京都大学防災研究所年報 第 53 号 B 平成 22 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 53 B, 2010

Roles of Riffle and Pool Structures in Particulate Organic Matter Dynamics
in the Downstream Reaches of Dam Reservoirs

Giyoung OCK, Yasunori MUTO, Tetsuya SUMI and Yasuhiro TAKEMON
Synopsis
The present study aims to elucidate the functions of riffle and pool structure in
retention process of the drifting plankton supplied from dam outlets, and to find better
configuration of the geomorphic features for increasing the retention efficiency. Two
dam tailwater reaches with different riffle and pool structures were selected and relative
contributions of three source origins to particulate organic matter (POM) were estimatedby means of a stable isotope mixing model. The relative contribution of the plankton
was reduced at riffles and that of terrestrial plant decreased in pools. In addition, theamount of reduction in the contribution of plankton was in proportional to the length ofriffle, but the retention efficiency was higher in short riffles than in long riffles. The
results indicate that riffle and pool structure and their morphological configuration haveroles in POM dynamics in stream ecosystem according to its source origins. Some
recommendations of geomorphological management for restoration of dam tailwaterreaches were discussed.
Keywords: particulate organic matter, plankton retention, stable isotope, channel
geomorphology

1. Introduction
River geomorphology and particulate organic
matter (POM) are indispensable to aquaticorganisms as matrix of habitat and primary energy
resource, respectively. Despite dam contributions
to water resources supplies, security from flooding
and generation of electricity, after several decadesof dam construction, modification of flow regimes
and interrupting the continuity of sedimenttransport by dam reservoirs have brought negative
downstream effects on channel geomorphology
and the quality and quantity of POM. Herein,
ecological deteriorations derived from the effects
have been pointed out as a social issue andrestoration measures to make river ecosystems
healthy are increasingly required in the world.
Dam tailwater ecosystems below dam
reservoirs are continuously disturbed by input oflarge amount of plankton from dam outflows(Richardson and Mackay 1991; Akopian et al.,
1999; Doi et al., 2008), and the nearest ecosystemfrom dam outlets is supposed to be highly reliant
on the plankton as energy source (Tanida andTakemon 1999; Hatano et al., 2005). Therefore, theretention 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, thefactors affecting or controlling the retention
efficiency in dam tailwater ecosystems have notbeen understood enough despite their applicable
importance for river managements.
In our previous study, we estimated the meantransport distance of lentic plankton as an indicatorof the retention efficiency in dam and lake tailwaterreaches and found that simplified channel
geomorphology (i.e., riverbed degradation, channel― 773 ―

incision, loss of riffle-pool structure anddisappearance of bar structure) by dam impacts canlead to reduce the retention efficiency of lentic
plankton (Ock and Takemon 2008; 2010). Ock et al.
(2009) also tried to elucidate deposition processes
in detail depending on the falling velocity of POM
particles and bed morphology through a hydraulicexperiment using pine pollens as surrogates,
showing that the deposition density of each pollen
was significantly different according to bed
morphology such as a damming pool and analternative bar channels. These finding indicatesthat the retention efficiency is highly related withthe channel geomorphological conditions, and alsosuggests that geomorphological management isrequired for recovering or restoring the degraded
tailwater ecosystem. In the present study, we focus
on understanding retention processes of particulateorganic matter (POM) and finding better
configuration of geomorphic features for increasing
the retention efficiency.
Riverine POM in ordinary natural rivers istrophically the mixture comprised of two sources;
the allochthonous terrestrial inputs and the
autochthonous instream production, and thefractional contribution of allochthonous toautochthonous
providesafundamental
understanding on material cycling, food web
structure, biodiversity (Wallace et al., 1982;
Webster and Meyer 1997) and functional feedinggroups (FFGs) of benthic invertebrates (Merritt andCummins 1996; Takemon 2005). Estimation of thefractional contribution of the two sources has beendeveloped by application of stable isotopic
two-sources 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’,
togetherwithallochthonous
inputsandautochthonous 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 oflentic plankton in the tailwater POM by means of
the three-sources mixing model using combined
dual δ13C-δ15N, to investigate the longitudinalretention properties in riffle and pool structures,and finally to suggest recommendation forgeomorphological management of riffle and poolstructure in the tailwater ecosystem.
2. Materials and Methods
2.1 Site description
The filed study was conducted in two dam
tailwater channels with different riffle and poolstructures 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 (Photo 1). The otheris 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 (Photo 2). The flow
discharges in both channels are stably regulated bydam operation (Fig.1a). The climate of the basin istypical monsoon and the basin is surrounded ofmountain forest.
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 samplingsites in the field are shown in Fig.1. The Uji Riveris morphologically characterized by a combinationof short riffles and long-deep pools (Fig.1b)
probably due to riverbed degradation as a dam

