OPINION PAPER

The importance of morphodynamic processes at riffles usedas spawning grounds during the incubation time of nase(Chondrostoma nasus)

Christoph Hauer Æ Gunther Unfer ÆStefan Schmutz Æ Helmut Habersack

Received: 8 March 2006 / Revised: 23 August 2006 / Accepted: 15 September 2006 /Published online: 9 December 2006� Springer Science+Business Media B.V. 2006

Abstract An ecologically orientated flood pro-

tection project was implemented at the Austrian

lowland Sulm River. Habitat modelling was

conducted during a 3-year monitoring program

to assess the effects of river bed embankment

and the initiation of a new meander by con-

structing an initial side channel, and allowing

self-developing side erosion. Hydrodynamic and

physical habitat models were combined with

statistical methods. This study focused on the

necessity for including stability analysis when

modelling spawning grounds. The critical erosion

parameter of the cobbles at spawning grounds

was analysed using numerical modelling com-

bined with substrate maps of potential spawning

grounds of nase (Chondrostoma nasus). Due to

the specific characteristics of this reach of the

Sulm River, instability of the riffles can hamper

successful reproduction in nase. Spawning

grounds of Chondrostoma nasus and their sta-

bility must be guaranteed at least for the

duration of incubation. This morphodynamic

necessity should be incorporated in future res-

toration projects and when artificially restoring

spawning habitats.

Keywords Habitat modelling � Restoration

measures � River monitoring � Chondrostoma

nasus � Riffles � Spawning habitats

Introduction

Cyprinid habitat modelling studies are rare, but

even fewer studies have described the quantita-

tive effects of morphodynamic processes on

spawning ground quality. Habitat modelling has

generally focused on stable channels (Holzer

et al., 2002; Brodeur et al., 2004; Pasternack

et al., 2004; Gard, 2005), and omitted the effects

of sediment transport and stability of potential

spawning grounds. Knowledge of other species

requirements are more complete; for example,

the parameters of seasonal timing and homing

for salmonid spawning behaviour (Baril &

Magnan, 2002), and the physical characteristics

Handling editor: D. Ryder

C. Hauer � G. Unfer � S. Schmutz � H. HabersackDepartment of Water, Atmosphere and Environment,BOKU – University of Natural Resources & AppliedLife Sciences Vienna, Vienna, Austria

C. Hauer (&) � H. HabersackInstitute of Water Management, Hydrology andHydraulic Engineering, Muthgasse 18, 1190 Vienna,Austriae-mail: christoph.hauer @boku.ac.at

G. Unfer � S. SchmutzInstitute of Hydrobiology and Aquatic EcosystemManagement, Max-Emanuellstrasse 17, A-1180 Wien,Austria

Hydrobiologia (2007) 579:15–27

DOI 10.1007/s10750-006-0406-7

123

of successful spawning grounds (Ottaway et al.,

1981; Heggberget et al., 1988; Grost et al., 1991;

Elliot, 1994; Darchambeau & Poncin, 1997; Rubin

et al., 2005). In the case of cyprinid fish, however,

most studies deal with physiological and time-

related observations of the spawning act (Baras,

1994, 1995; Rinchard & Kestemont, 1996; Fred-

rich et al., 2003). Relatively few studies describe

the abiotic characteristics of the spawning habi-

tats (Keckeis, 1991; Maier et al., 1992; Stein, 1992;

Melcher, 1999).

The occurrence of riffles, which are used as

spawning grounds by hiders (salmonids) and

stickers (rheophilic cyprinids), was investigated

using a broad database relating to spatial scale

including bedform type and frequency by Emery

et al. (2004) and Sear (1996). Further studies on

historical river morphology maps were deemed

necessary for understanding river dynamics and

as a tool for successful restoration measures

(Hudson, 2002; Hohensinner et al., 2004). Previ-

ous studies have dealt with the geomorphologic

and hydraulic effects of such riffle restorations

(Pretty et al., 2003; Walker et al., 2004; Brooks

et al., 2005), including the movement and stability

of riffles (Wilkinson et al., 2004). Beyond such

morphodynamic processes, Kappesser et al.

(2002) defined a Riffle Stability Index (RSI)

which estimates the mobility of cobbles in riffles.

The effects of restored riffles as spawning

grounds, in accordance with the European Water

Framework Directive, have been discussed in the

scientific literature less for rheophilic cyprinids

than for salmonids (Rubin et al., 2004; Zeh &

Donni, 1994).

