River Sedimentation – Wieprecht et al. (Eds)© 2017 Taylor & Francis Group, London, ISBN 978-1-138-02945-3

Effects of sediment bypass tunnels on sediment grain size distribution andbenthic habitats
C. Auel, S. Kobayashi, T. Sumi & Y. Takemon
Water Resources Research Center, Disaster Prevention Research Institute, Kyoto University, Uji, Japan

ABSTRACT: Four reservoirs in Japan and Switzerland with sediment bypass tunnels (SBT) as strategy against
sedimentation were monitored to analyze their effects in terms of up- to downstream morphological and bioticchanges. Sediment grain size distribution (GSD), local bed characteristics, microhabitat abundance and invertebrate richness were analyzed. It was found that GSD at reservoirs with newly established SBTs are fine in the upand coarse in their downstream waterway due to lack of conveyed sediments in the past. Analysis of biotic data
directly below the dams reveal that microhabitat richness is low and lentic species abundance is high compared
to upstream, while these differences decrease further downstream. Microhabitat and invertebrate richness in thedownstream adjust to the upstream values with increasing operation years clearly showing the positive effectsof long-term SBT operation.

1

INTRODUCTION

Anthropogenic impacts have altered river systems
worldwide with about 15% of the global annual river
runoff stored in manmade reservoirs (Nilsson et al.
2005). A dam impounding a river interrupts the natural flow and hinders continuous sediment transport
resulting in changes in downstream flow regime, bedmorphology and ecosystem (Poff et al. 1997, Brandt
2000, Kondolf et al. 2014, Aristi et al. 2014). A reservoir traps the incoming sediments leading to sedimentstarving conditions and degradation downstream if
no appropriate action is taken. Sedimentation management encompasses a large variety of strategies
including flushing, sluicing and bypassing (Morris& Fan 1998, Kondolf et al. 2014). Sediment bypass
tunnels (SBT) are one strategy routing sediment loadaround reservoirs thereby avoiding accumulation (Vischer et al. 1997, Sumi et al. 2004). The number oftunnels worldwide is still limited due to high construction and maintenance cost. Most tunnels are locatedat gravel bed rivers (GBR) in mountainous regions
at small to medium-size reservoirs where a considerable amount of coarse material is entrained. A SBT is
operated in supercritical free-surface flow conditions
with flow velocities up to 15 m/s with tunnel lengths
ranging from some hundred up to 4000 m (Auel &Boes 2011). The tunnel intake is located either at thereservoir head or inside the reservoir depending on
the desired operational conditions. A SBT has theadvantage that only newly entrained sediment fromthe upstream reach is diverted into the downstream
thereby reestablishing the sediment connectivity. The
sediment pulse is therefore of natural character as thepre-dam conditions are reestablished during floods.
Already accumulated sediments in the reservoir are

normally not mobilized. Auel et al. (2016) show for
two Japanese SBT that they are an effective strategy tocounteract sedimentation thereby considerably enlarging reservoir life. The downstream released sediments
lead to morphological changes in the river bed withformerly degraded sections being filled (Fukuda et al.
2012, Facchini et al. 2015). Besides morphologicalchanges, also the benthic habitat is largely affected byflushing or bypass operations (Robinson et al. 2003,
2004, Martin et al. 2015).
In this paper we analyze the downstream effectsof SBT operation on bed morphology and benthic
habitats exemplary for four different reservoirs in
Switzerland and Japan, the two countries with highest
number of SBT in operation and planning.
2

