Roles of riffle and pool structure for increasing retention of lentic
plankton in dam tailwater reaches
Giyoung Ock
Disaster Prevention Research Institute, Kyoto University
Gokasho, Uji, Kyoto, 611-0011, Japan
Yasunori Muto
Disaster Prevention Research Institute, Kyoto University
Gokasho, Uji, Kyoto, 611-0011, Japan
Ichiro Tayasu
Center for Ecological Research, Kyoto University
Hirano-Nichome, Otsu, Shiga 520-2113, Japan
Tetsuya Sumi
Disaster Prevention Research Institute, Kyoto University
Gokasho, Uji, Kyoto, 611-0011, Japan
Yasuhiro Takemon
Disaster Prevention Research Institute, Kyoto University
Gokasho, Uji, Kyoto, 611-0011, Japan
Abstract: As dam tailwater ecosystems depend on lentic plankton supplied from dam outflows as a major
trophic source, the retention efficiency of lentic plankton, an ability of the channel to reduce the driftingplankton, is critical for recovering a normal state of trophic structure in the stream ecosystem. The present
study aims to estimate the functions of riffle and pool structure in transport and retention of the lentic
plankton in the field, and to find configuration of riffle and pool geomorphs increasing the retention
efficiency. Two dam tailwater reaches with different riffle and pool structures were selected andlongitudinal changes in relative contribution of lentic plankton were estimated by means of theconcentration-weighted stable isotope mixing model using dual δ13C-δ15N. Relative contribution of lentic
plankton was generally reduced at riffles and that of terrestrial plant was decreased in pools. Amount of
reduction in the contribution of lentic plankton was in proportional to the length of riffle, but the retention
efficiency was higher in short riffles than in long riffles. The results indicate that riffle and pool structure
and their configuration have roles in particulate organic matter dynamics in stream ecosystem according
to its source origins. Some recommendations of geomorphological management for restoration of dam
tailwater reaches were discussed.
Keywords: particulate organic matter (POM), retention efficiency, stable isotope, concentration weighted
model, riffle-pool, sediment management

Introduction
Dam tailwater ecosystems below dam reservoirs are continuously disturbed by input of large amount ofplankton from dam outflows (Richardson and Mackay 1991; Akopian et al., 1999; Doi et al., 2008), andthe nearest ecosystem from dam outlets is supposed to be highly reliant on the plankton as energy source(Tanida and Takemon 1999; Hatano et al., 2005). The retention efficiency of lentic plankton, an ability of
the channel to reduce the drifting plankton, is critical for recovering a normal state of trophic structure in
the stream ecosystem. However, the factors affecting or controlling the retention efficiency in dam
tailwater ecosystems have not been understood enough despite their applicable importance for rivermanagements.
In our previous study, we estimated the mean transport distance of lentic plankton as an indicator of theretention efficiency in dam and lake tailwater reaches and found that simplified channel geomorphology639

(i.e., riverbed degradation, channel incision, loss of riffle-pool structure and disappearance of bar
structure) by dam impacts can lead to reduce the retention efficiency of lentic plankton (Ock and
Takemon 2010). This finding indicates that the retention efficiency is highly related with the channel
geomorphological conditions, and also suggests that geomorphological management is required forrecovering or restoring the degraded tailwater ecosystem. In the present study, we focus on understanding
retention processes of particulate organic matter (POM) and finding better configuration of geomorphic
features for increasing the retention efficiency.
Riverine POM in ordinary natural rivers is trophically the mixture comprised of two sources; theallochthonous terrestrial inputs and the autochthonous instream production, and the fractional
contribution of allochthonous to autochthonous provides a fundamental understanding on material cycling,food web structure, biodiversity (Wallace et al., 1982; Webster and Meyer 1997) and functional feedinggroups (FFGs) of benthic invertebrates (Merritt and Cummins 1996; Takemon 2005). Estimation of thefractional contribution of the two sources has been developed by application of stable isotopic twosources mixing model using single δ13C.
In dam tailwaters, the POM is supposed to be three sources mixture due to introduction of lentic plankton,
which means ‘dam originated source’, together with allochthonous inputs and autochthonous production.
Thus effect of lentic plankton on tailwater ecosystems can be quantified by its relative contribution to
POM. The present study aims to estimate the relative contribution of lentic plankton in the tailwater POM
by means of the three-sources mixing model using combined dual δ13C-δ15N, to investigate thelongitudinal retention properties in riffle and pool structures, and finally to suggest recommendation forgeomorphological management of riffle and pool structure in the tailwater ecosystem.