The present study therefore highlights the

needs of numerical models for morphodynamic

processes in restoration ecology and specifically

when dealing with spawning grounds of rheo-

philic cyprinids. Riffle sections play a major role

for reproduction of rheophilic cyprinids. Their

spawning grounds are characterized by adhesive

eggs on the bed sediments. For nase, we

hypothesised that the erosion of sediment

particles of the 2–6.3 cm size class at riffle

sections could mechanically damage the affixed

Chondrostoma nasus eggs and reduce spawning

success.

Methods

Study reach

The study reach is situated in the southern-east

part of Austria (–62950, 5181450; Gauss–Kruger/

national grid Austria). The Sulm River is formed

by two main tributaries originating from the

Central Alps and discharges into the Mur River,

a further tributary to the Danube. The biocenotic

description is defined as epipotamal (Muhar et al.,

1998) and barbel-region (Illies & Botoseanu,

1963). The riparian vegetation is defined based

on the main species by Otto (1981) as Salicetum

cinereae, Fraxino-Populetum, Salicetum albea for-

ests. After the confluence of the two tributaries

from the Central Alps, the Sulm River historically

resembled a meandering river (Muhar et al.,

1998). The bankfull discharge of the historical

Sulm was 40–45 m3 s–1 with an average slope of

0.0003 m m–1 (Habersack & Hauer, 2004). River

bed straightening in 1965 led to a reduction in

river sinuosity, with the slope increased to

0.001 m m–1 by meander cut-offs (Habersack &

Hauer, 2004). Within a further flood protection

project in 1999, the river channel was widened,

leading to an increased discharge profile of

80 m3 s–1. During the river widening and revital-

ization project, two small meanders were con-

structed. One of the meanders (Fig. 1) at river

station 12.960–13.356 km was modelled to analyse

the stability of the spawning habitats of the

rheophilic cyprinid nase (Fig. 1). The regulated

river bed was cut off and serves as a backwater in

low-flow situations. When the discharge increases

to more than 10 m3 s–1, the former river bed

functions as a flood channel (Fig. 1). This study

reach is located approximately 12.8 km upstream

of the mouth into the Mur River. Hydrological

data for the monitoring area were provided by the

Regional government of Styria (2001) and are

depicted in Table 1. The flow regime is indicated

as a pluvio-nivale 3 (i.e., the occurrence of three

equivalent flood peaks per annum is characteris-

tic; Mader et al., 1996).

Additionally, the Pielach River is presented

based on the fact that the main parameters

(velocity, depth and substrate) for the study were

16 Hydrobiologia (2007) 579:15–27

123

analysed in this Danube tributary (Fig. 1). Both

rivers (Sulm/Pielach) show similar hydro-

morphological characteristic (Table 1), where

nase is also among the most abundant species

(Schmutz et al., 2002; Zitek et al., 2004).

Modelling

One- and two-dimensional depth-averaged

hydrodynamic numerical models were used for

the hydraulic simulations. The geometric bound-

ary conditions for the investigated season (2002)

were based on morphological monitoring from

2001 to 2003, obtained by a terrestrial survey

(total station, Leica TC805). Furthermore, sub-

strate maps (describing the armour layer on each

terrestrial survey point) for 2002 were derived

with additional six volumetric sediment samples

(Wolman, 1954) in riffle 1 (3 samples = armour

layer; 3 samples = subsurface layer), leading to a

calculated critical shear stress scr of characteristic

Nr. of Cross sections: 22

upstream boundaries

J 19

J 20

J 21

J 23

J 22

J 24

J 25

J 26

J 27

J 28

Cross section 18

Cross section 17

Cross section 16

riffle 1

Inundation channel

> 10 m3s-1 discharge

Stone weir

riffle

riffle

. gauging station

Study reach: Pielach

Study reach: Sulm

Fig. 1 (Top) Location of the Sulm and Pielach River inAustria (Europe) presented in the national grid (–62950,5181450; Gauss-Kruger); (Left) Investigated river section

of this study (22 cross sections); (Right) Simulation grid ofthe 2-dimensional HN model for stability analysis includ-ing the spatial boundaries of riffle 1

Table 1 Comparison of the River typologies Sulm andPielach including the main morphological parameters,precipitation data and the mean discharge

Sulm Pielach

Catchment area 1.113 km2 591 km2

Altitude 272 m.a.sl. 230 m.a.sl.Mean precipitation per year 1160 mm 875 mmChannel slope 0.0008–0.001 0.002River length About 50 km 67 kmMean river width 18.5 m 22.5 mDominating substrate Microlithal MicrolithalMorphological type Meandering MeanderingMQ 8.85 m3 s–1 6.47 m3 s–1

MQ = mean discharge

Hydrobiologia (2007) 579:15–27 17

123

grain diameters with the formula of Meyer-Peter

& Muller (1949) in the selected riffles (Fig. 1).

scr ¼ 0:047 � ðqF � qWÞ � g � dm ð1Þ

where qF = density of sediment (2,665 kg m–3),

qW = density of water (1,000 kg m–3), dm = aver-

age diameter of sediment (m), g = gravity force

(9.81 m s–1).