METHODS

Four reservoirs, where SBTs as a sedimentation strategy are applied, were monitored in 2014 to analyze
the effects of tunnel operation and resulting sediment augmentation in the downstream. These were
Pfaffensprung and Solis in Switzerland, and Asahi
and Koshibu in Japan (Tab. 1). Sediment grain size
distribution (GSD), local bed characteristics, microhabitat and invertebrate richness were analyzed upand downstream of these reservoirs. The SBTs Asahi
and Pfaffensprung are already operated for 17 and93 years, respectively (termed old hereafter), whereas
Koshibu and Solis are new with no and two years ofoperation, respectively.
The benthic biota can be mainly divided into lentic
and lotic species corresponding to biota preferring stillor flowing water, respectively. Whereas upstream ofdams lotic species dominate, lentic ones are abundant825

downstream of dams where high discharge is scarce.
We hypothesize that the longer the SBT is in operation,
the lower the up- to downstream difference in GSD,
microhabitat and invertebrate richness.
2.1

Study area

2.1.1 Asahi reservoir
The 86 m high Asahi dam is located in the Kii Peninsula, Nara Prefecture, Japan, and operated by Kansai
Electric Power. The reservoir is part of the Okuyoshino
pump storage hydropower scheme together with theupper Seto dam. Operation started in 1978 with an
original reservoir volume of 15.5 × 106 m3 . The catchment area is 39.2 km2 and mostly covered by forest with a maximum altitude of 1,800 m a.s.l. The
upstream Asahi river reach is a GBR with steep slopesof 2% at the reservoir head up to about 4% further
upstream. Due to large landslides in the catchment,
reservoir sedimentation progressed by about 0.5% per
year. Therefore, construction of a SBT was initiatedand put into operation in 1998 (Nakajima et al. 2015).
77% of the incoming sediments were bypassed sincethen (Auel et al. 2016). The tunnel design discharge
is Qd = 140 m3 /s, the gate is opened at Q > 5 m3 /s,
whereas smaller flows are directed into the reservoir
through a weir opening. From March to September, thetunnel is opened about 12 h during daytime to enhancedownstream ecological conditions. The mean annual
inflow is low with Q = 2.5 m3 /s, floods occur in therainy season from July to September partially until
November with an annual mean of Q = 299 m3 /s.
2.1.2 Koshibu reservoir
The 105 m high Koshibu dam, constructed in 1969,
is located in Nagano Prefecture, Japan, and operated
by the Ministry of Land, Infrastructure, Transport andTourism. The 58 × 106 m3 large reservoir is used for
power generation, flood control and irrigation (Kashiwai & Kimura 2015). The Koshibu River is located in
an area of high erosional activity due to both heavy
rainfall (1600-2000 mm/a) and large seismic activity.
The catchment area is 288 km2 with a maximum altitude of 3,120 m a.s.l. Sedimentation in 2012 reached
17 × 106 m3 despite of frequent dredging operations(Kashiwai & Kimura 2015). Hence, 29% of the original reservoir volume was filled by then with a meanannual sedimentation rate of 0.68%. The constructionof a SBT was initiated in 2009, is currently in its final
stage and will presumably start regular operation in2017. An upstream check dam will retain the coarsest
sediment fractions to be used as construction material. The tunnel design discharge is Qd = 370 m3 /s,
while the gate is planned to be opened at Q > 80 m3 /s(Kashiwai & Kimura 2015). The mean annual inflow
and flood discharge is Q = 3.4 m3 /s and Q = 317 m3 /s,
respectively. As one reservoir purpose is flood control,
the mean flood outflow is lower with Q = 203 m3 /s.
2.1.3 Solis reservoir
The 61 m high Solis dam, built in 1986, is locatedin the Swiss Alps, canton of Grisons. It is operated
by the Electric Power Company of Zurich and used