Materials and Methods
Site description
The filed study was conducted in two dam tailwater channels with different riffle and pool structures in
the Yodo River system in central Japan; the first is Uji River reaches (34°52´ N, 135°49´ E) below the
Amagase Dam (the arch dam with 73 m height and 26.3 million m3 storage capacity) constructed in 1964.
The other is Nunome River reaches (34°42´ N, 135°58´ E) below Nunome Dam (the concrete gravity dam
with 72 m height and 17.3 million m3 storage capacity) constructed in 1992. The flow discharges in bothchannels are stably regulated by dam operation (Fig.1a). The climate of the basin is typical monsoon andthe basin is surrounded of mountain forest.

pool(b) Uji River(a) Key map(c) Nunome River
Fig.1 Map of study sites in the downstream reaches of the Uji River and Nunome River. Seven sites
were established at the border of riffle and pool in each river

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The study sites in the both rivers were selected at each border of riffle and pool within 2 km reaches of
distance from dam outlets. The longitudinal profiles of the reaches and sampling sites in the field areshown in Fig.1. The Uji River is morphologically characterized by a combination of short riffles andlong-deep pools (Fig.1b) probably due to riverbed degradation as a dam impact, whereas the Nunome
River is characterized as long riffles and short-shallow pools (Fig.1c), as a result of peakcut dam
operation and partially of artificial sediment replenishment works conducted for 6 years before thepresent study.
Field survey
Suspended POM samples were collected 7 sites for each river using a POM net sampler with 100㎛ inmesh size and 30cm in diameter of flame size in January to February 2009. Three replicates werecollected at each site. Each suspended fine POM (S-FPOM) samples was filtered in situ using the sieve of
1.0 ㎜ mesh size to separate it from suspended coarse POM (S-CPOM).
Terrestrial plant leaves, epilithic algae and lentic phytoplankton were also collected as potential POM
source origins. Special care was paid to when taking epilithic algae, because of difficulty for separating
pure algae from biofilm including other several materials (Finlay 2001; Hamilton et al. 2004). The
collected epilithic algae were washed several times in the river water and rinsed using distilled water inlaboratory to remove impurities. The purity was confirmed using a microscope in laboratory. Samples for
lentic plankton were taken in the nearest site from dam outlet and then the lentic plankton was extracted
from other seston particles by means of repetitive settling process where rapidly sinking particles wererepetitively excluded in a settling chamber. After that the extraction were also confirmed using a
microscope. For preventing from decomposition before analysis, collected samples for stable isotope
analysis were preserved in a ice box during transportation to the laboratory.
Stable isotope analyses and Concentration-weighted three-source mixing model
All isotopic samples were dried at 60℃ for 24 hours and then ground to homogenized bulk samples witha mortar and pestle in the laboratory. Stable isotope ratios of carbon and nitrogen were measured by acontinuous-flow isotope ratio mass spectrometry system with an elemental analyzer composed of EA1108(Fisons, Milan, Italy), ConfloII and Delta-S (Finnigan MAT, Bremen, Germany). The concentration oforganic carbon and nitrogen were measured simultaneously. Stable isotope ratios are expressed by thestandard δ notation as the following equation (Eq.1):
δ13C or δ15N (‰) = (Rsample ⁄Rstandard - 1) ⅹ1000(Eq.1)

where R is 13C⁄ 12C for δ13C or 15N⁄ 14N for δ15N. The standards were PeeDee Belemnite for δ13C andatmospheric nitrogen for δ15N. DL-alanine was used as working standard and the analytical precision was±0.2 ‰ for the δ13C and the δ15N.
We assumed that POM drifting in dam tailwater ecosystem is a mixture comprised of three origins; lentic
plankton supplied from dam reservoir, terrestrial plant products and instream epilithic algae. In order toestimate relative contribution of each source, the concentrated-weighted three sources mixing modelusing dual δ13C-δ15N were applied since the elemental (C-N) concentrations is distantly different among
the three end members (Phillips and Koch, 2002). The following equations (Eq.2,3,4) were modified from
Phillips and Koch (2002) and Newsome et al. (2004).