Habitat modelling

To define the potential spawning grounds, a

third specific model (CASIMIR) was imple-

mented in the monitoring process. This model

has no separate module to calculate the hydrau-

lic conditions (Jorde, 1999). The geometric

boundaries and the one-dimensional (1D) sim-

ulated water surface were therefore necessary to

initiate the habitat modelling. To obtain the

two-dimensional velocity distribution from the

average value in the cross section, the habitat

model uses the formula of Darcy–Weissbach

(Schneider, 2001).

vm ¼1ffiffiffi

kp �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

8 � g � Rhy � IE

p

ð2Þ

where vm = average velocity (m s–1), k = resis-

tance coefficient of Darcy–Weisbach (–), g =

gravity force (9.81 m s–1), Rhy = hydraulic radius

(m), IE = energy slope (m).

A possibility of using this equation for the

description of local currents consists of dividing

the wetted area in strips. This organization

becomes already pre-determined by the selected

vertical-gradual space resolution z which was

defined as 0.03 m.

mi ¼1ffiffiffiffi

ki

p �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

8 � g � hi � IE

p

ð3Þ

To link the hydraulic simulation results with

suitability indices, the method of Bovee (1986)

was used. This method of multiplying suitabil-

ity indices was also utilized in the PHAB-

SIM model (Milhouse, 1989) and is shown in

Eq. 4.

(a) (b)

(c)

Station (m)

276

270271272273274275

0 10 20

)m(

noitavelE

8Trap Nr. 7 6 5 4 3 2 1

0

5

10

15

20

25

30

35

40

0246810121416182022

Cross sections

Wid

th [

m]

200120022003

Meander

-10

0

10

20

30

40

50

60

22 20 18 16 14 12 10 8 6 4 2

Cross sectionsagg

rad

atio

n /

ero

sio

n [

cm]

Morphological changes [cm]

Fig. 2 Morphological development 2001–2003 of theinvestigation area (Sulm River); (a) the change of Riverwidth (m) for mean flow (8.1 m3 s–1), (b) mean aggrada-

tions & erosion (m) in the monitored cross sections for thetime period 2001–2003, related to the mean flow watersurface level; (c) cross section 17 with sediment traps 1–8

18 Hydrobiologia (2007) 579:15–27

123

SIges ¼ SId �SIv �SICI or general SIges ¼Y

I

i¼1

SIi

ð4Þ

where SId = Suitability Index depth, SIv = Suit-

ability Index velocity, SICl = Suitability Index

channel index, SIges = Suitability Index total,

SIi = Suitability index variable.

Snorkelling observations on the habitat use of

the spawning ground by nase in the Pielach River

in 1997 were used to generate suitability indices

for this age-class (after Melcher, 1999; Table 2).

This function of multiplying suitability indices

allowed the areas of potential spawning grounds

in the 2002 season to be defined. The decision

to apply this method (multiplying suitability

indices) was based on the habitat demands of

this specific life-stage being well documented for

three parameters: velocity, depth and substrate

(Melcher, 1999). Moreover, the PHABSIM

method (Bovee, 1986) was an adequate tool

for predicting fish habitat. A number of studies

(Shuler & Nehring, 1993; Elliot et al., 1996;

Harby & Arnekleiv, 1994; Huusko & Yrjana,

1996) have used PHABSIM to evaluate the

effectiveness of stream restoration projects post-

construction. To gain further quantitative mod-

elling results, the method of weighted usable

area (WUA) (Bovee, 1986) was implemented in

the modeling process (Eq. 4).

WUA ¼X

n

i¼1

HSIi:Ai ð5Þ

where n = total number of grid cells (–),

HSIi = habitat suitability index (–), Ai = area of

single grid cells (m2).

Shear stress modelling

For simulating the stability of the potential

spawning grounds, the depth-averaged two-

dimensional hydrodynamic model was used. The

model was developed by Nujic (1999) and calcu-

lates the hydraulic conditions on a linear grid by a

finite volume approach. The convective flow is

based on the Upwind-scheme by Pironneau

(1989) and the discretisation of time is done by

an explicit Runge Kutta method in second order.

