LEGACY EFFECTS OF COLONIAL MILLPONDS ON FLOODPLAIN SEDIMENTATION,BANK EROSION, AND CHANNEL MORPHOLOGY, MID-ATLANTIC, USA1

Edward R. Schenk and Cliff R. Hupp2

ABSTRACT: Many rivers and streams of the Mid-Atlantic Region, United States (U.S.) have been altered bypostcolonial floodplain sedimentation (legacy sediment) associated with numerous milldams. Little ConestogaCreek, Pennsylvania, a tributary to the Susquehanna River and the Chesapeake Bay, is one of these streams.Floodplain sedimentation rates, bank erosion rates, and channel morphology were measured annually during2004-2007 at five sites along a 28-km length of Little Conestoga Creek with nine colonial era milldams (onedam was still in place in 2007). This study was part of a larger cooperative effort to quantify floodplain sedi-mentation, bank erosion, and channel morphology in a high sediment yielding region of the Chesapeake Baywatershed. Data from the five sites were used to estimate the annual volume and mass of sediment stored onthe floodplain and eroded from the banks for 14 segments along the 28-km length of creek. A bank and flood-plain reach based sediment budget (sediment budget) was constructed for the 28 km by summing the net vol-ume of sediment deposited and eroded from each segment. Mean floodplain sedimentation rates for LittleConestoga Creek were variable, with erosion at one upstream site ()5 mm ⁄ year) to deposition at the otherfour sites (highest = 11 mm ⁄ year) despite over a meter of floodplain aggradation from postcolonial sedimenta-tion. Mean bank erosion rates range between 29 and 163 mm ⁄ year among the five sites. Bank heightincreased 1 m for every 10.6 m of channel width, from upstream to downstream (R2 = 0.79, p < 0.0001) result-ing in progressively lowered hydraulic connectivity between the channel and the floodplain. Floodplain sedi-mentation and bank erosion rates also appear to be affected by the proximity of the segments to one existingmilldam, which promotes deposition upstream and scouring downstream. The floodplain and bank along the28-km reach produced a net mean sediment loss of 5,634 Mg ⁄ year for 2004-2007, indicating that bank erosionwas exceeding floodplain sedimentation. In particular, the three segments between the existing dam and theconfluence with the Conestoga River (32% of the studied reach) account for 97% of the measured net sedimentbudget. Future research directed at understanding channel equilibria should facilitate efforts to reduce thesediment impacts of dam removal and legacy sediment.

(KEY TERMS: sediment; fluvial processes; geomorphology; rivers ⁄ streams; dams.)

Schenk, Edward R. and Cliff R. Hupp, 2009. Legacy Effects of Colonial Millponds on Floodplain Sedimentation,Bank Erosion, and Channel Morphology, Mid-Atlantic, USA. Journal of the American Water ResourcesAssociation (JAWRA) 45(3):597-606. DOI: 10.1111/j.1752-1688.2009.00308.x

1Paper No. JAWRA-08-0032-P of the Journal of the American Water Resources Association (JAWRA). Received February 12, 2008;accepted September 29, 2008. ª 2009 American Water Resources Association. Discussions are open until six months from print publi-cation.

2Respectively, Ecologist (Schenk and Hupp), U.S. Geological Survey, 12201 Sunrise Valley Drive Mail Stop 430, Reston, Virginia 20192(E-Mail ⁄ Schenk: eschenk@usgs.gov).

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INTRODUCTION

The legacy effects from postcolonial agriculture onstream sediment loads are increasingly relevant assediment loads remain high in many watersheds evenafter extensive soil conservation practices are imple-mented (Meade, 1982; Knox, 2002). This is a potentialproblem in many watersheds across the UnitedStates (U.S.), where postcolonial agricultural prac-tices resulted in accelerated erosion and sedimenta-tion and where much of the sediment remains withinfloodplains (Knox, 2002). Postcolonial anthropomor-phic changes were numerous and include channeli-zation, damming and un-damming waterways, and avariety of other activities that have disrupted normalchannel equilibrium, often with unpredicted conse-quences (e.g., Simon, 1989a,b; Walling, 1999; Pizzuto,2002; Fitzpatrick, 2005; Knox, 2006; Walter and Mer-ritts, 2008; Kroes and Hupp, In press).

