Advances in River Sediment Research – Fukuoka et al. (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00062-9

Missing link of coarse sediment augmentation to ecological functions
in regulated rivers below dams: Comparative approach in Nunome
River, Japan and Trinity River, California, US
G. OckInstitute of Urban and Regional Development, University of California, Berkeley, USA

G.M. Kondolf
Landscape Architecture and Environmental Planning, University of California, Berkeley, USA

Y. Takemon & T. Sumi
Disaster Prevention Research Institute, Kyoto University, Japan

ABSTRACT: This study investigated gravel augmentation activities undertaken in Nunome River andTrinity River in relation to ecological functions of instream geomorphological features from the empirical perspectives of habitat diversity and organic matter interactions. It was found that riffles supplemented by high-flow stockpile in Nunome River functioned as an important natural filter for removing
reservoir-derived plankton, and then subsequently contributed to macroinvertebrates species richness
and functional feeding group. In the Trinity River, we found that an island and a point bar, which recreated by high-flow injection and direct placement respectively, had a distinct retention capacity for suspended particulate organic matter, and played an important role in inducing hyporheic flows providing
simplified habitat with hydrological and thermal heterogeneity. These findings of the ecological rolesof inchannel riffles and gravel bars could yield insights that will inform environmental management ofdesign, implementation of rehabilitation activity and their maintenance.

1

INTRODUCTION

1.1 Sediment starvation in regulated channels
below dams
Discontinuity of sediment transport by reservoir
sedimentation and subsequent sediment deficit
downstream have triggered a chain of riverine
geomorphic changes such as riverbed degradation,
channel incision and bank erosion. Such sedimentstarvation downstream is reported to cause loss of
in-channel geomorphological features (Kondolf
1997, Graf 2006), accompanied by a subsequent
simplification of habitat structure to aquatic communities and degradation of ecological functions inriverine ecosystem (Ligon et al. 1995, Power et al.
1996). In particular, loss of shallow geomorphic
features such as riffle-pool sequence and several
types of riverine bars is known to lead to degrade
fish spawning habitat (Kondolf 2000) and toelongate a transport distance of reservoir derived
plankton leading to disturb trophic balance in riverine foodweb system (Ock & Takemon 2010).
In response to the sediment deficit downstream, sediment augmentation, which artificiallyintroduce coarse sediment into regulated channels

in a variety ways, has been increasingly applied
worldwide. Nunome River in Japan has performed
the sediment augmentation activities, and a total of
2,585 m3 of sediment were replenished downstream
for 6 years since 2004, and resulted in lengthening
riffle zones in the regulated channel (Kantoush
et al. 2010, Ock et al. 2010). In the Trinity River innorthern California, a total of about 122,000 m3 ofcoarse sediment has been added to stream channels since 1972 (Gaeuman 2012). In recent years,
the Lowden Rehabilitation project in the Trinity
River recreated gravel bar features through coarse
sediment augmentation (TRRP 2012).
There has been a missing link in much riverrestoration projects between geomorphologicalchanges caused by gravel augmentation and subsequent contribution to ecological functioning.
The study provides an opportunity to interpretthe link by evaluating the effects of the long riffles
in Nunome River and new gravel bars in Trinity
River on ecological benefit for the downstream
ecosystems. Better understanding of the ecologicalrole of inchannel riffles and gravel bars in relation
to river sedimentation could yield insights thatwill inform environmental management of design,1531

implementation of rehabilitation activity, and theirmaintenance.
1.2

Ecological importance of POM retention

The trophic origin and composition in riverine
Particulate Organic Matter (POM) is important
indicator as an integrated record of naturaland anthropogenic activities in river basins(Kendall et al. 2001, Hamilton et al. 2004). POM
retention functioning, an ability of the channel
to reduce the POM concentration, plays a rolein supporting river and stream ecosystems as a
primary energy resource (Tockner et al. 1999).
However, in regulated rivers below dams, inputof large amount of plankton from dam outflows
influences POM quantity and quality, and subsequently could disturb downstream ecosystem
foodweb (Richardson & Mackay 1991, Akopian
et al. 1999). Thus, the retention efficiency of reservoir derived plankton can be important component for recovering a normal state of trophic
structure. In this sense, this ‘natural filtering’
function, that is, to reduce POM concentrations
and reservoir plankton from running water, issignificant for enhancing self-purification alongchannels and restoring downstream ecosystem
foodweb.
1.3