f B , p + f B ,t + f B ,s = 1

f C ,i =f C , p + f C ,t + f C ,s = 1 (Eq.2)
f N , p + f N ,t + f N ,s = 1

f B , i × [C ] i

∑(f

B ,i

i

f N ,i =f B ,i × [ N ] i

∑(f

B ,i

δ 13C POM = ∑ ( f C ,i ×δ 13C i )× [C ] i )× [ N ]i )(Eq.3)iδ 15N POM = ∑ ( f N ,i ×δ 15N i )(Eq.4)ii

where fB,i , fC,i and fN,i represents a fractionation of biomass, carbon and nitrogen, respectively, contributed
to the POM by source i ; herein, composed of lentic plankton (p), terrestrial plant (t) and instream algae(s). Values of fC,i and fN,i were given by the concentration-dependent mass balance equations. In addition,[C]i and [N]i represents the C and N concentration, which is a percentaged fraction of elemental mass to641

total mass in the source i, respectively. POM source combinations were considered to be feasible
solutions if the predicted POM isotopic signatures δ13CPOM and δ15NPOM matched the observed values.
Retention efficiency of lentic plankton
For quantitative analysis of downstream reduction patterns of lentic plankton, the relative contribution oflentic plankton at the uppermost site was set to 100% and then relative ratios were calculated
longitudinally. It is known that concentration of FPOM particles decreased exponentially downstream(Jones and Smock 1991; Cushing et al. 1993; Thomas et al. 2001), indicating that the fraction of lentic
tracer plankters would follow a negative exponential function with channel distance as shown in Eq.5.

F(x) = exp(-kx)(Eq.5)

where F(x) is the fraction remaining at downstream sites, x is downstream distance and k is thelongitudinal loss rate. In the study, the k value was used as a retention coefficient (m-1), which is related tothe proportion of contribution diminishing per metre, for indicating ‘retention efficiency of the river
channel’. Larger values of k indicate higher rates of retention.

Results and Discussion
Longitudinal changes in the relative contribution of three sources along riffles and pools.
The contribution changes of S-FPOM along both channels showed spatial trends according to riffle andpool structures. In the Uji River, it was found that even within a distance of a few reaches’s scale of 1.6
km, S-FPOM composition can be dynamically changed being strongly affected by the riffle and poolstructure. During passing through both riffle zones, fB,p decreased about 9.7% points from 67.5 to 57.8%
in St1-St2 and 11.9% points from 46.1 to 34.2% in St3-St4. In contrast, fB,s increased 10.1 and 14.1% in
the riffles respectively. There were little variations in fB,t in the riffles and in the glide zone (St5-St6) madeof partially exposed bedrock. In the pool zones, on the other hand, fB,t showed various patterns in the
Pool1(St2-St3) and Pool2 (St6-St7). fB,t increased 12.6% points during passing the Pool1, whereas it
decreased 7.0% points in the Pool2 (Fig.2a).

Terrestrial plant(a) Uji River

Relative contribution.

100%

Epilithic algae
Lentic plankton

80%
60%
40%
20%
0%
0

400

800

1200

1600
Terrestrial plant(b) Nunome River

Relative contribution.

100%

Epilithic algae
Lentic plankton

80%
60%
40%
20%
0%
0

200

400

600

800

1000

Fig.2 Downstream changes in relative contributions of the three sources to S-FPOM in the Uji Riverand Nunome River. Dotted-rectangulars represent the riffle zones.

