Feb 21, 2014 (3 years and 3 months ago)



Ellen WOHL and Sara RATHBURN

Many reservoirs currently in operation trap most or all of the sediment entering the reservoir,
creating sediment-depleted conditions downstream. This may cause channel adjustment in the form of
bank erosion, bed erosion, substrate coarsening, and channel planform change. Channel adjustment
may also result from episodic sediment releases during reservoir operation, or from sediment
evacuation following dam removal. Channel adjustment to increased sediment influx depends on the
magnitude, frequency, duration and grain-size distribution of the sediment releases, and on the
downstream channel characteristics. Channel adjustment may occur as a change in substrate size-
distribution, filling of pools, general bed aggradation, lateral instability, change in channel planform,
and/or floodplain aggradation. The increased sediment availability may alter aquatic and riparian
habitat, reduce water quality, distribute adsorbed contaminants along the river corridor, and provide
germination sites for exotic vegetation. Mitigation of these sedimentation hazards requires: (1)
mapping grain-size distribution within the reservoir and estimating the grain-size distributions of
sediment that will be mobilized through time; (2) mapping shear stress and sediment transport
capacity as a function of discharge on the basis of channel units for the length of the river likely to be
affected; (3) mapping potential depositional zones, and aquatic habitat and “acceptable losses,” along
the downstream channel, and comparing these volumes to the total sediment volume stored in the
reservoir as a means of estimating total transport capacity required to mobilize reservoir sediment
delivered to the channel; (4) designing discharge and sediment release regime (magnitude, frequency,
duration) to minimize adverse downstream impacts; and (5) developing plans to remove, treat, contain,
or track contaminants, and to restrict establishment of exotic vegetation. The North Fork Poudre River
in Colorado is used to illustrate this approach to mitigating sediment hazards downstream from

Key Words: Reservoir sediment, Hazards, Downstream channel adjustments

Increasing attention is being given to sedimentation hazards downstream from reservoirs as dams built
during the past century accumulate progressively greater volumes of sediment. The sediment storage both
decreases reservoir capacity and operating efficiency of the dam, and creates a “sediment-shadow”
downstream where sediment-starved flows commonly erode channel boundaries and create long-term
channel instabilities. Numerous studies have documented downstream channel changes resulting from
sediment depletion and altered annual hydrograph associated with a dam. These changes include channel
narrowing, reduction in braiding, and associated loss of habitat complexity (Ligon et al., 1995; Van
Steeter and Pitlick, 1998a,b; Surian, 1999), bed erosion and a reduction in the overbank flooding that is
critical to many riparian species (Baxter, 1977; Brooker, 1981; Lagasse, 1981; Erskine, 1985; Ligon et al.,
1995; Friedman and Auble, 2000), substrate coarsening (Collier et al., 1997), and bank erosion (Petts,
1984; Williams and Wolman, 1985). The specific changes occurring downstream from a dam as a result
of reservoir sediment trapping will depend on: the changes in flow regime and sediment transport
capacity downstream from the dam; the erodibility of the downstream channel boundaries, as governed
by the presence of vegetation and the grain-size distribution of the channel substrate; the presence of
tributaries, hillslope mass movements, or other sources of sediment input to the main channel; and the
amount and size distribution of sediment released from the reservoir.