Photo1 Amagase Dam in the Uji River and its
downstream channel with long pool

Photo 2 Nunome Dam in the Nunome River and its
downstream channel with long riffle― 774 ―

(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

impact, whereas the Nunome River is characterized
as long riffles and short-shallow pools (Fig.1c), asa result of peakcut dam operation and partially of
artificial sediment replenishment works conducted
for 6 years before the present study as shown in Fig
2.

Fig.2 Temporal information about the date andvolume of the replenished sediment since 2006 andfiled survey in the Nunome River. The values
below dates represent amount of supplied sedimentin the day.
2.2 Field survey
Suspended POM samples were collected 7 sites
for each river using a POM net sampler with 100㎛ in mesh size and 30cm in diameter of flame sizein January to February 2009. Three replicates werecollected at each site. Each suspended fine POM(S-FPOM) samples was filtered in situ using thesieve of 1.0 ㎜ mesh size to separate it fromsuspended coarse POM (S-CPOM).

Terrestrial plant leaves, epilithic algae andlentic phytoplankton were also collected as
potential POM source origins. Special care waspaid to when taking epilithic algae, because ofdifficulty 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 andrinsed using distilled water in laboratory 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
were repetitively 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 duringtransportation to the laboratory.
2.3 Stable isotope analyses and a concentration
weighted three-source mixing model
All isotopic samples were dried at 60℃ for 24hours and then ground to homogenized bulk
samples with a mortar and pestle in the laboratory.
Stable isotope ratios of carbon and nitrogen were
measured by a continuous-flow isotope ratio mass
spectrometry system with an elemental analyzer― 775 ―

composed of EA1108 (Fisons, Milan, Italy),
ConfloII and Delta-S (Finnigan MAT, Bremen,
Germany). The concentration of organic carbon
and nitrogen were measured simultaneously.
Stable isotope ratios are expressed by the standard
δ notation as the following equation (Eq.1):
δ 13C or δ 15N (‰) = (Rsample ⁄Rstandard - 1) ⅹ1000(1)
where R is 13C⁄ 12C for δ 13C or 15N⁄ 14N for δ 15N.
The standards were PeeDee Belemnite for δ 13C andatmospheric nitrogen for δ 15N. DL-alanine wasused as working standard and the analyticalprecision was ±0.2 ‰ for the δ 13C and the δ 15N.
We assumed that POM drifting in dam
tailwater ecosystem is a mixture comprised ofthree origins; lentic plankton supplied from dam
reservoir, terrestrial plant products and instream
epilithic algae. In order to estimate relative
contributionofeach
source,theconcentrated-weighted three sources mixing modelusing dual δ 13C-δ 15N were applied since theelemental (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 , p + f C ,t + f C ,s = 1(2)

f N , p + f N ,t + f N ,s = 1
f C ,i =2.4 Retention efficiency of lentic plankton
For quantitative analysis of downstream
reduction patterns of lentic plankton, the relative
contribution of lentic plankton at the uppermost
site was set to 100% and then relative ratios were
calculated longitudinally. It is known thatconcentration of FPOM particles decreased
exponentially downstream (Jones and Smock1991; Cushing et al. 1993; Thomas et al. 2001),
indicating that the fraction of lentic tracer
plankters would follow a negative exponentialfunction with channel distance as shown in Eq.5.

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

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

f B , i × [C ] i

∑(f

B ,i

× [C ] i )if N ,i =(t) and instream algae (s). Values of fC,i and fN,iwere given by the concentration-dependent massbalance equations. In addition, [C]i and [N]irepresents the C and N concentration, which is apercentaged fraction of elemental mass to total
mass in the source i, respectively. POM source
combinations were considered to be feasible
solutions if the predicted POM isotopic signatures
δ 13CPOM and δ 15N POM matched the observed values.

f B ,i × [ N ] i

∑(f

B ,i

3. Results and Discussion(3)× [ N ]i )i

δ 13C POM = ∑ ( f C ,i ×δ 13Ci )i

δ N POM = ∑ ( f N ,i ×δ 15N i )
15

(4)

i

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

3.1 Longitudinal changes in the relative
contribution of three sources along riffles andpools.
The contribution changes of S-FPOM along
both channels showed spatial trends according toriffle and pool 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 theriffle and pool structure. During passing throughboth riffle zones, fB,p decreased about 9.7% points― 776 ―

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 andin the glide zone (St5-St6) made of 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.3a).
Terrestrial plant

Relative contribution.