Objectives

This study focuses on evaluation for gravelaugmentation activities undertaken in Nunome
River and Trinity River in relation to ecological
functions of instream geomorphological features
such as riffles-pools and gravel bars. The objectives of this study are to make a general review
of implementation process of gravel augmentation applied in the Nunome River by comparative
approaches with Trinity River, and to investigate
the instream features’ ecological functions in relation to particulate organic matter retention andhyporheic exchange in the regulated channel.
2

merits to be relatively cost efficient and to provide
immediate benefit, for instance, improvement of
spawning habitat.
Despite conceptual strategies for satisfying
both objectives to prevent reservoir sedimentation and to mitigate downstream impacts ofdams and reservoirs, there remains still something
to be systematically developed in stages of planning, implementation techniques, monitoring andevaluation, and adaptive management (Harvey
et al. 2005); i.e., magnitude and frequency of
flushing flow, quality and quantity of coarse sediment, source of evacuated sediment, sediment
treatment; selecting appropriate methods; monitoring and evaluation for physical and ecologicalchanges; adaptive management for feedback via
science-based progress in the face of uncertainty.
Coarse sediment can be replenished mainly
through some different injection types, each withspecific advantages (Kondolf & Minear 2004,
Harvey et al. 2005). First, ‘direct placement’ isrelatively past implementation method where
over-size sediment larger than 15 cm is directly
placed within low-flow channel normally to build
permanent features, e.g. constructed riffles, grade
control structures or constructed point bars,
which are largely immobile under the current flow
regime. Second, ‘high-flow stockpile’ placed gravel
along the channel bank margin to be distributed
downstream by high flows. It is common type currently applied in Japan. This approach can add
relatively large amounts of gravel at relatively low
cost, but limited to volume piled and the numberof suitable sites. Third, ‘inchannel bed Stockpile’,as a variant on the high-flow stockpile, spread
gravel on the river bed to facilitate transport even

CLASSIFICATION OF COARSESEDIMENT AGUMENTATION

Comprehensive sediment management in dams
and rivers has been developed worldwide including ‘sediment replenishment’, ‘sediment bypassing tunnel’, ‘flood mitigation dam’ without
impoundment and ‘dam removal’ (Sakamoto
et al. 2005, Sumi & Kantoush 2010). Comparedwith sediment bypassing tunnel and flood mitigation dam, the gravel augmentation is likely
unsustainable for maintenance because of repetitive needs periodically. However this approach has

Figure 1. Sediment augmentation methods dependingon implementation techniques.

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by lower flows than the flow volume needed forhigh-flow stockpile. But it involves inchannel
work at low flows, which may increase turbidity
downstream. Fourth, ‘high-flow injection’ introduces directly gravel to river channel during ahigh flow event. Comparing high-flow stockpilethat is quickly exhausted in a large flow, high-flow
injection can allow new sediment to replace thematerial already transported, and larger volume
can be introduced during the course of a flood.
Finally, the gravel augmentation can be combinedwith mechanical floodplain reshaping work toTable 1.

increase bed mobility, so-called ‘bank rehabilitation’ (Fig. 1).
3

COMPARATIVE ANALYSES OF GRAVELAUGMENTATION IN THE NUNOME
RIVER AND TRINITY RIVER

Table 1 summarized the sediment augmentation
applied in Nunome River and Trinity River withcomparison of the purposes, implementation,
flooding flow and added sediment characteristic.

Comparisons of coarse sediment augmentation activities between Nunome River and Trinity River.