The majority of the Mid-Atlantic Region was con-verted from forest to intensive row crop agriculturein the 18th century (Merritts et al., 2004). The lackof soil conservation practices led to substantial ero-sion from the Piedmont uplands and subsequentdeposition of sediment in the floodplains and streamchannels (Jacobson and Coleman, 1986). This thicksediment layer is pervasive along the floodplains ofthe Eastern U.S. (Hupp, 2000) and is often referredto as legacy sediment (Walter et al., 2007).

The trapping of legacy sediment was particularlypronounced throughout the Conestoga Riverwatershed (tributary to the Susquehanna and eventu-ally the Chesapeake Bay) because of numerous dams.By the mid-19th century the Conestoga Riverwatershed was impounded approximately every2.5 km creating slackwaters extending upstreamapproximately 2.4 km, effectively creating a series ofslackwater reservoirs (Merritts et al., 2004). The ser-ies of slackwater reservoirs effectively trapped andstored legacy sediment as it was eroded off of thenewly tilled fields. With time many of these low-head(<5 m high) milldams have been removed or havebeen breeched through neglect. In other stream sys-tems, dam removal, or failure, caused episodes ofincreased downstream sedimentation, bank erosion,and change in floodplain dynamics (Shuman, 1995;Fitzpatrick, 2005; Fitzpatrick et al., 2007). Each sedi-ment pulse from dam removal, and subsequent head-cut retreat, may take decades to flush through asystem (Simons and Simons, 1991) channel adjust-ments, including bank erosion of legacy sediments,may take even longer to reach equilibrium.

The Little Conestoga Creek (LCC), Pennsylvaniawas heavily impacted over the last 150 years byland use change and the construction of numerous

mill dams. The LCC floodplain is relatively discon-nected from the channel due to the elevation gaincaused by the deposition of 1-2 m of legacy sedimentduring initial land clearing. The milldams along theLCC are in various stages of function. With theremoval of most dams the channel elevation has theability to return to presettlement levels throughincision through typically fine grained in-channellegacy sediment deposits. Legacy sedimentationalong the LCC has elevated the floodplain thusdiminishing the sediment sink function of the flood-plain, possibly similar to the creation of river ter-races adjacent to an incising river (Hupp, 2000;Jackson et al., 2005; Hupp and Rinaldi, 2007; Walterand Merritts, 2008). This is one of several researchprojects assessing high sediment yields produced byPennsylvania streams tributary to the ChesapeakeBay (Gellis et al., 2004; Hubbard, 2006; Walter andMerritts, 2008).

FIGURE 1. Study Site and River Segment Locations on the LittleConestoga Creek. Red circles represent study sites, solid blackhashes represent river segment divisions; river segments betweenstudy sites are represented by the mean floodplain deposition andbank erosion rates from the two adjacent study sites (see inset).

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The objectives of this study are to quantify flood-plain sedimentation and bank erosion rates on theLCC to facilitate an understanding of sedimentsources and sinks in watersheds impacted by low-head dams and exhibiting substantial postcolonialsedimentation tributary to the Chesapeake Bay. Sedi-mentation and erosion rates were examined spatiallyand temporally in comparison with other factors,including discharge, floodplain area, and channelmorphology, to determine trends or processes (chan-nel incision, floodplain connectivity, and flow dura-tion) that might affect stream sediment loads.

STUDY AREA

The LCC, a Piedmont stream, drains predomi-nately fractured carbonate bedrock (limestone, dolo-stone, and small amounts of shale) in southeastPennsylvania (Berg and Dodge, 1981; Loper andDavis, 1998). The streambed consists primarily of finesand in the upstream section of our study reachcoarsening up to gravel for the downstream sections.The LCC is a tributary of the Conestoga River (Fig-ure 1), which drains into the Susquehanna River, thelargest river by discharge to the Chesapeake Bay.The Conestoga River watershed has the highest per-centage of tillable land (52%) of any river basinwithin the Susquehanna watershed (Ott et al., 1991).The riparian zone consists of a scattered woodlands,

manicured lawns, and old fields. A detailed long pro-file of the LCC, including the majority of our studysite, is available in Walter and Merritts (2008).