The shear stress was calculated for each node of

the grid with the following formula (Nujic, 2004):

s ¼ q � g � h � Ir ð6Þ

Ir ¼v2

k2str � h

43

ð7Þ

where q = density of water, v = velocity at node

(m s–1), kstr = roughness number by Strickler (1/

Manning n), h = water depth (m), g = gravity

force (9.81 m s–1).

The simulations were performed with the

parabolic eddy viscosity model and a constant

turbulent viscosity coefficient of 0.6.

Validation of initiation of sediment transport

For validating the simulated initiation of motions

of bedload transport, eight sediment traps were

installed in the river bed at cross section 17

(Fig. 1). The hydraulic efficiency of the traps

(diameter 0.4 m; depth 0.25 m) was analysed in

the laboratory. Several cases of filling 30, 60 and

90% were investigated by vertical velocity distri-

bution. The P.-EMS, developed by DELFT-

Hydraulics, was used to measure flow velocities.

Table 2 Suitability Indices (SI) of Chondrostoma nasus which were developed by Melcher (1999) by snorkelling at thePielach River

SI Very high High Low No SI

Water depth (cm) 15–30 30–45 45–90 <15, >90Current velocity (cm s–1) 100–110 80–120 70–80/130–140 <70, >140Substrate Microlithal Microlithal mesolithal pelal

Ad. Table 1: pelal (<0.063 cm), psammal (0.063–0.2 cm), akal (0.2–2 cm), microlithal (2–6.3 cm), mesolithal (6.3–20 cm),macrolithal (20–40 cm), megalithal >40 cm//very high (SI: 1–0.8), high (SI: 0.8–0.4), low (SI: 0.4–>0), no SI (SI: 0)

Hydrobiologia (2007) 579:15–27 19

123

The P.-EMS is capable of measuring velocity

components in a 2D-plane (Delft Hydraulics,

2006). Data were transported to data-acquisition

equipment (LabVIEW). The P.-EMS allows flow

velocity measuring within a range of 0–2.5 m s–1

(optional 0–5.0 m s–1) bi-axial in a four quadrant

range. Verticals were predefined according to

stationing of the installed sediment traps and

were probed by seven point sampling measures,

with main focus on the near-bottom flow pro-

cesses.

Hydrologic data

Hydrologic data for modelling the spawning

period of nase were taken from the gauge

Heimschuh, which was installed on a bridge

300 m upstream of the study area (–63067/

5181216; Gauss Kruger/national grid Austria).

The data were analysed for a two-month period in

2002 and 2001. The decision to run the modelling

with a 2-month time series was based on existing

correlations between spawning and water tem-

perature (Lelek & Penaz, 1963; Maier, 1993).

Depending on the weather conditions, spawning

in the study region can take place in April or

May. The hydrologic gauging data for 2001 were

also analysed for the defined time period to use a

different discharge data set for further interpre-

tation of possible erosion processes over spawn-

ing grounds.

Results

The calculation of the critical shear stress by the

formula of Meyer-Peter & Muller (1949) yielded

a certain classification of critical shear stress

values for the different sediment size classes

(Table 3). As the main available choritope type

in the study riffles the microlithal size class was

documented. The critical shear stress for this

substrate classification is defined between

15.21 N m–2 and 47.92 N m–2 (Table 3). Further

the small existing sediment size classes in the

possible spawning grounds akal (1.51–15.21 N m–2)

and mesolithal (47.92–152 N m–2) are listed.

In a second step, potential spawning grounds

were defined by analysing suitability curves of

nase (Melcher, 1999) (Table 2). The investigation

area features two riffles (Fig. 3) which are

suitable as nase spawning grounds; the use of

riffles as reproduction areas has been described in

other studies (Keckeis, 1991; Stein, 1992; Maier

et al., 1992). Both riffles contain the necessary

abiotic conditions for successful spawning (Fig. 3,

Table 2). Riffle 1 was selected for the numerical

stability analysis. It extends from nearby cross

sections 18 to 16 (Fig. 3).

The habitat model, which used the suitability

curves as biotic input, calculated the number

(WUAs) of potential spawning grounds of nase in

the 2002 season (Fig. 4). Figure 4 shows the total

weighted useable area of the investigation reach

and the specific values for riffle 1. The results

indicate an increase in potential spawning areas

(riffle sections) with increased discharge.