for hydropower generation. The dam retains the waterfrom the Albula River, resulting in a 4.1 × 106 m3 large
reservoir of narrow shape. The mean annual inflow is
25 m3 /s, coming from the Albula and Julia Rivers witha total catchment area of 900 km2 as well as upstreamhydropower plants (Auel et al. 2011). The mean catchment elevation is 2150 m a.s.l. Both rivers are GBR
with mean slopes of about 1.5% and 5% for the Albula
and Julia, respectively. The HQ100 (flood event with
100 year return period) accounting for both rivers wasreported with 280 m3 /s (Auel et al. 2010, 2011) basedon data from 2003 by the Federal Office for the Environment (FOEN) but increased to 428 m3 /s in 2015
due to updated statistical data for the Julia River. The
average annual sediment aggradation in the reservoir
is 8.0 × 104 m3 (Auel et al. 2010, 2011).
In 2012 about half of the reservoir was filled(Oertli & Auel 2015) corresponding to an annual sedimentation rate of 1.9%. In the same year, constructionof a SBT was completed. Since then, the tunnel successfully operates and already diverted a major floodof 288 m3 /s onAugust, 13th 2014 (Oertli &Auel 2015).
The tunnel design discharge is Qd = 170 m3 /s corresponding to a five year flood event, the mean annual
flood is Q = 131 m3 /s.
2.1.4 Pfaffensprung reservoir
The 32 m high Pfaffensprung dam is located in the
Swiss Alps in the canton of Uri, commissioned in
1922 and operated by the Swiss Federal Railways. The
1.55 × 105 m3 small reservoir has the function of acompensation basin as part of the Reuss River cascadehydropower schemes (Müller & Walker 2015). The
SBT was constructed at the same time as the reservoir
itself. The tunnel design discharge is Qd = 220 m3 /sand the catchment is 390 km2 with a mean elevationof 2300 m a.s.l. The Reuss upstream of Pfaffensprungis a GBR with a mean slope of 4%. The Pfaffensprung SBT is in operation for about 135 days per year
during the high flow period from April to November(Müller & Walker 2015). The nearest gauging station Reuss-Andermatt operated by FOEN is locatedabout 9 km upstream, while about 20 small tributaries drain into the Reuss in between Andermatt
and the reservoir, which are not accounted for. The
mean annual flood at that station is Q = 101 m3 /s,and the HQ100 = 251 m3 /s, while Müller & Walker(2015) report HQ100 = 460 m3 /s most likely taking
the tributaries into account.
2.2

Data collection

Data was collected on May 13th and July 24–25th 2014
at Asahi and Koshibu, and on August 28th and 30th
2014 at Solis and Pfaffensprung reservoirs. Samples
were taken at up to four different locations at eachsite (Fig. 1): Upstream of the reservoir (US), directly
downstream of the dam (DS-D), downstream of the
SBT outlet (DS-SBT), and downstream of the first
tributary confluence (DS-TC).
At each location, the local bed slope Sb and riverwidth B were measured at least three times using the826Table 1.

Reservoir characteristics.

Original volume
Dam completion
Catchment size
SBT completion
SBT length
SBT QdFlood HQ100
Mean an. Flood HQ1Mean an. inflow[106 m3 ][km2 ][m][m3 /s][m3 /s][m3 /s][m3 /s]Asahi

Koshibu

Solis

Pfaffensprung

15.5
1979
39
1998
2383
140
1200
299
2.5

58
1969
288
2016
3982
370
1300
317/203**
3.4

4.1
1986
900
2012
968
170
280/428*
131*
25

0.15
1922
390
1922
282
220
251*/460
101*
7.1*

Data taken from Auel & Boes (2011), Kashiwai & Kimura (2015), Müller & Walker (2015), Nakajima et al. (2015). * Data
from FOEN ** Data from MLIT (inflow/outflow).

Figure 2. Photographs of macro invertebrate samples: a)mayfly, b) stonefly, c) caddisfly (courtesy of S. Kobayashi).