The LCC had approximately one dam every 2.5river kilometers, during the mid-1800s (Figure 1)(Merritts et al., 2004; Walter et al., 2007). Each ofthese low-head milldams was capable of filling withlegacy sediment, storing this sediment for decades,and then eroding when the dam either breechedthrough disrepair, or was intentionally removed.Some, if not many, of the old reservoirs still retain aconsiderable amount of legacy sediment, despite par-tial or complete dam breaches. A drained reservoirlocated in a tributary of the LCC was determinedto have retained over 50% of its trapped sediment(approximately 170,000 Mg) despite the dam’sremoval in 1901 (Walter et al., 2007). Currently, theonly known existing dam along the creek is the 3 mhigh run of river dam downstream of the U.S. Geologi-cal Survey (USGS) streamgage (Figure 1) (DorothyMerritts, personal communication; Pennsylvania Fishand Wildlife Commission 2007).

One USGS streamgage operated on the LCC dur-ing the study period. The streamgage (site number:01576712) was immediately downstream of Site 4(river km 15.5) (Figure 1) and recorded an annualmean streamflow of 2.04, 1.54, and 1.59 m3 ⁄ s (71.9,54.5, and 56.1 cubic feet per second) for calendaryears 2004, 2005, and 2006, respectively. Figure 2provides a hydrograph during our study period.Annual precipitation, measured in Millersville, Penn-sylvania, was 108.1, 98.73, and 123.01 cm in 2004,

FIGURE 2. Hydrograph of Daily Mean Discharge (m3 ⁄ s) Measured at the Millersville U.S. Geological Survey (USGS)Streamgage. Black squares represent field visits to measure floodplain and bank rates. The dashed line represents

the discharge required to overtop the bank as determined from the streamgage’s stage discharge curve.

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2005, and 2006, respectively. The mean precipitation,between 1914 and 2007, is 104.29 cm ⁄ year. The high-est mean monthly precipitation during the study was33.88 cm in June 2006, whereas the lowest was2.59 cm in September 2005 (Millersville UniversityWeather Information Center, 2007).

METHODS

Five sites were selected for intensive study of flood-plain sedimentation and bank erosion along a 28-kmlength of the LCC (Figure 1). Sites were selectedbased on land access and the location of pre-existinggeomorphic studies by Robert Walters, Dorothy Mer-ritts, and students from Franklin and Marshall Col-lege. Transects were placed along representativesections of the floodplain and bank within each site.Clay pads and erosion pins were installed along twotransects per site in July 2004 and measured annu-ally through June 2007. The transects were perpen-dicular to the channel, approximately 100 m apart,and extended the width of the floodplain. Each tran-sect consisted of one to six feldspar clay pads spacedalong the floodplain of each transect on both sides ofthe stream in an approximately 0.5 m diameter circle,2-4 cm deep. Five to nine erosion pins (1-m long ironrods) were also installed in each transect normal tothe ground profile along the bank face, bank top, andinto the floodplain adjacent to the floodplain feldsparclay pads. Exposed pins (‡20 cm of erosion) werereset to the soil surface during each field visit. Sedi-mentation and erosion rates at each site were calcu-lated by dividing the total change by the timebetween installation and the final measurements inJune 2007.

The study reach was divided into 14 river seg-ments to aid in estimating sediment retention andexport from the bank and floodplain. The river seg-ments were created by dividing the section of streambetween each site into three equidistant river seg-ments with one segment upstream of Site 1 (river km1.9) and one segment downstream of Site 5 (river km26.3). The upper boundary of the study area isapproximately 1.5 km upstream of Site 1 with thelower boundary at the confluence with the ConestogaRiver (Figure 1 and Table 1).

Floodplain area, for each river segment, was delin-eated from five foot (1.5 m) contour intervals on topo-graphic maps provided by the Lancaster CountyPlanning Department (2007). Channel length wasdetermined for each river segment using Map sourcedigital topographic maps. Clay pad, erosion pin, andbank height measurements made at study sites were

extrapolated to adjacent river segments to determinereach based bank and floodplain erosion and deposi-tion rates and volumes. River segments that were notadjacent to a study site were assigned a mean flood-plain deposition and bank erosion rate and a meanbank height based on the two bounding study sites.Bank erosion for segments was calculated by multi-plying the erosion rate by the segment bank heightand channel length. Segment floodplain depositionwas calculated by multiplying the segment floodplaindeposition rate by the floodplain area.