Such increases for potential spawning areas are

limited by the initiation of motion of sediment

particles. The calibration of the two-dimensional

Table 3 Critical shear stress values based on the formulaof Meyer-Peter & Muller for characteristic sediment sizediameters

Choriotope Sediment size Critical shear stress(N m–2) (after MPM)

Pelal <0.063 mm <0.48Psammal 0.063–2 mm 0.48–1.52Akal 2–20 mm 1.52–15.21Microlithal 20–63 mm 15.21–47.92Mesolithal 63–200 mm 47.92–152Macrolithal 200–400 mm 152–304.28Megalithal >400 mm >304.28

MPM: Meyer-Peter & Muller formula

269.4

269.6

269.8

270

270.2

270.4

270.6

270.8

0246810121416182022

Cross sections

]ls.a.m[

noitavele

minimum channel

water surfaceelevation

riffle

Q = 1 m3s-1

riffle 2

Fig. 3 Longitudinal profile of the study reach (SulmRiver) with potential spawning areas (riffle sections)including the bed elevation and the water surface levelfor 1 m3 s–1

20 Hydrobiologia (2007) 579:15–27

123

hydrodynamic model for riffle 1 required the

calculation of the simulated water surface level

for measured values at 2.66 m3 s–1 discharge. The

sum of roughness influences showed the best

correlation with n = 0.045 (n = manning rough-

ness factor). At this low discharge, the influences

of form roughness are much higher than the grain

roughness. The calculation of the n based on

volumetric sediment samples by the formula of

Meyer-Peter & Muller (1949) yielded n = 0.029 as

a roughness factor with d90 as an input parameter.

In a second step, the 2-dimensional hydrodynamic

numerical model was calibrated based on the

depth-averaged flow velocities. Figure 5 shows

the final calibration results for cross sections 17

and 19, which are situated in the middle of the

riffle and immediately upstream of the potential

spawning site (Fig. 3).

The total shear stress was calculated for the

predefined model grid and is listed in Table 4.

Riffle 1 included 143 grid areas (Fig. 1). Each line

of Table 4 shows the final results for a simulation

step of 1 m3 s–1. Beneath the wetted areas, the

maximum shear stress and the amount of rectan-

gular areas within critical conditions for the

microlithal sediment size class are shown. The

formula of MPM calculated 15.21 N m–2 as the

critical value (sediment diameter = 2 cm) for the

possible initiation of sediment motion in riffle 1

(Table 3). Furthermore, 47.92 N m–2 were calcu-

lated for 6.3 cm sediment; representing the upper

limit of the microlithal (Table 3). The simulation

results show that, at 7 m3 s–1 discharge, more than

half of the wetted area has a significant trans-

gression of the defined critical value of

15.21 N m–2. At 10 m3 s–1, 66% of the predefined

riffle area was within the possible beginning of

sediment transport.

Erosion, however, is usually the result of

complex interactions between a number of mech-

anisms that act on the bed sediments either

simultaneously or sequentially (Sear et al.,

2003). It was therefore necessary to validate the

numerical results for the initiation of motion.

Validation by sediment traps

Eight sediment traps were monitored for three

time periods (22.09–30.09.2002; 11.10–31.10.2002;

17.11–25.11.2002) with 73, 56 and 10 m3 s–1

respectively as maximum discharge. The results

0

20

40

60

80

100

120

2.66 6.30 10.00

discharge [m³/s]

]²m[

AU

W

riffle 1 riffles 1 & 2

Fig. 4 Weighted useable areas (WUA) (m2) of spawninggrounds of nase in 2002 for the whole study reach (riffle 1and 2; grey bars) and for riffle 1 (black bars) related to theanalysis of three characteristic discharges (2.66 m3 s–1;6.3 m3 s–1; and 10 m3 s–1)

Cross section 17 Cross section 19

00 05 510 1015 15 20

0.10.20.30.40.50.60.70.80.9

station [m]

velo

city

[m

s-1]

measuredcalculated

0

0.1

0.2

0.3

0.4

0.5

0.6

station [m]

]1-sm[

yticolev

measuredcalculated

Fig. 5 Calibration of the depth average velocities (m s–1)over the cross sections 17 (Left) and 19 (Right) for2.66 m3 s–1 discharge. Whereby the dashed line represents

the measured depth average velocities compared to themodelled velocity distribution (continuous line)

Hydrobiologia (2007) 579:15–27 21

123

of the flow event with 10 m3 s–1 as maximum

discharge showed that the transport of bed load

material starts in deeper areas of the cross section

(Table 5; Fig. 2c).