Figure 1. Sketch of sample locations.

distance laser Impulse 200 from Laser Technology Inc.
B was measured from bank to bank, defined as theorigin of major vegetation growth, e.g. shrubs or trees.
The GSD was analyzed with minimum 10 photographs
taken at each location using manual counting with a 25
node grid superimposed. Only grains below the nodes
were accounted for, leading in total to 254 ± 66 stones
at each location. Possible armoring layers were notremoved prior to the photo shot.
We defined 18 types of microhabitats: A splash
zone, B leaves pack, C bedrock, D tree root, E mossmat, F wood, G cobbles, H embedded cobbles, I
gravel, J sand, K emerged plant, L submerged plant,
M deposited leaves pack, N mud, O filamentous algae,
P spring, Q seepage, R plant cover. The abundanceof each microhabitat was recorded by four levels:
0-no, 1-minor, 2-frequent, 3-major. Benthic invertebrate samples were taken at riffle and pool sections toaccount for variation in microhabitat structure. Sample collection was done with a D-framed net (opening:
500 × 500 mm2 , mesh size: 0.5 mm) for 10 to 20 minutes by two persons. Samples were separated for lotic(riffles and fast flows) and lentic species (pools andother slow flows). Invertebrates were preserved by
Ethanol and taxa were identified in the laboratory
using a microscope. We focused on macro invertebrates such as mayflies, stoneflies, caddisflies, andcoleopterans, we did not analyze small invertebrates
such as worms and midges (Fig. 2).
We separated the abundance of each taxon into
4 levels: 0-not present, 1-less than 9 individuals, 2less than 99 individuals, 3-more than 100 individuals.
Each taxon was classified by its life type and habitat specialty according to Merritt & Cummins (1996),
Takemon (2005) and our data set. Life types areclassified as swimmers, gliders, clingers, burrowers,

net-spinners and case makers. Habitat specialties are
riffle specialists with taxa being more in riffles thanpools across all locations, pool specialists with taxa
more in pools than riffles, and generalists with taxa
other than the previous two conditions.

2.3 Data analysis
GSD were plotted at each location and diameters D16 to
D99 where 16% to 99% of the particles pass the sieve,
were analyzed. In Japan,D60 is widely used as a representative for a gravel bed (Yamamoto 2010), whereas
the effective diameter Dm is used in Switzerland(Meyer-Peter & Müller 1948):

where dmi = mean D of fraction i and pi = weight
proportion. Dm does not equal D50 , the median diameter, but represents a mean value. Furthermore, theGSD pattern is described by the uniformity coefficientσu = (D84 /D16 )0.5 .
The microhabitat richness σmh is defined as thenumber of bed types present at the site. The higher σmh
the more diverse microhabitats are present in the river
reach. The bed coarseness index σbed was estimatedfrom the weighted average of the relative abundanceof five bed material types as:

The relative abundance of organic versus inorganic
habitats is defined as the sum of the relative abundance of organic over the total number of microhabitat

827

types as:with 0 = fully inorganic covered bed to 1 = fullyorganic covered bed.
The invertebrate species richness σinv is defined as
the number of taxon found at each location. The higher
σinv the more diverse species are present. The relative
abundance of net-spinners versus gliders follows as:

The net spinner prefers stable beds for their living,whereas the glider prefers clean stones surfaces, which
is typical of a frequently disturbed bed. Thus, a highindex shows the preference of the community for bed
stability. The relative abundance of pool versus riffle
specialists is defined as:
Figure 3. GSD at riffle and pool locations of a) Koshibu,
and b) Asahi dams.

The higher σpo the higher the preference of thecommunity for slow-flow conditions.
The similarity of the composition in microhabitats and invertebrate community between upstream
and downstream locations (at DS-SBT) was analyzed
using the Bray-Curtis similarity index (Bray & Curtis
1957):

where ni = abundance of each microhabitat type orinvertebrate taxon, and Ni = sum of the abundance of
all microhabitat types or all invertebrate taxa. IBC = 0
if no common type or taxon between the two sites wasfound, and IBC = 1 if exactly the same composition
between the two exists.
3
3.1