The channel and floodplain were surveyed at eachsite along transects during visits to document bankheight, channel width, and changes in channel mor-phology in addition to sediment pin and pad measure-ments. Surveys were completed using a surveyingrod and an optical level and were referenced to tem-porary benchmarks for each transect. Bank heightand channel width measurements were extrapolatedto adjacent river segments and averaged for the non-adjacent river segment between measurement sites.A linear regression was used to explore the statisti-cally significant relation between surveyed bankheight and channel width. A simplified long profilewas also developed using elevation data from Lancas-ter County (Lancaster County Planning Department,2007).

Floodplain sediment characteristics were deter-mined from eight soil samples per transect takenfrom the top 5 cm of each soil column using a 5-cmdiameter stainless steel sleeve. Four of the soil sam-ples were collected from near the clay pads and foursamples were collected from approximately mid-bankbetween erosion pins. The organic content of the top5 cm of floodplain sediment was determined usingloss on ignition (LOI) methods (Nelson and Sommers,1996). Approximately 5 g of each soil sample wasdried for 24 h at 110�C. The samples were thenallowed to cool in a dessicator, weighed to within0.01 g precision and burned for 16 h at 400�C in amuffle furnace. The cooled sample was re-weighedand the percent mass lost was recorded as theorganic content of the sample.

TABLE 1. Study Site Locations and Mean FloodplainDeposition and Bank Erosion Rates Measured at the FiveSites Between July 2004 and June 2007; Study Sites From

Upstream to Downstream Order.

Site River km

FloodplainDeposition(mm ⁄ year)

BankErosion

(mm ⁄ year)

1 1.9 )5 )712 7.7 11 )843 11.9 1 )294 15.5 10 )1635 26.3 3 )94

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Soil bulk density was measured to determine thesediment mass eroded or deposited; the methods areoutlined in the U.S. Department of Agriculture’s SoilSurvey Manual (Burt, 2004). The cores were dried at110�C and weighed every 24 h until the weight stabi-lized within a hundredth of a gram. The dried weightof the core was then divided by the known core vol-ume to provide a bulk density. A paired mean t-testwas performed to compare mean bulk density of bankand floodplain deposits. Bulk density measurementswere extrapolated to adjacent river segments and amean bulk density measurement was developed forthe river segment that was not adjacent to eitherbounding measurement site.

Sediment particle size was determined by grindinga known amount of oven-dried sediment with a mor-tar and pestle and then placing the sediment througha sonic sieve for 12 min. Sieved sediment was exam-ined under a low power microscope to check forbound particles and clay colludes, which wouldrequire further sediment grinding and re-sieving.Samples were sieved until approximately 95% of theparticles were independent for accurate sieving. Sedi-ment size is reported as a weight fraction with sedi-ment divided into categories of greater than 0.5 mm(coarse sand), between 0.25 and 0.5 mm (sand), 0.125and 0.24 mm (fine sand), 0.062 and 0.124 mm (veryfine sand), and less than 0.063 (silt and clay). A modi-fied sediment size index was used to classify the soiltexture for each site. The index is detailed in Kroesand Hupp (In press) and makes use of phi size classunits (Krumbein, 1936). The index multiplies the sizeproportion by the inverse of its phi unit; resulting ina simple quantitative measure of soil texture. The

coarse sand fraction is multiplied by five, sand byfour, fine sand by three, very fine sand by two, andsilt and clay by one. The index was used to providean intuitive measure of soil texture: the index valueincreases with increasing grain size.

The study area is 28.1 river km long with approxi-mately 126.4 ha of floodplain with a mean activefloodplain width of 45 m, as determined from USGStopographic maps and geographic information system(GIS) data provided by Lancaster County. Leopold(1973) noted the difficulty in distinguishing flood-plains from terraces in an incised Mid-Atlanticstream. We defined the LCC floodplain area as theextent of the valley floor with an elevation betweenthe bank top and 1.5 m higher than the bank top.Terrace surfaces were defined as being higher than1.5 m above the bank top. The break between flood-plain and terrace was determined by the highestinstantaneous stage from the USGS streamgage dur-ing the study period (1.3 m above top of bank)(Figure 3).