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In the Nunome River, the contributions of lentic plankton decreased in all riffles, where those ofterrestrial plant increased in two of the three riffles probably due to supply from adjacent forest andriparian plant. However, the contributions of terrestrial plant decreased in all pools. But the contributionof instream algae showed an increase in the short pools but no trend in the long riffles. The relative
contribution of lentic plankton decreased 6.8-8.0% points during passing riffles, whereas that of theterrestrial plant source decreased 2.3-24.1% points during passing pools (Fig.2b). Unlike the Uji R.,
however, any certain patterns in contribution of autochthonous epilithic algae along reaches was difficult
to be found in association with riffle and pool. The contribution of epilithic algae, however, gradually
increased along reaches irrespective of riffle and pool.
Reduced contribution of lentic plankton in the riffles may be attributed to both physical filtering bysubstrate and biological filtering by filter-feeding animals. Also reduced contribution of allochthonous
terrestrial source during passing pools could be due to settling down by comparatively deep depth andlow velocity. These findings can provide an interesting implication that riffles tend to remove lentic
plankton, and pools can trap the terrestrial plant particles, consequently riffle-pool sequences in dam
tailwater channels may play a role in spatially different contributions of S-FPOM according to its source
origins.
Retention efficiency of lentic plankton depending on riffle-pool structure
Reduction patterns of contribution of lentic plankton were compared between the two rivers. Althoughboth rivers revealed overall downstream decreasing pattern along channel, the retention coefficient (k)was approximately 2 times larger in Nunome R. (0.491) than in the Uji R. (0.243) as shown in Fig.3. The
result means that the downstream channel of the Nunome R with relatively long riffles-short pools
structure has higher retention efficiency than that in the Uji R. with short riffle-long pool structure,
implying that configuration of geomorphic features is also important for retention efficiency of driftingplankton.

Fig.3 Reduction patterns in contribution of lentic plankton between the Nunome River and the Uji
River. The retention coefficient (k) in the Nunome R. and Uji R. were calculated as 0.491 and 0.243
respectively.
Table 1 Retention coefficient and hydro-geomorphological parameters between the Uji River andthe Nunome River
Retentioncoefficientk (km-1)

RiffleFraction
FR (%)

RiffleshR (m)Depth
Pools
hP (m)Uji River

0.243

15.5

0.67

Nunome River

0.491

68.4

0.34

643

hP/hR

Hydraulicradius
Rh (m)

Mean
velocityu (m/s)

2.20

3.30

1.87±0.91

0.61±0.23

0.61

1.76

0.34±0.13

0.47±0.15

In the study, differences in the configuration of riffle-pool feature can be characterized by somemorphological indices (Table 1); Riffle fraction in reaches (FR ), which represents the relative size of riffle
to pool can be quantified by the ratio of the riffles distance to the total distance, characterize thelongitudinal configuration to influence the filtering rate of riffle for drifting plankton. The FR in the
Nunome R. (68.4%) was much higher than that in the Uji R. (15.5%). In addition, the depth ratio of pooland riffle (hp/hR) characterize the vertical configuration to affect the settling rate of pool for terrestrial
plant particles. The hp/hR in the Nunome R. (1.76) was much less than that in the Uji R. (3.30).
Subsequently increasing FR and reducing hp/hR can lead to decrease the values of hydraulic radius (Rh),which characterize the cross sectional difference. Rh of the Nunome R. was 0.34 m, which was 5.5 times
lower than that of the Uji R. of 1.87 m.
Such configuration differences between the two rivers characterized by longitudinal, vertical and crosssectional indices may attribute the driving factors to the degree of river-bed degradation and sedimentmanagement in the two rivers. The Amagase Dam in the Uji River has stopped the bed-load transport over
40 years since its construction so that the downstream reaches has been degraded in bed to an extent of
bedrock exposure and pool may deepen and lengthen. Ock and Takemon (2010) showed that more
simplified changing channel cross-section represented by lower Rh can lead to increase the mean transportdistance of lentic plankton indicating of the low retention efficiency. In contrast, the downstream reachesof the Nunome Dam has shallow-short pool and long riffle maybe due to diminishing flood flushing bypeakcut dam operation and partially the sediment supplies artificially by sediment replenishment works.
Higher retention efficiency in the Nunome R. can partially attribute to effect of the sediment
replenishment and consequently it will be a necessity and importance for sediment management for
downstream ecosystem.
Geomorphological management for increasing retention efficiency of lentic plankton
Recently, some mitigation measures through sediment management are increasingly planed or already
implemented in dam tailwaters to restore continuity of sediment transport and budget between upstream
to downstream of dams for restoration of geomorphology as well as ecology in dam tailwaters. For
instance, a sediment replenishment, a sediment bypass tunnel and a flood mitigation dam without
impoundment (Sumi, 2005). However, assessing the effects of the mitigation measures on downstream
ecosystems were hardly implemented yet due to ambiguity of ecosystem response and lack of its
quantification methods. Our results derived from the present study can provide conceptual criteria forevaluation the measures and also give practical suggestions in designs for increasing the retention
efficiency.
(a) Riffles were found to have higher filtering capacity than other features. Therefore, in order to increase
the filtering capacity of riffles, it is more efficient to increase the fraction of riffles (FR) in channel (Table1). In case the FR is fixed or same condition, the length, numbers of riffles and their arrangement inchannel should be considered. In both of the two rivers, the short riffles with high velocity appeared to behighest retention efficiency (∆fB,p/m) than those in riffles with longer distance (Table 2). Thus several
numbers of short riffles will be more efficient than one long riffle when the total channel length is limited.
However, since the maximizing FR may result in adverse effect on habitat for benthic communities due toloss of pools, the optimum FR adjusted in the channel should be applied.
Table 2 Comparison of retention efficiency of plankton in the riffles between the Nunome River andthe Uji River
Water
Water
surfacesurface
Reachesvelocity
gradient(m/s)(%)
St1-St2
Riffle0.93
0.43
68
Uji R.
St3-St4
Riffle
180
0.78
0.28
St1-St2
Riffle
300
0.41
0.48
Nunome R.
St3-St4
Riffle0.69
0.69
50
St5-St6
Riffle
300
0.53
0.60
*
∆fB,p : retention rate means the reduction rate of fB,p in the riffle**∆fB,p/m : retention efficiency means the reduction rate per metre of fB,p in the riffle
Geomorphologic
features