Traps 1 and 8, which were positioned at the

edges of the cross section, remained empty at a

discharge of 10 m3 s–1. The fact that trap 3 was

empty, and that the adjacent trap showed the

second highest transport rate (4.6 kg), reflected

the high variability of sediment transport pro-

cesses within the riffle. The average diameters of

sediments of sample units 31, 32, 33 and 34 are

distorted by few cobbles of size class >31.5 and

>56 mm. This had a pronounced influence on the

whole sample because of the lower cumulative

weight of these samples. Based on a well-known

discharge regime (Mader et al., 1996) and the

continuous control of the traps, the initiation of

sediment motion (sediment size class: 2–6.3 cm)

was defined at 7–10 m3 s–1. Figure 6 shows the

lateral variability of the transported sediments for

sample periods 1 and 2. The analysis of the

surface sediment layer of riffle 1 by the method of

Wolman (1954) in 2002 yielded the following

characteristic sediment size classes: d10 =

8.70 mm, d50 = 38.21 mm, d90 = 68.58 mm,

dm = 42.10 mm. Therefore, most of the trapped

sediments were transported from the upper reach

section of the riffle because the comparison of the

medium diameter of the surface layer and the

transported material shows a significantly finer

transported sediment class. Nevertheless, the

Table 4 Results of the stability analysis in riffle 1 by two-dimensional numerical modelling including theovertopping of the critical shear stress value for thedocumented grain size distribution

Discharge(m3s–1)

Wettedarea (m2)

Maximal value(N m–2)

>15.21N m–2

(%)

>47.92N m–2

(%)

1 490.2 32.13 20.93 02 615.6 43.05 25.93 03 793.8 54.36 30.16 1.444 843.6 60.97 40.54 5.415 866.4 66.81 42.11 6.586 900.6 70.53 49.37 7.607 934.8 74.53 56.10 7.318 946.2 101.56 62.65 7.239 969 101.78 65.88 7.0610 980.4 102.34 68.60 8.14

Total area of riffle 1 = 1436 m2

Table 5 Sedimentsamples during themonitoring periodincluding the samplenumber, the weight of theprobe, the characteristicsediment diameter andthe critical abioticparameters of sedimentmovement for threedifferent periods

(1) sample nr. 1–8, max.discharge = 73 m3 s–1; (2)sample nr. 9–16, max.discharge = 56 m3 s–1; (3)sample nr. 17–21, max.discharge = 10 m3 s–1

Trap nr. 1 2 3 4 5 6 7 8

Sample nr. 1 2 3 4 5 6 7 8Weight (kg) 15.6 16.1 16.4 16.9 15.9 13.8 15.0 14.7dm (mm) 9.5 4.2 10.6 7.1 5.0 6.1 9.0 7.7scrit 7.0 3.1 7.8 5.2 3.7 4.5 6.7 5.7vcrit 1.1 0.7 1.1 0.9 0.8 0.9 1.1 1.0s1D 8.4–10.5v1D 0.6–0.8

Emptying of trapsSample nr. 9 10 11 12 13 14 15 16Weight (kg) 12.2 14.2 12.9 14.6 15.5 17.5 16.7 18.7dm (mm) 7.9 5.6 9.1 5.9 5.6 9.1 8.1 13.5scrit 5.9 4.1 6.7 4.3 4.1 6.7 6.0 10.0vcrit 1.0 0.8 1.1 0.9 0.8 1.1 1.0 1.3s1D 8.4–10.5v1D 0.6–0.8

Emptying of trapsSample nr. – 17 – 18 19 20 21 –Weight (kg) – 4.6 – 1.3 1.8 4.8 3.0 –dm (mm) – 3.2 – 2.0 21.6 26.1 35.0 –scrit – 3.1 – 5.2 3.7 4.5 6.7 –vcrit – 0.6 – 0.5 1.6 1.8 2.1 –s1D 8.48–13.5v1D 0.7–0.8

22 Hydrobiologia (2007) 579:15–27

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10 m3 s–1 sample showed the partial initiation of

movement of cobbles out of the surface layer. The

lateral variability, which was found in all samples,

can be caused by secondary flow in the cross

sections (Gunther, 1971).

Discussion

The potential spawning grounds of nase (Chond-

rostoma nasus) were investigated in the frame-

work of a 3-year monitoring program. During that

time the development of the cross sections

(Fig. 1) was analysed, and the results are pre-

sented in Fig. 2, whereby in Fig. 2a the changes of

River width (m) for mean flow (8.1 m3 s–1) are

described. The cross sections show an average

reduction of 3.3 m of the former width and

further in Fig. 2b the mean aggradations (m) an

increase of the minimum channel level of the

monitored cross sections (max. 0.49 m). More

detailed information about the morphological

development can be found in Hauer et al.