RESULTS

The highest and lowest bank erosion rates were atadjacent sites; the lowest mean bank erosion ratewas at Site 3 (river km 11.9, 29 mm ⁄ year), which issituated between two closely spaced historic dams.The highest bank erosion rate was at the Site 4 nearthe USGS streamgage (river km 15.5, 163 mm ⁄ year).Mean floodplain deposition rates varied from ero-sional at Site 1 (5 mm ⁄ year of erosion) to depositionalat Site 2 (river km 7.7, 11 mm ⁄ year of deposition; thehighest rate in the study). The active portion of thefloodplain, within 1.5 m vertical of the top of bank,had a mean deposition of 5 mm ⁄ year. Mean deposi-tion and erosion rates by site are provided in Table 1.

Bank height and channel width increase at aknickpoint around river km 6; bank height increasesas channel width remains relatively constant nearthe existing dam at river km 22 (Figures 4 and 5).Bank height and channel width increase linearly(R2 = 0.78, p = 0.048) from 0.8 and 3.3 m to 1.9 and14.5 m, respectively (Figure 4) from upstream todownstream, despite the geomorphic adjustments atriver km 6 and 22. Floodplain width was highly vari-able but trended narrower downstream (Figure 4).Surficial bank and floodplain sediments become coar-ser downstream, however all sediment samples areconfined among fine sand, very fine sand, or silt andclay. The bank material is slightly coarser than thefloodplain sediments at each site (Figure 6). Thesmall difference in texture does not seem to affect

FIGURE 3. A Representative Survey Cross-Section From Site 4Including Bank Height and Floodplain Length Determinations

and Mean and Maximum Water Stage [as interpreted fromthe nearby U.S. Geological Survey (USGS) streamgage].

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sediment density, which was similar between thebanks and floodplain (p = 0.16, t-test: paired mean).Sediment density was similar among all sites atabout 1.04 g ⁄ cm3, except for at Site 1 where the den-sity was 0.77 g ⁄ cm3 (Table 2). The mean organic con-tent in bank and floodplain samples ranged between3.52 and 4.24%, within the observed range for Pied-mont streams (Table 2, Axt and Walbridge, 1999).

An approximate mass of sediment deposited andremoved from the floodplain and banks along each ofthe 14 segments was determined using the densityand volume of accumulated and eroded sedimentobserved at the five study sites. This sediment budgetis negative (net erosional) for the two upstream mostsegments (Segments 1 and 2) and is either positive(net deposition) or relatively stable for Segments 3-11. The only existing dam along the creek is near theupstream boundary of Segment 12 (Figure 1); allthree segments downstream of this dam are highly

FIGURE 4. (A) Extrapolated Bank Height, Floodplain, andChannel Width Trends With Distance Downstream and (B)

Linear Regression of Surveyed Bank Height vs. Channel Width.

FIGURE 6. Sediment Particle Size Index for the Bank and Flood-plain at the Five Little Conestoga Creek (LCC) Sites, the Higherthe Index Number the Coarser the Sediment. All sites had fourfloodplain and bank samples except Site 5 (river km 26.3) wherethere was only one sample for the floodplain and three samples forthe bank.

TABLE 2. Mean Sediment Bulk Density and PercentOrganics for the Five Little Conestoga Creek (LCC)Study Sites Based on Floodplain and Bank Samples.

Site Percent Organic n Density (g ⁄ cm3) n

1 3.96 7 0.77 32 3.52 3 1.04 33 3.91 8 1.04 34 4.24 7 1.05 35 4.10 4 1.03 3

Notes: Organic content was measured using loss on ignition meth-ods and bulk density was measured using a 5 cm deep core.

FIGURE 5. Lower Bank Elevation (msl) for the Study Area asDetermined Using Lancaster County 1.5 m Contour Data. Figureincludes the location and elevation of the five study sites as well asthe existing dam and the knickpoint downstream of Site 1 (riverkm 1.9).

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net erosional (Figure 7). Segment sediment budgetsare detailed in Table 3. The mean approximateannual sediment budget for the 28-km length of theLCC is )5,634 Mg ⁄ year (erosional). The sedimentbudget reflects only sources and sinks from the banksand floodplain and is part of a more encompassingsediment budget that could include: (1) inputs fromtributaries, (2) bed load depositional and erosionalprocesses, or (3) adjacent upland contributions to theobserved floodplain depositional rates. The sedimentbudget was formed assuming that average depositionand erosion rates adequately characterize the reachand that rates from the five measurement sites canbe extrapolated to the rest of the study site.

