idaho Water Quality Board Standards) are certainly thriving.

Three species have not been reported previously from the area. The Mossy valvata

(Valvata sincera sincera) has not been found alive in Idaho before. The Sharp sprite

(Promenetus exacuous) is also new to the fauna: previous Idaho records are from Neitzel

& Frest (in press) and Frest & Johannes (1993). Both species are more typically found

at sites E. of the Rocky Mountains. The third form, Pyrgulopsis n. sp., is likely an

undescribed spring taxon so far found only as "river" (Lake Walcott) drift.

It is unlikely that any of the noted mollusc faunal changes below Minidoka Dam are

attributable solely or largely to the recent low flows. The listed and candidate taxa

survive relatively well in a few unimpacted sites downstream despite low flows. At least

two such taxa also survive immediately upstream. If it is desirable to restore, maintain,

or enhance the native cold water biota, further degradation of the study area should be

avoided: this is especially true for Lake Walcott. Improvements in flow would be helpful;

but much more important would be improvements in water quality, including decreased

average temperature, increased dissolved oxygen, and decreased nutrient input. Cold

spring influx, with its notable buffering effects, is relatively limited in comparison to

the Hagerman-Bliss Reaches. The area below Minidoka Dam is a presage of what

conditions in the rocky portions of the middle Snake River could rapidly become. Given

the pressures that the downstream reaches already face, deterioration of the reaches

immediately upstream from many to all surviving populations of the five listed species

should be avoided.

ACKNOWLEDGEMENTS

Bob Adair of the U. S. Bureau of Reclamation provided maps and background information

concerning the proposed project. We especially thank Al Van Vooren and William Horton

of Idaho Department of Fish and Game for expediting our application for a scientific

collecting permit for this survey.

SITE DESCRIPTIONS

Legal descriptions are based on current USGS 7.5' topographic quadrangle maps;

river mile (RM) designations derive from recent USGS corrections, which may not be

incorporated on latest available maps. Collectors' name abbreviations: TF- Terrence

Frest; EJ- Edward Johannes.

1. N ~ N G NW, sW, NW, sec. 1, T9S R25E, Lake Walcott West quad., Minidoka Co., Idaho. Boulder rapids on the N. side of Snake River at RM 574.6, near public boat launch above pool, 0.4 mi. downstream (W.) of Minidoka Dam. Sample taken 9' from N. shore and 19' from high-water mark. Elev. 4140'. Depth 2". Cobble-boulder substrate with pockets of mud, sand and fine gravel on bedrock (basalt) substrate. Scattered Potamogeton pectinatus patches; mixed epiphytic crust on boulders. Abundant and diverse epiphytic algae; rare sponge, fungal patches; rare blue-green algae ("Nostoc"). Sample is gravel-mud from pockets between boulders. Probably partially older drift. Abundant dead molluscs (mostly lymnaeids); a few live lymnaeids, physids, sphaeriids. Common long-dead Fluminicola hindsi and Vorticifex effusa. Dip net collection. 9/26/92 TF, EJ!

2. S W ~ NW/4 N d sW^ Nh$ sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Boulder rapids at about mid-channel in the Snake River at RM 574.6, near public boat launch just above pool, 0.4 mi. downstream (W.) of Minidoka Dam. Sample taken 75' from N. shore and 84' from high-water mark. Elev. 4140'. Depth 18". Rapid flow rate. Cobble-boulders on bedrock (basalt) substrate. Cobbles and boulders are cemented to the bottom, most encrusted with diverse epiphytic algae. Abundant Cladophora, flat green algae, uncommon Potamogeton pectlnatus, rare blue-green algae ("Nostoc"}. Physella only live, rare. Hand, brush & tray, dip net collections. 9/26/92 TF, EJ!

3. sW, sW~ N d sW, NW, sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. N. side of Snake River at RM 574.7, 0.1 mi. upstream from rapids, 0.3 mi. downstream (W.) of Minidoka Dam. Sample taken 100' from N. shore and 11 1' from high-water mark. Elev. 4140'. Depth 1 2 . Cobble- boulders on bedrock (basalt) bottom, with minor potholes in which sand and gravel substrate occur. Most larger boulders are cemented to the bottom (some with their tops above water). Potamogeton pectinatus in sand-gravel patches; all exposed surfaces (except seasonally aerially exposed) covered with algae. Mostly Cladophora, blue green algae, fungi, ?sponge patches, uncommon blue-green algal balls ("Nostoc"). Sample probably partially older drift. Abundant dead molluscs (mostly lymnaeids); few live lymnaeids, physids, sphaeriids. Common long-dead Fluminicola hindsi and Vorticifex effusa. Hand, brush & tray, dip net collections. 9/26/92 TF, EJ!