(2004). Beyond examining the interesting changes

in the morphology of the study reach, the present

paper discusses the effects of possible erosion at

nase spawning grounds. Spawning in nase takes

place in spring and is highly correlated with water

temperature (Lelek & Penaz, 1963; Maier, 1993).

Kuttel et al. (2002) defined a minimum of 12�C

water temperature as necessary condition for

successful development of the affixed fish eggs

of the nase Muller (1997) found that 12–17�C are

optimal for growth and development of eggs. At

an average value of 14�C, the 350 daily degrees

which are needed for the incubation defined in

the study of Pokorny (1999) are attained in

25 days. Therefore, the stability of the spawning

habitat has to be guaranteed for nearly 4 weeks

for successful spawning. The hydro-morphologi-

cal characteristics of the Sulm River pointed to

April and May as the best period to investigate

spawning and incubation of eggs (Fig. 7).

Discharge clearly increased for nearly one

week in spring 2002 (Fig. 7). During that time,

sediment cobbles were eroded from the riverbed

surface in the riffle area. This is supported by the

numerical results and sediment trap measure-

ments. Church et al. (1991) used a similar method

(sediment traps) to investigate sediment transport

with particles <10 mm. Influenced by near-bot-

tom turbulent flows, the bedload transport can be

separated into sediment transects. This effect can

further trigger a spatial variation of secondary

flows and transport forms, which were investi-

gated by Tsujimoto (1989). In other studies, the

position of these transport stripes was found to be

very unstable (Sohngen, 1988; Nezu & Nakagawa,

1989).

When compared with the discharge in 2001,

spawning success (through riffle stability) in 2002

depended on the hydrologic situation. The artifi-

cial morphologic characteristic of the investiga-

tion area (flow split up >10 m3 s–1) yielded a

00 2 4 6 8 10 12 14

10

20

30

40

50

60

station [m]

dx

]m

m[

dm sample 1dm sample 2d90 sample 1d90 sample 2

Fig. 6 Grain size distribution over cross section 17 forcharacteristic grain size diameters dx; where dm = mediandiameter of the sample; d90 = 90% of the cumulativeweight of the sample

0

5

10

15

20

25

1.4 6.4. 11.4. 16.4. 21.4. 26.4. 1.5. 6.5. 11.5. 16.5. 21.5. 26.5. 31.5.

date

m[e

grahcsi

d3 s

1 -]

20012002

τcr

Fig. 7 Discharge situation during the potential spawningperiods (begin of April to the end of May) in 2001 and2002. The continuous line is representing the occurreddischarges (m3 s–1) in 2001 compared to the dashed graphwhich shows the discharge situation in spring 2002

Hydrobiologia (2007) 579:15–27 23

123

specific morphodynamic situation for riffle 1.

During high flow events (bankfull discharge) the

meander section examined in this study acted as a

sediment trap (Fig. 2a, b), depositing material in

the riffle section. This material will be eroded at

discharges of approximately mean flow. Such

non-uniform sediment transport processes pro-

hibit the build up of a well-developed armour

layer (Parker & Sutherland, 1990). This was also

supported by the detailed documentation of the

morphological development over the monitoring

period (Hauer et al., 2004). Where they described

the development of the bottom shear stress at

bankfull discharge which was reduced from

76 N m–2 (cross section 22) to 10 N m–2 (cross

section 18) and is therefore responsible for

aggradation in the study reach. Armouring is

crucial for the stability of bed materials of aquatic

habitats in river systems (Sear et al., 2003).

Armouring greatly reduces bed material loads,

limits scour, and provides spawning sites for fish

and secure substrate for invertebrates and benthic

algae (Sear et al., 2003). These benefits to the

ecosystem are lost if the armour is removed

during a major flood, gravel mining or dredging.

In the area investigated during this study, the

artificial meander functioned as a sediment trap.

This prohibited the build-up of the necessary

armour layer in the spawning riffles. Deposition

and superposition of bed load at higher dis-

charges, caused by the reduced sediment trans-

port capacity, and erosion processes, which start

very early at mean discharge made the entire

reach very unstable for spawning of nase. Thus,

the restored river section was still morphologi-

cally dynamic with habitat changes expected

through time. This study shows that short-term

effects (3 years) of river restoration on nase

(Chondrostoma nasus) and their spawning

grounds (riffle sites) should be interpreted with

caution. When restoring spawning grounds, long-

term morphological development is an important

issue. The functionality of a restored spawning

area has to be maintained for future generations

of nase, which might carry out homing migrations

to their place of birth. Generally homing brings

an individual fish back to an environment which is

known to be suitable for reproduction (Wootton,

1992). The ability to home to particular spawning

locations is well documented for salmonid species

(e.g., Hasler & Scholz, 1983; Quinn, 1993).