100%

Epilithic algae
Lentic plankton

80%
60%
40%
20%
0%
0

400

800

1200

1600

(a) Uji River
Terrestrial plant

100%
Relative contribution.

epilithic algae, however, gradually increased alongreaches irrespective of riffle and pool.
Reduced contribution of lentic plankton in theriffles may be attributed to both physical filtering
by substrate and biological filtering byfilter-feeding animals. Also reduced contribution of
allochthonous terrestrial source during passing
pools could be due to settling down bycomparatively deep depth and low velocity. Thesefindings can provide an interesting implication thatriffles 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 contributionsof S-FPOM according to its source origins.

Epilithic algae
Lentic plankton

80%
60%
40%
20%
0%
0

200

400

600

800

1000

(b) Nunome River
Fig.3 Downstream changes in relative contributionsof the three sources to S-FPOM in the Uji River andNunome River. Dotted-rectangulars represent theriffle zones.
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 ofterrestrial plant decreased in all pools. But thecontribution of instream algae showed an increasein 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 the terrestrial plant source
decreased 2.3-24.1% points during passing pools(Fig.3b). Unlike the Uji R., however, any certain
patterns in contribution of autochthonous epilithic
algae along reaches was difficult to be found inassociation with riffle and pool. The contribution of

3.2 Retention efficiency of lentic plankton
depending on riffle-pool structure
Reduction patterns of contribution of lentic
plankton were compared between the two rivers.
Although both 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) asshown in Fig.4. The result means that thedownstream channel of the Nunome R withrelatively long riffles-short pools structure hashigher 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.4 Reduction patterns in contribution of lentic
plankton between the Uji River and the Nunome
River. The retention coefficient (k) in Uji R. andNunome R. were calculated as 0.243 and 0.491,
respectively.

― 777 ―

In the study, differences in the configuration ofriffle-pool feature can be characterized by somemorphological indices (Table 1); Riffle fraction inreaches (F R ), which represents the relative size of
riffle to pool can be quantified by the ratio of theriffles distance to the total distance, characterize
the longitudinal configuration to influence thefiltering rate of riffle for drifting plankton. The FR
in the Nunome R. (68.4%) was much higher thanthat in the Uji R. (15.5%). In addition, the depth
ratio of pool and riffle (hp/hR) characterize thevertical configuration to affect the settling rate ofpool 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 andreducing hp/hR can lead to decrease the values ofhydraulic radius (Rh), which characterize the crosssectional difference. Rh of the Nunome R. was 0.34m, which was 5.5 times lower than that of the Uji R.
of 1.87 m.
Such configuration differences between the tworivers characterized by longitudinal, vertical andcross sectional indices may attribute the driving
factors to the degree of river-bed degradation andsediment management 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 bylower Rh can lead to increase the mean transportdistance of lentic plankton indicating of the low
retention efficiency. In contrast, the downstream
reaches of the Nunome Dam has shallow-short pooland long riffle maybe due to diminishing flood
flushing by peakcut 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 sedimentmanagement for downstream ecosystem.
3.3
Geomorphologicalmanagementforincreasing retention efficiency of lentic plankton
Recently, some mitigation measures throughsediment management are increasingly planed or
already implemented in dam tailwaters to restore
continuity of sediment transport and budget
between upstream to downstream of dams forrestoration of geomorphology as well as ecology in
dam tailwaters. For instance, the sediment
replenishment, a sediment bypass tunnel and theflood 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 for evaluation themeasures and also give practical suggestions indesigns for increasing the retention efficiency.
(a) Riffles were found to have higher filtering
capacity than other features. Therefore, in order toincrease the filtering capacity of riffles, it is more
efficient to increase the fraction of riffles (FR) in
channel (Table 1). In case the FR is fixed or same
condition, the length, numbers of riffles and theirarrangement in channel should be considered. In
both of the two rivers, the short riffles with highvelocity appeared to be highest 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 totalchannel length is limited. However, since themaximizing FR may result in adverse effect on

Table 1 Comparison of retention coefficient and hydromorphological parameters between the Uji Riverand Nunome River
Retentioncoefficientk (km-1)

RiffleFraction
FR (%)

RiffleshR (m)Depth
Pools
hP (m)hP/hR

Hydraulicradius
Rh (m)Uji River

0.243

15.5

0.67

2.20

3.30

1.87±0.91

0.61±0.23

Nunome River

0.491

68.4

0.34

0.61

1.76

0.34±0.13

0.47±0.15― 778 ―

Mean velocityu (m/s)Table 2 Variation of retention rate and efficiency of plankton in the riffles between Uji River andNunome River
Geo-morph
Reachesologicfeatures

Uji R.