RESULTS AND DISCUSSION
Grain size distribution

Figures 3 and 4 show the GSD for the Japanese andSwiss sites at up – and downstream locations for bothriffles and pools. Table 2 lists all D for each location.
The pool GSD mostly follows the riffle ones, however
some, as Solis US and Pfaffensprung DS-SBT, considerably differ. This is due to fines that tend to settlein pool areas.
The two new SBT Koshibu and Solis show similar
behavior, with fine GSD in the up- and coarser onesin the downstream. GSD at Koshibu dam reveals aremarkably finer distribution upstream (D60 = 57 mm)compared to DS-SBT (D60 = 100 mm). This is to be

Figure 4. GSD at riffle and pool locations a) Solis, and b)
Pfaffensprung dams.

expected as the SBT is still not operating, and sediments are entirely trapped in the reservoir or excavated.
GSD at Solis is fine upstream (D60 = 71 mm), coarser
below the SBT outlet (D60 = 95 mm), and again getting finer below the confluence (D60 = 61 mm). The
tunnel was operated only three times before the sample
date, and the SBT intake is not located at the reservoir

828

Table 2.

Characteristic diameter for riffle locations.
Asahi

D16 [mm]
D50 [mm]
D60 [mm]
D84 [mm]
D99 [mm]Dm [mm]σu

Koshibu

Solis

Pfaffensprung

US

DS-TC

US

DS-SBTUS

DS-SBT

DS-TC

USDS-SBT

DS-TC

21
75
86
130
280
81
2.5

12
51
63
105
218
60
3.0

2
42
57
119
759
72
7.3

17
84
100
168
365
97
3.2

8
50
71
126
300
68
4.0

27
71
95
173
402
98
2.5

6
45
61
132
571
76
4.6

7
98
132
238
564
130
6.0

26
130
177
289
643
163
3.4

33
104
146
230
513
138
2.7

head but only 450 m upstream of the dam. Hence theupstream bed had to adjust to the bypass operationin the first events where almost no coarse sediment
was bypassed (Hagmann et al. 2015). During the floodon August, 13th 2014, with a peak of Q = 288 m3 /s,
about 80,000 m3 sediment were bypassed (Facchini
et al. 2015). This event seems to have mobilized coarse
material from the reservoir head consequently explaining the coarse GSD at US-SBT. However, the fine
GSD at the confluence is not consistent as the sediment should be coarse either due to coarse sediments
entrained after the bypass operation or to the lackof sediments from the past. A possible explanation
is an open gravel pit in the sample location vicinity
entraining most likely fines into the river. The studyof Facchini et al. (2015) showed decreased bed elevation after the event of August, 13th in the upper sectionof the downstream reach indicating that sediment andwater from the tunnel passed through and eroded
the bed.
The two old SBT Asahi and Pfaffensprung show
ambiguous behavior. It is expected that all coarse
sediments are transported through the SBTs, hence
the GSD should be of similar size up- and downstream. However, GSD at Asahi is coarse upstream(D60 = 86 mm) and finer at DS-TC (D60 = 63 mm),while at Pfaffensprung the GSD upstream and
DS-TC are similar (US: D60 = 132 mm, DS-TC:
D60 = 146 mm), but coarser directly below the SBT
outlet (DS-SBT D60 = 178 mm). In case of Asahi, thetributary 1.2 km below the dam may provide finermaterial, in case of Pfaffensprung the coarse material
may be due to the locally steep bed slope.
Comparison of data in Table 2 reveal that Dm lays in
the range of D60 rather than D50 with a deviation of only
9±8% compared to 35±20% for the latter, supporting
the idea that D60 ≈ Dm and that Dm is equally valid torepresent a gravel bed GSD. Furthermore, it is foundthat D99 is very large being 428±315% higher thanD60 and still 186±132% than D84 due to their wide
distribution with σu = 3.9 ± 1.5 typical for GBR.
Figure 5 shows D60 as a function of Sb for thefour SBTs together with data from Japanese GBR(Mikuniya & Chibana 2011). In general, Sb decreases
along a stream, and accordingly the material getsfiner due to sorting and abrasion (e.g. Gomez et al.
2001), which is supported by our data and Mikuniya &Figure 5. D60 as a function of Sb with data from both 43
Japanese GBR (Mikuniya & Chibana 2011) and the SBT
river sections studied herein.