DISCUSSION

Results from our study indicate that the remainingdam on the LCC is affecting modern floodplain sedi-mentation and erosion rates (Figure 7). The channeland floodplain sediment particle size increases down-stream, while the increase is relatively small, it ischaracteristic of dammed rivers; dammed waterwayspreferentially transport fine sediments allowing bankmaterial to become coarser over time (Poff and Hart,2002). The reservoir (water surface) behind the exist-ing run of river dam extends for approximately 3 km,assuming a hydraulic gradient of 0.001 calculatedfrom the detailed long profile provided in Walter and

Merritts (2008). This reservoir extends upstream tothe approximate location of the next historical mill-dam, providing grade control for further upstreamchannel incision. River segments between the damand the confluence with the Conestoga River (32% ofthe studied stream reach) export the majority of theeroded sediment of the system (97% of the measurednet sediment budget) while river segments above thedam, with the exception of the two upstream mostsegments, are net depositional or exhibit low net sedi-ment yields. Channel elevation downstream of thedam is subjected to the base elevation of the Cones-toga River as well as scouring effects of the dam. Thedam provides grade control preventing further ero-sion upstream including the channel bed and storedlegacy sediment in both the banks and the floodplain.The vast amount of stored legacy sediment through-out the LCC system likely affects a greater volume of

TABLE 3. River Segment Location (river km denotes downstreamboundary of each segment), Floodplain and Bank Erosion and

Accretion Rates and Segment Net Sediment Budget in Mg ⁄ year.

River km

Erosion ⁄ Accretion(Mg ⁄ year)

Net BudgetFloodplain Bank

2 )450 170 620 Upstream4 )677 170 8466 279 286 78 896 441 4559 481 324 157

11 679 228 45112 96 121 2513 65 121 5614 265 353 8815 568 626 5819 1,110 626 484

Existing dam23 975 1,542 56726 316 2,714 2,39828 198 2,714 2,515 Downstream

4,802 )10,437 5,634 Total

Note: The location of the existing dam is also noted.

FIGURE 7. (A) River Segment Net Sedimentation or Erosion inKilograms Per Year and (B) River Segment Net Sedimentation orErosion Per Square Meter of Floodplain Area. River segment calcu-lations are based on floodplain deposition and bank erosion ratesand bank height, floodplain area, and river length. The one remain-ing dam on the Little Conestoga Creek (LCC), the knickpoint, andthe river segment numbers are all displayed for reference.

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eroding sediment between the dam and the Cones-toga River, especially near breeched historical dams,and creates the potential for a higher magnitude ofsediment transport upstream of the existing damwhen the structure eventually breeches.

The remaining dam may control, to some degree,the upstream sediment budget while affecting down-stream net erosion (Figures 7 and 8). The system iscloser to a zero net sediment budget upstream of theremaining dam, with a lack of dam influence nearSite 1. Bank height is highest near the confluencewith the Conestoga River where the channel isincised to match the velocity and elevation of theriver and is simultaneously affected by the upstreamdam (Ligon et al., 1995).

The LCC channel expands in width and depth as itflows toward the Conestoga River. The increase issymptomatic of the channel degradation stage of sev-eral channel evolution models (Schumm et al., 1984;Simon and Hupp, 1992; Rinaldi, 2003), has beenobserved after a low-head dam removal in Wisconsin(Doyle et al., 2002, 2003), and has been identified onthis system as an incising channel (Walter andMerritts, 2008). Site 1, River Segments 1 and 2,appears to be upstream of the channel degradationknickpoint from previous dam failures. The channel isapproximately 10 m wider and 0.5 m deeper at Site 2,River Segments 4 and 5 than upstream; there appears

to be a knickpoint near river km 6 between the twomeasurement sites (Figures 4 and 5). The short-termerosional effect of the knickpoint can be deduced fromTable 3 and Figure 7 where the river segments down-stream of the knickpoint are depositional or near zeroand the segments above the knickpoint are erosional.Thus, the Site 1 river reach should eventually becomeincised without some form of grade control (e.g., bed-rock outcrop or low-head dam).