4. SE- SE: NV\^ s'& NW, sec. 1, T9S R25E. Lake Walcott West quad., Cassia Co., Idaho. Middle of Snake River at RM 574.7, 0.1 mi. upstream from rapids, 0.3 mi. downstream (W.) of Minidoka Dam. Sample taken 200' from N. shore and 211' from high-water mark, near edge of deeper channel (thalweg). Elev. 4140'. Depth 18". Bare basalt bedrock with uncommon boulders, cobbles, sand and gravel in potholes. Most large liths are cemented to the bottom. Scattered Potamogeton pectinatus in sand-gravel patches. Abundant epiphytic algae (many kinds including blue-green) on all exposed

rock sides (rarer on rocks likely to be exposed). Small patches Of fungi-bacterial slime on rocks; uncommon Nostoc, ?Rivularia. Live Physella and others very spotty; locally common; mostly juveniles. Hand, brush & tray, and dip net collections. 9/26/92 TF, EJ!

5. sV$ sE; N E ~ S$ N$ sec. 1, T9S R25E, Lake Walcott West quad., Minidoka Co., Idaho. N. side of Snake River at RM 674.8, 0.2 mi. above rapids, 0.2 mi. downstream (W.) of Minidoka Dam. Sample taken 100' from N. shore and 106' from high-water mark. Elev. 4140'. Depth 1 8 . Mud, gravel, cobbles and boulders (many welded to the bottom) over a basalt bedrock substrate. Potamogeton pectinatus common at this site. Algae cover most boulders. Molluscs collected by hand tray and brush, and dip net. 9/26/92 TF, EJ!

6. N d N E ~ S& sW, N$ sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Mid-channel of Snake River at RM 674.8, 0.2 mi. above rapids, 0.2 mi. downstream (W.) of Minidoka Dam. Sample taken 150' from N. shore and 156' from high-water mark, at edge of main channel (thalweg). Elev. 4140'. Depth 18-38". Potamogeton pectinatus beds in gravel and mud with larger liths common on bedrock (basalt) surface; boulders and cobbles almost completely submerged; slightly turbid; moderate velocity. Common epiphytic algae on Potamogeton pectinatus; abundant and diverse on exposed rock surfaces; uncommon Nostoc. Snails collected from Potamogeton pectinatus, boulders and cobbles. Abundant Gyraulus parvus on Potamogeton pectinatus; lymnaeids and physids most common on rocks. Hand, brush & tray, and dip net collections, 9/26/92 TF, EJI

7. sV$ S& S$ S& N$ sec. 1, T9S R25E, Lake Walcott West quad., Minidoka Co., Idaho. N. side of Snake River at RM 674.9, near Minidoka Dam outfall (0.1 mi. below). Sample taken 9' from N. shore and 12' from high-water mark at edge of deep channel (thalweg) and rapids. Elev. 4140'. Depth 18". Mostly boulders; few scattered cobbles, gravel patches; uncommon Potamogeton pectinatus; rapid velocity; algal flora diverse on exposed rock; common Rivularia and Nostoc. Molluscs rare; physids and rare Gyraulus parvus. Dip net or brushed into tray from rocks. 9/26/92 TF, EJ!

8. NE, SE, S E ~ NE, NE: sec. 2, T9S R25E, Lake WalCOtt West quad., Minidoka Co., Idaho. Pool about 0.1 mi. below rapids on N. side of Snake River at RM 674.5, below public boat ramp, about 0.7 mi downstream (W.) of Minidoka Dam. Sample collected 200' from N. shore, 212' from high-water mark. Elev. 4140'. Depth 36". Deep, relatively quiet pool; gravel substrate (mixed lithologies); low velocity; no macrophytes, although common Ceratophyllum occurs E. of the pool; bare gravel with almost no epiphytes. No live ~ O I I U S C S seen. Old, long-dead Fluminicola hindsi; some in pool relatively recent dead. Dip net collection. 9/26/92 TF, EJ!

9. N E ~ S E ~ SE- NE- NG sec. 2, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Boulder rapids ca. 100' from spillway edge (basalt cliff, ca. 10' in height) on S. side of Snake River at RM 674.6, opposite sites 1 and 2, about 0.4 mi downstream (W.) of Minidoka Dam. Elev. 4140'. Depth 3 6 . Common loose and cemented boulders; bedrock not obvious; common sponge, abundant epiphytic algae on exposed rock surfaces; few macrophytes (some Potamogeton pectinatus); some fungi on under surfaces; some Rivularia (small); swift current. Snails rather uncommon. Hand, brush & tray, and dip net collections. 9/26/92 TF, EJ!