Factors of sediment augmentation
River and Dam
River
Basin and location
Length/basin area
Upstream dam
Name/year built/height/
storage capacity
Type/purpose
Reservoir
sedimentation risk
Sediment augmentation
Objectives
Physical objectives
Ecological objectives
Implementation characteristics
Year started
Frequency
Main methods applied
Number of sites
High-flow discharge characteristics
Discharge released
Peak magnitude
Peak duration
Sediment added characteristics
Source excavated
Amount added per a year
Grain size

Nunome River

Trinity River

Kizu River basin in Nara, Japan
30 km long/86.2 km2

Klamath River basin in California, US
266 km long/7,389 km2

Nunome Dam/1994/
72 m high/17.3 MCM*

Trinity Dam/1962/164 mhigh/3,019 MCM
Lewiston Dam/1963/28 mhigh/18.1 MCM
Earthfill/diversion, hydropower

Concrete gravity/mainly
flood control
High

Low

To reduce reservoir sedimentation rate
To protect riverbed degradation
To improve bed mobility
To detach nuisance attached algae

To recreate gravel bar features in partthrough natural fluvial forming
process
To make complex habitat
structure for salmonids fish

2004
Variable (once or twice a year insummer) depending on flooding
eventsHigh-flow stockpile

1972
Once a year in spring (April)

A single site just below dam
Flood-control discharge release bypeak cutting
Approx. 80 m3/s, but varied depending
on inflow volume intensity inmonsoon climate
2–4 hours
From check dam located at endof reservoir
Max 500 m3
Mixed sand and gravel (0.075–19 mm)*MCM means a million cubic meter.

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High-flow injection
Bank rehabilitation
Direct placement
Four sites along channel below dam
Environmental flow release set up by
water year types
Max to 311 m3/s in extremely wet year
Over 5 days
From floodplain rehabilitation work
Max 51,224 m3
Mixed gravel and boulder (15–102 mm)

3.1

Purposes specific to reservoir sedimentationand sediment deficit downstream

Nunome River was a tributary of Kizu River basin
in Nara Prefecture in Japan. The concrete gravity
dam (72 m high and 17.3 mcm storage capacity)
built in 1994 was planned to set sedimentation
capacity to 100 years (1,900,000 m3), but sedimentation in the reservoir had been progressed
faster than that of original plan, and sedimentation rate in 2006 already reached to 13 percent(243,000 m3) of the planned sedimentation(Kantoush et al. 2010). The rate would be accelerated unless appropriate sediment management inreservoir and upstream basin had been employed.
As an objective for reducing reservoir sedimentation rate, inflowing sediment to reservoir has been
trapped at the check dam located at upstream endof reservoir, and then the excavated sediment has
been used for source of downstream augmentation.
For objectives in downstream river channels, mitigation for bed degradation and improvement of
nuisance attached algae were expected.
The Trinity River located in northwest
California is the largest tributary to the Klamath
River. Since Trinity Dam and Lewiston Dam construction in 1962, about more than 50 percent
of water in the Trinity Basin was diverted to the
Sacramento River basin. Subsequent reduction ofboth flow and sediment downstream have resulted
in the loss or degradation of salmon habitats andcaused naturally produced salmon populations to
decline up to 60 to 96 percent (USDOI 2000). Trinity
River Restoration Program (TRRP) have mademuch of efforts to restore salmonids habitats requiring hydro-geomorphological complexity corresponding to diverse stages of spawning, incubation,
rearing and outmigration, and adopted restoration
strategies to induce fluvial process morphologicallyfor gravel bar restoration by coarse sediment augmentation (USFWS & HVT 1999, USDOI 2000).
3.2

Implementation characteristics

3.2.1 High flow stockpile in the Nunome River
The sediment mechanically excavated at upstream
check dam moved to downstream river below damby dump truck, and then placed down on the rightbank far from 300 m below Nunome Dam (Fig. 2).
The placed sediment consist mainly of sand andgravel with 0.38 mm median size (d50) ranging 0.25
to 19 mm. The volume of replenished sediment ina single implementation varied 80 to 500 m3, which
depended on magnitude and duration of flushing flow. Based on the event in October 2009, theamount of 500 m3 of coarse sediment were transported downstream by high flow recorded of 81 m3/sof peak flow for 4 hours (Kantoush et al. 2010).

Figure 2. Nunome River project site; high-flow stockpile was done at the 300 m downstream form dam, andthree riffles were developed along about 1000 m channel.