The floodplain, at least near the USGS streamgageat Site 4, was inundated almost annually for shortperiods. Flow overtopped the bank 0.13, 0, 0.60, and0.002% (11, 0, 48, and 1 h exceedance, respectively)of the year from 2004 through June 6, 2007, as mea-sured at the USGS streamgage. Four of the five studysites exhibited floodplain deposition, suggesting thatthe floodplain still functions as a sediment sink,despite channel incision. Floodplain deposition pat-terns can be assessed at Site 4; the site had at leastthree pins and pads on the right bank for both tran-sects. This site clearly shows that deposition is high-est near the channel and decreases with distancefrom the channel (Figure 9), supporting the assertionthat the floodplain above the dam still functions as asediment sink for the creek and not as a discon-nected terrace, as assumed in Walter and Merritts(2008). The floodplain does, however, tend to becomenarrower downstream, with a width consistently less

FIGURE 8. Representation of the Affect of the Existing Dam on Channel Sediment Processes (not to scale), Including UpstreamGrade Control, Downstream Incision, and an Upstream Knickpoint From Previous Upstream Dam Failures. Three survey

cross-sections are also displayed from top of the figure to bottom: river km 1.9 (above knickpoint), 15.5 (between knickpoint anddam), and 26.3 (between the dam and the Conestoga River). The vertical and horizontal scales are 11 and 55 m, respectively.

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than 50 m downstream of the dam (Figure 4). Thesediment storage potential between the dam and theConestoga River is likely much lower than the widerfloodplain observed upstream of the dam.

Agriculture has been cited as contributing high sed-iment loads due to the large amount of tillable landwithin the river basin (Ott et al., 1991; Gellis et al.,2004). According to recent sediment work conductedat the streamgage, approximately 37% of the sus-pended sediment load on the LCC can be attributed toupland sources (Gellis et al., 2009). The majority(63%) of the present annual sediment yield is likelydue to continued erosion of stored legacy sedimentfrom within the channel, bank erosion and loss offloodplain function downstream of the existing dam,and channel and floodplain erosion near the migratingknickpoint. The sediment yield may likely be muchgreater without the existing dam retaining and stabi-lizing upstream in-channel sediment and bank slopes.

CONCLUSIONS

The remaining dam on the LCC appears to affectmodern floodplain and bank sediment dynamics bypromoting an erosional environment downstream andproviding grade control for the upstream system. Aknickpoint approximately 9 km upstream of the damhas produced a net erosional environment in theupstream two river segments. The floodplain experi-enced short periods of inundation nearly annually atthe USGS streamgage, between the knickpoint and

the dam, despite the heightened banks from postcolo-nial sedimentation and subsequent dam removals.Sediment trapping was recorded at four of the fivestudy sites, indicating that the aggraded LCC flood-plain still functions as a sediment sink.

Sediment issues resulting from dam removals willbecome more relevant as the pace of removal increasesdue to the number of aging dams, and the increasedconcern for fish passage and unimpeded river flow andrecreation (Pizzuto, 2002; Poff and Hart, 2002). Damremovals have many benefits, but they come with thecost of remobilizing large amounts of sediment.Managers and policy makers in the Northeast andMid-Atlantic U.S. will have the additional burden ofmanaging the storage and transport of legacy sedi-ment. Dam removals in these regions can lead to largeand sustained sediment pulses as legacy sediment isremobilized and transported further downstream,where increased sedimentation is a critical concern forimperiled estuarine resources, in this case the Susque-hanna River and the Chesapeake Bay.

ACKNOWLEDGMENTS

We would like to thank Dan Kroes and Mike Shackleford fortheir extensive field work and data analysis as well as Faith Fitz-patrick, Allen Gellis, and three anonymous reviewers for their com-ments and suggestions. Funding for this research came from theU.S. Geological Survey’s Priority Ecosystems and NationalResearch Programs.

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Berg, T.M. and C.M. Dodge, 1981. Atlas of Preliminary GeologicQuadrangle Maps of Pennsylvania (Map 61). Pennsylvania Geo-logic Survey, Harrisburg, Pennsylvania.

Burt, R. (Editor), 2004. Soil Survey Laboratory Methods Manual(42nd Edition). Vol. 4. Natural Resources Conservation Service,U.S. Department of Agriculture, 700 pp.

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