Chibana (2011). However, as we analyzed only local
values, some downstream Sb are steeper and/or coarser
compared to its upstream (e.g. Solis at TC and Pfaffensprung) due the fact that fining and river bed
flattening are large scale processes being relevant in
the catchment scale.
3.2

Habitat features

Figure 6 shows the three habitat parameters σmh , σbed ,and σorg as functions of the stream location. Figure
6a reveals that the microhabitat richness σmh drops
for Asahi and Solis directly downstream of the dam
likely due to the lack of discharge in the dam vicinity
hindering morphological changes, but recovers further downstream. Sediment supply through the tunnel
might have supported increasing richness downstream
for these two dams.
Data of Koshibu reveals that σmh at DS-SBT ishigher than at US, disclosing that it is not an adequate
dam impact indicator in that case as the SBT is still
not in operation. Furthermore, at Pfaffensprung, therichness at DS-SBT was slightly lower than upstream
although the bypass is operated often (∼135 days/a)and for a long time (96 years). Therefore, the effect of829

Figure 7. Taxonomic richness as function of stream location
for a) riffles, and b) pools.

Figure 8. Life type proportion at a) riffle, and b) pool,and habitat specialty proportion at c) riffle, and d) pool asfunctions of stream location.
Figure 6. a) Microhabitat richness σmh , b) bed coarseness index σbed and c) relative abundance of organic versus
inorganic habitats σorg as functions of stream location.

bypass operation was apparent in some but not all of
the four cases in terms of microhabitat richness.
Figure 6b shows that the bed coarseness index σbed
is in general higher down- than upstream, with the
exception of DS-D at Solis. Also at Koshibu, σbed washigher at DS-SBT than US, whereas it is almost similar
between US and DS for Pfaffensprung. These results
may be explained by the fining process after bypass
operation. However, at Solis, σbed is low at DS-D presumably due to flushing of fine sediments from thereservoir trough the bottom outlets in 2007 and 2008.
Figure 6c shows that the organic index σorg tends
to be higher at DS than US except for Pfaffensprungand DS-D of Solis. At Koshibu, σorg is substantiallyhigher down- than upstream. This is likely related to
an increased abundance of mud, moss, and filamentous
algae at DS-SBT probably due to reduced flow velocity
and increased bed material stability.
Figure 7 gives an overview of the taxonomic richness as a function of the stream location for riffles(a) and pools (b). Data reveal that invertebrate species
richness, especially mayfly and caddisfly species, wasgenerally lower in Swiss than in Japanese sites. However, our study was conducted in a period when
the lowest richness out of five surveys in 2014 wasrecorded (Martin et al. 2015). At Asahi, the richnessof riffle communities was substantially low at DS-D
but as high as upstream at DS-SBT. This indicates the
positive effect of the SBT on species richness supporting results from Mitsuzumi et al. (2009) reporting
increase of species richness since 1998. However, ingeneral no clear difference between US and DS locations is observed. For example Koshibu dam does notreveal lower richness (but instead a slight increase) as
one may expect as a consequence of permanent lackof sediments.
Figure 8 shows the invertebrate life type and habitat specialty as functions of the stream location. Figure
8a and b reveal that no net-spinners were found at the
Swiss sites and clinger species are abundant throughout all locations. At Koshibu at DS-SBT, the proportion of net-spinners was high and the ones of gliders
low, compared to its upstream location.AtAsahi at DSD, the proportion of net-spinners was higher compared
to its up- as well as further downstream. Increase ofnet-spinners and decrease of gliders below dams without SBT have been revealed at several dams in Japan(Hatano et al. 2005).
Figure 8c and d reveal the evident finding that lotic
species are abundant in riffle areas, whereas vice versa
lentic ones are more present in pools. Data ofAsahi andSolis reveal that in dam vicinity (DS-D) the abundanceof lotic species is minimal regardless of specialty but

830

Figure 10. Bray-Curtis similarity index IBC of a) microhabitat, and b) invertebrate community as functions of years of
SBT operation.