For the magnitude of flushing flow and amountof coarse sediment added, the largest amount ofcoarse sediment replenished in a single event wasrecorded as 500 m3 on August and October of 2009.
The grain size consisted mainly of mixed sandand gravels less than 10 mm. The frequency andamount of the sediment replenishment has been
dependent of flushing flow released downstream
during flooding season in July to October. The
replenishment activities have been available in the
aspects of riverbed degradation and detachment
of attached algae. Despite deposition of sand bar
at meandering bend, the amount and grain size of
sediment replenished is likely to be insufficient tocreate gravel bar feature in channel. Occasionallysand bed could be problematic in habitat condition to make spawning bed buried and to let rearing pool filled. And thus in order to transport thecoarser sediment much farther downstream, themuch higher magnitude of the flushing flow andlonger duration time of the peak flow should beconsidered.
It has mainly used the high-flow stockpile during the flooding event by intensive rainfall andtyphoon. Although the method is effective forlarge amount of stocked sediment to be replenished downstream in a single event with relatively
short peak duration, the downstream length foradded sediment to be transported was relatively
short comparing to the high-flow injection. Major
deposition was actually found at just 300 m below
the implementation site below Nunome Dam. To
maximize transport distance of replenished sediment, the high-flow injection would be effective,
because the method is able to add sediment directly
to strong thalweg of the channel using a heavyequipment like conveyor machine. It also has merits to control total amount of gravel added depending on peak duration and thus enables much moreamount of gravel supplied downstream than highflow stockpile (Kondolf & Minear 2004).

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3.2.2High-flow injection and direct placement
at the Lowden rehabilitation projectin Trinity River
The Lowden Ranch rehabilitation project reaches,
located 11 km downstream from Lewiston Dam.
A mid-channel bar (island) was deposited in 2011
downstream from a high-flow gravel injection site.
The highest peak flow of 311 m3/s to date wasreleased from Lewiston Dam and approximately
1,567 m3 of gravel was injected from upstream siteand distributed downstream by the high flows.
Another point bar with an alcove at the downstream end was mechanically constructed by‘direct gravel placement’ in 2010 (Fig. 3). About
6,880 m3 of gravel was placed in a straight channel
in order to force a meander bend in the main channel (TRRP 2011, TRRP 2012).
For the flooding flow downstream, TRRP
determined the five water year types based solely
on Trinity Reservoir inflow. The probability ofthese water year types occurring is based on historical data over eight decades past. Total allocation ranges from 1,005 mcm in an ‘extremely wet
year’ to 455 mcm in a ‘critically dry year’. Basedon average inflow to Trinity Reservoir, allocation
to Trinity River represents 43 percent of total
inflow into Trinity Reservoir. Once the water year
type was designated and the allocation to Trinity
River has been established, the discharge patternproposed (USFWS & HVT 1999). Peak dischargevaries depending on water year type. Volume andgrain size of sediment added were consideredof pre dam topography and sediment transportcapacity reduced under the current flow regime(Kondolf & Minear 2004). Gravel added usedover size of 15 mm after removing fine sediments,
ranging approximately 25 to 102 mm of large cobble size, which is able to mobile under anticipatedflow regimes (Gaeuman 2008). In addition, foursites were selected as implementation locations forgravel augmentation in consideration of sedimentdelivery from tributary, sediment transport capacity below injection site.