Although evidence of homing during spawning

migrations in non-salmonid species is sparse

(Lucas et al., 1998), studies recently carried out

in Austria clearly supported homing migrations of

barbels and nase (Wiesner et al., 2006).

The European Water Frame Directive aims to

achieve a good ecological status for the rivers of

Member States. This requires healthy fish popu-

lation structures to guarantee sustainability for

this part of the aquatic ecosystem. Fish spawning

grounds and their stability must be guaranteed at

least for 4 weeks (Chondrostoma nasus). This

morphodynamic necessity must be incorporated

in future restoration projects and when artificially

restoring spawning grounds.

Acknowledgements The author wishes to thank DIKaroline Maierhofer and DI Novak Irene for support,DI Rudolf Hornich from the Regional Government ofStyria for funding the monitoring.

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Christoph Hauer is a Research ass-itant in Department of Water,Atmosphere and Environment,BOKU – University of NaturalResources & Applied Life SciencesVienna, Vienna, Austria. Hisresearch expertise in hydrodynamicnumerical modelling: Application of1D, 2D and 3D models in terms of

river restoration. Habitat modelling: Application ofhydrodynamic numerical models for habitat analysesincluding morphodynamic processes. Development of aconceptual model. Fishecological monitoring: Fish popu-lation surveys, evaluating ecological integrity based on fish.Ecomorphology: Leitbild development, monitoring ofnear-natural hydrologic engineering projects. Restorationof running waters: Theoretical concepts of running waterrestoration, methodological developments, referencedefinition and restoration evalution.

Gunther Unfer is a Research assis-tant in Department of Water,Atmosphere and Environment,BOKU – University of Natural Re-sources & Applied Life SciencesVienna, Vienna, Austria. His re-search expertise in projectmanage-

ment. Fishecology: fishmigration, population ecology,ecology of native species. Fishecological monitoring:Functionality of fish migration aids, fish population sur-verys, evaluating ecological integrity based on fish. Fish-eries: Ecologically oriented fisheries concepts, fish farming.Ecomorphology: Leitbild develompment, monitoring ofnear-natural hydrologic engineering projects. Specialmethodological expertise: Electrofihing, radio telemetry,fish tagging, fish traps, diving.

Stefan Schmutz is a Faculty memberin Department of Water, Atmo-sphere and Environment, BOKU –University of Natural Resources &Applied Life Sciences Vienna, Vien-na, Austria. His research expertise inAutecology of native fish species:

Migratory behaviour (spawning runs, migrations of juve-niles, larval and juvenile drift). Radio telemetry, devel-opment of field sampling devices (automatic driftsampler). Habitat preference (methodological develop-

ments, multivariate habitat models). Development of fish-based methods to assessing the ecological integrity of run-ning waters: Standardisation of field sampling protocols,theoretical assessment concepts, assessment criteria, fishcoenotic reference conditions, historic fish fauna, referencemodels, assessment applications, development of an Aus-trian-wide Fish and Habitat Database (HaFiDat). Resto-ration of running waters: Theoretical concepts of runningwater restoration, methodological developments, refer-ence definition, restoration evaluation. Fish-based assess-ment of human impacts in running waters: Riverengineering, fish migration barriers, water abstraction,hydro peaking, flushing of impoundments (development ofmultivariate statistical habitat models for assessing flowand physical habitat alterations). Fish migration devices:Development and evaluation of nature-like fish passes,downstream fish passage solutions. Fisheries managementof running waters: Impact of stocking on autochthonouspopulations, effects of fishing regulations.

Helmut Habersack is a Facultymember in Department of Water,Atmosphere and Environment,BOKU – University of NaturalResources & Applied Life SciencesVienna, Vienna, Austria. His re-search expertise in River morphol-ogy: Monitoring and documentationof the morphological development inAustrian rivers. Sediment transportmeasurements: Development and

application of measuring techniques for bed load andsuspended load transport. Flood analyses: Documentationand analyses of the catastrophic flood events 2002 and2005. Development of guidelines. Hydrodynamic model-ling: Application of 1D, 2D and 3D models in terms ofriver restoration. Ecomorphology: Leitbild development,monitoring of near-natural hydrologic engineering pro-jects. Restoration of running waters: Theoretical conceptsof running water restoration, methodological develop-ments, reference definition, and restoration evaluation.

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