Figure 9. a) Relative abundance of net-spinners versus gliders, b) relative abundance of pool versus riffle specialists asfunctions of stream location.

increase again further downstream showing the positive effects of SBT operation. Data of Pfaffensprung
and Solis reveal that up- and downstream proportions are similar, indicating the positive SBT operation
effect. As the latter SBT only operated for two years,
the effect seems to be immediate. Koshibu data revealthat lentic species are of higher proportion downstream
compared to US in pools as well as riffles indicating lack of areas where lotic species may prosper.
Increase of pool specialists and decrease of riffle specialists below dams without SBT have been shown fora large number of dams in Japan (Kobayashi et al.
2016) supporting the findings herein.
The difference in life type proportion (Figs.8a, b) isalso reflected in the net spinner index σnet (Fig. 9a, calculated only for riffle community). Net spinners preferstable beds for constructing their nets while gliders
prefer unstable stones. σnet was higher for sites, which
are considered to have coarse and stable bed materials,
such as DS-D of Asahi and DS-SBT of Koshibu.
The difference in proportion of habitat specialty(Figs. 8c, d) is also reflected in the pool specialist index
σpo (Fig. 9b, calculated only for riffle community). σpo
was higher for sites, which are considered to have slow
flow conditions due to a coarse bed creating patches ofstagnant areas between the roughness elements, suchas DS-D of Asahi and Solis, and DS-SBT of Koshibu.
The similarity index IBC in microhabitat composition σmh (Fig. 10a) between up- and downstream(DS-SBT) shows a clear increasing trend with increasing bypass operation. This is partly due to similar
GSD and less organic microhabitats in the downstream
for dams with longer bypass operation. It appears
that habitat composition recovers toward the upstream
condition with increasing years of bypass operation.

Also IBC for the invertebrate composition σinv (Fig.
10b) between up and downstream (DS-SBT) increased
for dams with longer bypass operation. This is partly
related to the increase of gliders over net spinners for
the Japanese sites and the increase of riffle over pool
specialists for both Japanese and Swiss sites. A large
difference in similarity between Koshibu and Solis andsmall one among Solis, Asahi, and Pfaffensprung may
suggest that the recovery of invertebrate community
towards the upstream condition may occur within a
few years of bypass operation.
Both, microhabitat and invertebrate community
structure tend to be similar between US and DS-SBT
for dams with older SBTs. The environment has recovered substantially to a previous state for Asahi and
Pfaffensprung and is apparently improving at Solis.
4

CONCLUSION

Four reservoirs in Japan and Switzerland with adjacent
SBTs as a strategy against sedimentation are analyzed
in terms of up- to downstream morphological andbiotic changes due to tunnel operation. It was foundthat GSD at new SBTs (recently inaugurated) are finein the up- and coarse in their downstream due to lackof conveyed sediments. However, the old ones do notreveal similar GSDs between up- to downstream locations, suggesting that GSD changes are effected by further parameters such as downstream tributaries, slope
instabilities and dam operation. Analysis of biotic data
reveal that directly below dams microhabitat richness
is low and lentic species (e.g. net-spinning caddisfly)
abundance is high, while these differences decrease
further downstream, and riffle specialists dominate
in case of old SBTs. It is shown that microhabitatand invertebrate richness downstream of reservoirs
adjust to their upstream values with increasing SBT
operation. This finding clearly reveals the positive
ecological effects of SBT operation.
ACKNOWLEDGMENTS
The authors would like to thank Yosuke Awazu for collecting and analysing the field data during his Master831

thesis. The first author acknowledges the financial
support of the Japanese Society for the Promotion of
Science (JSPS).

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