4 EFFECTS OF GRAVEL AUGMENTATION
ON DOWNSTREAM ECOLOGICALFUNCTIONS
4.1 Field sampling and laboratory analyses
In order to investigate how the riffles elongatedby high-flow stockpile in the Nunome River andthe gravel bars recreated by high-flow injection
and direct placement in the Trinity River influence
downstream ecosystems, i.e., natural filtering functions, quality and quantity of POM were longitudinally analyzed in the both river sites. In addition,
thermal diversity within gravel bar features wasinvestigated in relation to hyporheic flow among
mainstem and side channel, alcove in the Trinity
River.
Suspended POM samples were collected ateither border of riffles or gravel bars using a POM
net sampler with 100 Mm in mesh size, in January to
February 2009 in Nunome River and July 2012 in
Trinity River. Three replicates were collected ateach site. Each suspended fine POM samples was
filtered in situ using the sieve of 1.0 mm meshsize to separate it from suspended coarse POM.
Terrestrial plant leaves, epilithic algae and reservoir phytoplankton were also collected as potential
POM source origins.
We traced the downstream changes in POM
trophic sources (composed of allochthonous terrestrial inputs, autochthonous instream production
and reservoir plankton from dam outflows), andestimated POM concentration (Ash Free Dry Matter (AFDM) of POM in a unit water volume) andqualitatively analyzed stable isotope values (D15N
and D13C) to effectively identify the three sources
and longitudinally estimate their relative contributions. The concentrated-weighted mixing modelusing dual D13C-D15N were applied (Phillips 2001,
Phillips & Koch 2002). In the Trinity River, watertemperatures were monitored using thermometer
installation along the wetted margins of the bothgravel bars. The temperature readings were taken
every 10 minutes continuously for approximately
one week in summer 2012.
4.2 Suspended POM retention capacityof riffle features in the Nunome River

Figure 3. Lowden Ranch rehabilitation site in the
Trinity River; a point bar and an island were created bydirect placement and high-flow injection, respectively.

Geomorphological monitoring research for crosssection bed deposition and grain size distribution
showed that downstream riffle was developed
along channel where riffles percentage accounts for
68% as shown in Figure 2 (Ock et al. 2010), and tiny
sand bar was deposited 300 m downstream fromthe implementation site (Kantoush et al. 2010).
But the research on ecological response to thegeomorphological changes was little investigated.

1535

In three riffles distributed longitudinally, there
was a downstream change in D15N-D13C values ofsuspended POM samples (Fig. 4). D15N decreased
and D13C increased in the riffles, and such downstream characteristics resulted in downstream
contribution changes of three trophic sources ofreservoir plankton derived from reservoir, terrestrial plant leaves and instream algae (Fig. 5).
Interestingly, the contributions of reservoir
plankton were found to be decreased in all riffles.
Almost 60% of reservoir plankton contributed
to suspended POM at the uppermost site near
dam outlet point, but the contribution gradually
decreased along three riffles to 52% (1st riffle),
46% (2nd riffle) and 37% (3rd riffle) within just
1 km channel length.
This finding indicates riffles have a retention capacity for removing reservoir-derived
plankton in the downstream regulated channels, and subsequently change gradually trophic
source in downstream foodweb system morebalanced by increasing contributions of terrestrial plant and instream algal sources (Takemon

Figure 4. Downstream changes in stable isotope ratiosof suspended POM surrounded by nonlinear mixing triangle composed of three trophic sources. The values ofratios composed of three sources are in order of terrestrial plant: epilithic algae: reservoir plankton.

Figure 5. Downstream changes in relative contributions of the three sources to suspended POM.

2005). Assuming that aquatic macroinvertebrates
communities could response to the downstream
trophic source changes in the channel, the species richness and functional feeding group wereanalyzed in the three riffles. Number of species
increased downstream almost more than twotimes from 13 species at 1st riffle and 20 at 2nd
riffle to 31 at 3rd riffle. It seems not to be immediate response of functional feeding groups. Species of filterers increased downstream in spiteof plankton contribution reduction, whereas
the rate and number of collectors and predators increased, showing gradually downstream
shifting to more balanced foodweb condition(Fig. 6).
4.3

Suspended POM retention capacityof gravel bar features in the Trinity River

It was found of reduction pattern in SPOM concentration estimated from the upstream end todownstream end in both gravel bars. In both thefirst and second samplings, the concentration
was reduced 4 to 10% in the constructed pointbar, whereas 17 to 14% in the deposited island(Fig. 7a). The difference could be differentiatedin a length-normalized (100 m) retention rate, theFigure 6. Downstream distribution of species richness
and functional feeding group for macroinvertebrates
communities.

Figure 7. Suspended POM concentration along gravelbar reaches (a) and retention rate normalized to a lengthof 100 m (b) in the first and second samplings.