Nunome R.
*

Water

Water

Retention

Retention
efficiency

Lengthsurfacesurfacerate(m)velocity

gradient

(%)

(m/s)

(%)

∆fB,p

(%/m)
*

∆fB,p/m**

St1-St2

Riffle

68

0.93

0.43

9.7

0.14

St3-St4

Riffle

180

0.78

0.28

11.9

0.07

St1-St2

Riffle

300

0.41

0.48

8.0

0.03

St3-St4

Riffle

50

0.69

0.69

7.1

0.14

St5-St6

Riffle

300

0.53

0.60

6.8

0.02

∆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

habitat for benthic communities due to loss of pools,
the optimum FR adjusted in the channel should beapplied (Fig. 5).

(a)deposition of terrestrial plant particles is required, adepth and low 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 (Fig. 6).

FR=(1+1)/10=0.2(a) low hP/hR(b)
FR=(3+3)/10=0.6(b) high hP/hR(c)
FR=(2+2+2)/10=0.6

Fig.5 Management of the fraction of riffles (F R ) for
increasing retention efficiency. The higher F R withlong riffles and short pool (b) is efficient for
retention of lentic plankton relative to short riffles
and long pool (a). If the FR is constant, several
numbers of short riffles and short pools are efficient
for retention for the plankton (c).
(b) Pools were found to be a settling place ofterrestrial plant due to relatively heavy specificweight compared with the lentic plankton. If theFig.6 Management of the depth of pools forincreasing retention efficiency. High h P /h R is
efficient for retention for terrestrial plant source.
(c) The hydraulic radius (Rh) defined by theratio of cross sectional area to wetted perimeter
showed a positive correlation with the meantransport distance. It indicates that the retention
efficiency for lentic plankton increases withdecreasing Rh. Hereby in order to decrease the Rh,
the cross sectional area should be either decreased
by reducing flow discharge or lengthen a wetted
perimeter by complicating river bed (Fig. 7).

― 779 ―

geomorphology in downstream reaches of dam
reservoirs is altered to more simplified such as theloss of riffle-pool structure and sand/gravel bar
structure by dam impacts. In this sense, the present
study provides some practical suggestions in
sediment management in dam tailwater channels forsustaining and enhancing the ecosystem.

(a) high Rh

Acknowledgement(b) low Rh

Fig.7 Management of hydraulic radius forincreasing retention efficiency. If a maximum depth
is constant, the low Rh with complex wetted
perimeter is efficient for retention for lentic
plankton

The authors are grateful to Dr. Yoshikazu Katoand Ms. Hiromi Uno for their assistance with fieldworks. Stable isotopes analysis was conductedusing 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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4. Conclusion
Retention is the process of which removes
matters from transport within the reaches so that it
can make available for reduction the suspended
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is important for recovering from trophicallyreservoir dependant state to a normal state of lotic
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貯水ダム下流河川の粒状有機物動態における瀬と淵の役割

玉 基英・武藤裕則・角 哲也・竹門康弘

要 旨
貯水ダム下流域の劣化した河川生態系を修復する上で不可欠となる貯水池由来物質の定量化方法を検討して河床による粒状有機物の捕捉効率と河道地形条件との関係を研究した。宇治川の天ヶ瀬ダム下流域と土砂還元が実施されている
布目川の布目ダム下流域を調査対象として, 炭素-窒素の安定同位体混合モデルを用して, 粒状有機物を構成する３種
起源物の寄与率が瀬-淵による変化を検討した。瀬ではダム由来プランクトンの捕捉率と付着藻類の放出率が高く、淵では逆にプランクトンの捕捉率が低く陸上植物の捕捉率が高いことを明らかにするともに, 瀬淵の長さ, 深さ, 径深, 水
面勾配, 流速などの水理地形特性がその捕捉率と強い相関を持つことを見出した。本論文は河道の粒状有機物の起源推定の定量的な手法の確立に寄与するともに, 貯水ダムの河川生態系への影響を軽減する際に, 目標となる河床地形や土砂供給量設定するための基礎を提供する。

キーワード: 粒状有機物, プランクトン捕捉率, 安定同位体, 河床地形

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