10. NW- N E ~ S$ Sk$ N$ sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Pool behind rock berm (loose boulders-possibly submerged in high water), just E. of spillway area along basalt "cliff' edge on the S. side of the Snake River at RM 674.65, 0.35 mi. downstream (W.) of Minidoka Dam. Elev. 4140'. Depth 36". Relatively cold pool (ground water inflow at cliff edge). Mud substrate (oxygenated) with gravel patches; scattered Potamogeton pectinatus; moderate velocity. Uncommon live Valvata utahensis in T-shaped area about 28' W.-E. and about 12' in average width; near center, width to 18'. Dip net collection. 9/26/92 TF, EJ!

11. sW- NW; S+ sW- N W ~ sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Shallow area about 150' E. of site 10, near S. shore of Snake River (ca. 60' N. of high-water mark) and 4 5 N. of edge of Typha marsh, Snake R. at RM 674.75, 0.25 mi. downstream from (W. of) Minidoka Dam. Elev. 4140'. Depth 6-12". Gravel-mud flat offshore from Typha marsh; slow current; no macrophytes present; minor epiphytic algae on sediment surface. Very few molluscs seen. Dip net collection. 9/26/92 TF, EJ!

12. SE- sW, sW- S@ NW^ sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Basalt bedrock in E. side of spillway channel (now dry), ca. 250' S. of spillway edge, S. side of Snake River. Elev. 4160'. Dry channel. Drift sample on dry Potamogeton pectinatus. Scattered Valvata utahensis, some recently dead. Hand collected, sieved in the field. 9/26/92 TF, EJ!

13. N E ~ SW; S& S$ N+ sec. 2, T9S R25E, Lake Walcon West quad.. Cassia Co., Idaho. Lower (W.) permanent lake in spillway area on S. side of Snake River channel. Elev. 4150'. Depth 36". Mud, sand and medium gravel; no macrophytes; no current; cold, slightly turbid lake. Single valve of freshly dead A. californiensis; several long-dead and three immature live Valvata utahensis; long-dead Fluminicola hinds/; a few live sphaeriids. Dip net collection. 9/26/92 TF, EJ!

14. S E ~ ME, N W ~ sW; S W ~ sec. 1, T9S R25E, Lake Walcott West quad., Cassia Co., Idaho. Upper (E.) permanent lake in upper spillway area on S. side of Snake River below Minidoka Dam, ca. 75' from S. shore. Elev. 4170'. Depth 13". Mud, sand, and fine gravel; scattered Potamogeton pectinatus elsewhere in lake; few epiphytic algae on sediment surface; no current; cold lake. Dip net collection, but no molluscs seen. 9/26/92 TF, EJ!

15. N E ~ S E ~ NE, N E ~ N d sec. 1, T9S R25E, Lake Walcott West quad., Minidoka Co., Idaho. Just W. of public boat ramp in Walcott Park, ca. 82' offshore (from current shore), 102' from high-water mark, N. shore of Lake Walcott (Snake River) at RM 675.7. Elev. 4195'. Depth 40". Poorly oxygenated mud offshore; sand and gravel near shore; abundant macrophytes such as Potamogeton crispus, Potamogeton pectinatus, Ceratophyllum ; some epiphytic algae on macrophytes. Thin-stemmed macrophytes up to 8' or more in length. Live Valvata utahensis, Valvata humeralis; recently dead Anodonta californiensis ; common Physella. Dip net collection. 9/26/92 TF, EJ!

16. S< st$ S& S& S& sec. 36, R25E, Lake Walcott West quad., Minidoka Co., Idaho. Beach at high-water mark, public boat ramp in Walcott Park, N. shore of Lake Walcott (Snake River) at RM 675.7. Elev. 4195'. Common recently and long-dead Valvata utahensis. Drift sample, partially sieved in field. 9/26/92 TF, EJ!

17. N E ~ SW SE; SE; S E ~ sec. 1, T9S R29E, Register Rock quad., Power Co., Idaho. Drift at high water mark just W. of boat ramp in Massacre Rocks State Park and opposite Mowers Spring, Lake Walcott (S. side of Snake River). Elev. 4195'. Uncommon recently and long dead (including possible Pleistocene fossil) Promenetus exacuous, Valvata humeralis, Valvata utahensis; recently dead Anodonta californiensis; long-dead Fluminicola hindsi rare. Drift sample, partially sieved in field. 9127192 TF, EJ!

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Bowler, P. 1990. The Rapid Spread of Potamopyrgus antipodarum (Gray) in the Middle Snake River, southern Idaho. Proc. Desert Fishes Council 21: 173-182.