1536

island formed by natural fluvial processes showedabout 3 times higher retention rate than the constructed point bar (Fig. 7b).
The results are a clear evidence that the gravelbar features in the reaches have a retention capacity and can act as a sink for suspended POM.
The reduction of POM concentration, which
is an indicator for enhancing material exchange
in river ecosystems, was found in both bars forthe two sampling times. Also the finding that thehigher retention efficiency occurred at the deposit
island bar could probably attribute to an active
trapping process in the shallow near-boundary
along the island bar, because of the relatively
long boundary length due to the contributionof side channels and wide shallow area (McNair
2006, Hunken & Mutz 2007). In contrast toisland bars, point bars that occur on the inside
meander bends, and are more likely to have passive deposition of suspended POM in slackwater,
whose retention efficiency is expected to varywith the sinuosity of the inshore zone (Schiemer
et al. 2001). Consequently, our results indicatethat gravel bars created through gravel augmentation activities contribute to enhancing suspended
POM retention, and suggest the importance ofboundary length, and shallow area in gravel bar
management and restoration.
4.4

Hyporheic flow effect between mainstemand subsurface waters beneath gravel bar

Water temperature data acquired showed distinct
thermal regimes inter-intra gravel bar features.
Diel fluctuation patterns interpreted by a sinefunction showed the same 24 hour frequency withamplitudes and lag times specific to each feature(Arrigoni et al. 2008). In the island bar, the temperature in the tail was slightly cooler than that in
the head during the daytime, resulting in the smallest amplitude of diurnal temperature (2.8nC) of
the thermometers placed in the head, side channel(Fig. 8a). On the other hand, water temperaturesin the point bars showed a diverse magnitude of
amplitude and shifting time in the fluctuation
function, mainly due to the influences of alcoves
derived from hyporheic flows The alcove wasabout 3.1nC cooler than the mainstem at the timeof mainstem maximum temperature of 15.1nC,
whereas it was much warmer at the time of mainstem minimum temperature in the constructed
point bar (Fig. 8b).
There was a time lag in temperature signals
between alcove and mainstem in the point bar,and between head and tail in the island. The timelags were shown differently in the two bars; thepoint bar alcove had a longer time lag of about
6.2 hours than the island tail with 0.2 hours.

Figure 8. Spatial and temporal variations of water temperature along a gravel bar shore. Data were recorded at
10-minute intervals.

Also, the temperature signal measured close to
the mouth of the alcoves appeared to converge
with that of the mainstem, showing a different
time lag than that observed between mainstem
and alcoves.
For maximum hyporheic cooling benefit in summer, it is a strategy to increase hyporheic flow rate
which is a function of hydraulic gradient and substrate permeability (Tonina & Buffington 2007).
Because both island and point bar were formed
mainly by coarse sized gravel from coarse sedimentaugmentation indicating allow a high permeability, the extent of hyporheic flow rate among thebars is likely dependent on a hydraulic gradient.
Field observation showed that the island showed
higher hydraulic gradient between head and tail,whereas the constructed point bar showed highhydraulic gradient between mainstem and alcove.
It indicates that naturally deposited islands withlongitudinally steep hydraulic gradients could
induce high hyporheic flow rates, while point bar
with laterally steep hydraulic gradient induce highhyporheic flow rate into alcove. Likewise, from aperspective of gravel bar management, hyporheic
flow contribution to thermal heterogeneity canbe promoted by maintaining high permeability
through regular substrate replacement and byincreasing hydraulic gradients within a gravel bar(Boulton 2007).

1537

5

CONCLUSION

Recent river restoration has shown a growing
concern for the instream features management
and restoration through sediment augmentation,
however recognition of recovering ecological
functions have not been assessed enough from theempirical perspectives of physical habitat diversity and organic matter interactions. This study
summarized augmentation processes by comparison of two different cases in Nunome River andTrinity River. Then, POM trophic composition
results estimated by stable isotope mixing model
showed that riffles in Nunome River functioned
as an important natural filter for removing reservoir plankton in the dam tailwater ecosystem,
subsequently contributed to macroinvertebrates
species richness and functional feeding group.
Also newly created gravel bars in the Trinity Rivernot only functioned as a strong natural filter forsuspended POM, but also play a role in inducing
hyporheic flows, providing simplified habitat withhydrological and thermal heterogeneity.
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