Length(m)

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Retention
rate(%)∆fB,p*
9.7
11.9
8.0
7.1
6.8

Retention
efficiency(%/m)∆fB,p/m**
0.14
0.07
0.03
0.14
0.02

(b) Pools were found to be a settling place of terrestrial plant due to relatively heavy specific weightcompared with the lentic plankton. If the deposition of terrestrial plant particles is required, a depth andlow velocity enough for settling should be sustained. In particular, if an appropriate structure of pools
were restored or created, sustainable management is required not to progress the river bed degradation.
(c) The hydraulic radius (Rh) defined by the ratio of cross sectional area to wetted perimeter showed apositive correlation with the mean transport distance. It indicates that the retention efficiency for lentic
plankton increases with decreasing Rh. Hereby in order to decrease the Rh, the cross sectional area shouldbe either decreased by reducing flow discharge or lengthen a wetted perimeter by complicating river bed.

Conclusion
Retention is the process of which removes matters from transport within the reaches so that it can makeavailable for reduction the suspended materials and for utilization by stream biota. In dam tailwater
ecosystem, the retention of lentic plankton is important for recovering from trophically reservoir
dependant state to a normal state of lotic ecosystem. The present study focused on elucidating theretention processes in relation to channel geomorphology, particularly in riffle and pool structure. Results
showed that the relative contribution of lentic plankton was generally reduced within the riffle zone andthat of terrestrial plant was decreased in the pool zone. Although the reduction rate in the contribution oflentic plankton was in proportional to the riffle fraction (FR) in the channel, the retention efficiency (k or∆fB,p/m) can be higher in several short riffles than in the long riffle when the total channel length is limited.
The results indicate that riffle and pool structure has an important role for transport and retention of POM
in downstream reaches below dam reservoir, and moreover riffle-pool structure contributes to increase
spatial heterogeneity of POM according to its source origins. Nowadays channel geomorphology indownstream reaches of dam reservoirs is altered to more simplified such as the loss of riffle-pool structureand sand/gravel bar structure by dam impacts. In this sense, the present study provides some practicalsuggestions in sediment management in dam tailwater channels for sustaining and enhancing theecosystem.
Acknowledgement
We are grateful to Dr. Yoshikazu Kato and Ms. Hiromi Uno for their assistance with field works. Stable
isotopes analysis was conducted using Cooperative Research Facilities of Center for Ecological Research,
Kyoto University. This work was partially supported by Grant-in-Aid for Scientific Research
No.19360224 and MLIT grant for the construction technology development program.

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