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Clarke, A. 1973. The Freshwater Molluscs of the Canadian Interior Basin. Malacologia 13: 1-509.

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TABLE 1. LISTED AND CANDIDATE MOLLUSCS AT COLLECTION SITES.

TABLE EXPLANATION: +=live specimen x=recently dead specimen (<1 year) o=iong dead specimen (>I year) T=total specimens at a site []=all or part not retained

TABLE EXPLANATION: +=live specimen x=recently dead specimen (<1 year) oblong dead specimen (>1 year) bold face=cold water taxon

TABLE 2. MOLLUSC FAUNA AT COLLECTION SITES.

TAXON NAME

Vallonia cyclophorella

NO. LISTED OR CANDIDATE:

NO. COLD WATER SPECIES:

LIVE AQUATIC DIVERSITY (+ 8. x)

TOTAL AQUATIC DIVERSITY

0

1

8

11

0 1

3

4

0

1

3

4

0 2

4

5

0 1

5

5

0

1

5 6

TABLE 2. MOLLUSC FAUNA AT COLLECTION SITES.

TAXON NAME

TABLE EXPLANATION: +=live specimen x-recently dead specimen (c1 year) oblong dead specimen (>I year) bold face=cold water taxon

TABLE 2. MOLLUSC FAUNA AT COLLECTION SITES. corn.

TAXON NAME

TABLE EXPLANATION: +=live specimen x=recently dead specimen ( < I year) o=Song dead specimen (>I year) bold face=cold water taxon

3 5

TABLE 3. COMPARISON OF HISTORIC AND CURRENT MOLLUSC FAUNA IN STUDY AREA.

TAXON NAME HISTORIC PRESENT LAKE COMMENTS MINIDOKA WALCOTT

(SITES (SITES

AQUATIC SNAILS: Valvata sincera sincera Say, 1824 Valvata humeralis Say, 1829 x Valvata utahensis Call, 1884 x Bliss Rapids snail 1 Fluminicola hindsi (Baird, 1863) x Stagnicola (H.) caperata (Say, 1829) x Stagnicola (S.) hinkleyl (Baker, 1906) x Fossaria (F.) modicella Say, 1825 x Radix auricularia (Lime, 1758) -I 1 Physella (P.) gyrina (Say, 1821) x Physella (C.) Integra (Haldeman, 1841) x Physa (H.j natricina Taylor, 1988 ? Planorbella (P.) subcrenatum (Carpenter, 1857) x Gyraulus (T.) parvus (Say, 181 7) x Promenetus exacuous (Say, 1821) Vorticifex effusa (Lea, 1856) x Ferrissia parallella (Haldeman, 1841) x Ferrissia rivularis (Say, 181 7) x

BIVALVES: Anodonta californiensis Lea, 1852 Gonidea anaulata (Lea. 1839) Sphaerium simile (say, '1816)' Musculium lacustre (Miiller, 1774) Musculium securis (Prime, 1852) Pisidium (C.) variabile Prime, 1852 Pisidium (C.) casertanurn (Poll, 1795) Pisidium (C.) compressum Prime, 1852 Pisidium (N.) punctatum Sterki, 1895

Extinct in middle Snake River. May be relict taxon in this area. Extinct in middle Snake River. Probably common locally until recently. Decreasing in Snake River. Probably dominant locally until recently. Decreasing in middle Snake River. No definite records in area. Decreasing in Snake River. Now rare locally; formerly dominant. Increasing in Snake River. Becoming common locally. Decreasing in Snake River. Probably common locally until recently. Stable in middle Snake River. Insufficient information locally. Introduced to system in 1930s. Increasing in Snake River. Increasing locally, Decreasing in middle Snake River. Stable locally. Decreasing in middle Snake River. No definite records in area.

x ~ncreasin~in Snake River. Increasing locally. x Increasing in Snake River. Increasing locally. x May be relict taxon in this area. x Decreasina in Snake River. Decreasina locallv. x ~ncreasin~in Snake River. Probably rare locally until recently x Increasing in Snake River; probably rare locally until recently.

x Decreasina in middle Snake River. Uncommon (decreasina) locallv -. ~ncreasin~in middle Snake River. Decreasing locally.

x Increasing in middle Snake River. Decreasing locally. x increasing in middle Snake River. lncreasing~ocally:

Increasing in middle Snake River. Increasing locally. x Stable in middle Snake River. Local status uncertain.

Increasing in middle Snake River. Local status uncertain. x Increasing in middle Snake River. Local status uncertain. x Stable in middle Snake River. Local status uncertain.

TOTAL DIVERSITY AND TRENDS

TABLE EXPLANATION: l=lntroduced x=present -=absent "=not previously reported from area