Department of Earth Resources, Colorado State University, Ft. Collins, CO, USA,
Note: Discussion open until June 2004.
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Sediment may be deliberately released from a reservoir in an attempt to either reduce downstream
channel instability, or to increase reservoir capacity. Large, episodic sediment releases from reservoirs
have received relatively little detailed study (Wohl and Cenderelli, 2000; Rathburn and Wohl, 2001).
However, channel response to such releases may be inferred from published studies of other large,
episodic sediment inputs resulting from dam failure (Jarrett and Costa, 1986; Pitlick, 1993; Cenderelli
and Wohl, 2001), dam removal (Williams, 1977), heavy rainfall and associated flooding (Shroba et al.,
1979; Lisle, 1982; Madej and Ozaki, 1996), mining (Pickup et al., 1983; James, 1991, 1993; Hilmes and
Wohl, 1995), and volcanic eruptions (Montgomery et al., 1999; Simon, 1999).
Channel adjustment to increased sediment influx depends on the magnitude, frequency, duration and
grain-size distribution of the sediment releases, and on the downstream channel characteristics (Fig. 1). If
the sediment introduction exceeds the transport capacity of the downstream channel, selective or general
sediment accumulation may occur. Selective sediment accumulation describes a situation where sediment
is stored at sites of locally reduced transport capacity, such as pools. Preferential pool filling is a common
response to sediment increase along pool-riffle channels, and is commonly used to assess channel
response to various land-use activities (Lisle, 1982; Madej and Ozaki, 1996; Montgomery and Buffington,
1997). More generalized sediment accumulation throughout a channel may result in: a change – usually a
fining – in streambed grain-size distribution (Wilcock et al., 1996); widespread bed aggradation (James,
1993); or a change in channel planform (Hilmes and Wohl, 1995), which commonly occurs as the
initiation of braiding in a formerly single-thread channel. Excess sediment may also be deposited on
adjacent floodplain surfaces, and this floodplain deposition may be substantial enough to reduce channel-
floodplain connectivity (Pickup et al., 1983). In addition to altering channel configuration and reducing
lateral and vertical channel stability, the introduction of excess sediment to a channel may substantially
impact aquatic and riparian ecosystems by altering habitat type and stability, reducing water quality,
distributing adsorbed contaminants such as heavy metals along the river corridor, and providing
germination sites for exotic vegetation (LaPerriere et al., 1985; Wagener and LaPerriere, 1985; Van
Nieuwenhuyse and LaPerriere, 1986; McLeay et al., 1987; Miller et al., 1999; Stoughton and Marcus,

Transport Aggradation Degradation
(during & after release) (during & after release) (after release)
when:when:occurs in
*steep stopes *low gradient 1) thalweg
*narrow gorges occurs in 2) pools
*no substantial areas of 1) pools 3) channel incision
flow separation 2) lateral bars 4) armoring
3) channel bed
4) floodplain (1=first response
5) tributary confluences 3=last response)
(1=low sediment supply,
5=high sediment supply)
Complex response
(Schumm, 1973)
Reach-scale respone to a sediment release

Fig. 1 Conceptual model of channel response (at the scale of a channel reach) to a sediment release
(after Rathburn, 2001; Rathburn and Wohl, in press)

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The United States presently has an estimated 80,000 dams that are together capable of storing a volume
of water almost equaling one year’s mean runoff (Graf, 1999). Most rivers in the Northern Hemisphere
are segmented by dams (Dynesius and Nilsson, 1994), and dams are increasingly being built in the
Southern Hemisphere (Goldsmith and Hildyard, 1984). The widespread presence of dams and reservoirs
suggests that it is imperative that a systematic, rather than haphazard, approach be taken to addressing
downstream sedimentation hazards associated with these structures. This in turn requires careful
consideration of both general patterns, and site-specific characteristics.

Sedimentation hazards downstream from dams and reservoirs can be mitigated using a five-step
procedure (Fig. 2).

Map grain-size distribution within reservoir &
estimate grain-size distributions of sediment that will be mobillzed through time
three-dimensional map of grain size within a reservoi
Map shear stress and sediment transport capacity as a function of discharge
on the basis of channel units for length of river likely to be affected
downstream distribution of channel units
map of channel units and shear stress for differing discharges
Map potential depositional zones, aquatic habitat & acceptable losses
Compare these volumes to the total sediment volumes stored in the reservoir to
stimate the total transport capacity to mobilize sediment delivered to the channe
downstream distribution of depositional
volume/acceptable loss per channel unit:
map of depositional volumes & acceptable losses: total vs. sediment stored
sum of acceptable losses =30;
volume in reservoir =20
Design discharge & sediment release regime (magnitude, frequency, duration )
to minimize adverse downstream impacts
list of discharge and sediment discharge characteristics
Develop plans to remove, treat, contain, or track contaminants,
and to restrict establishment of exotic vegetation
list of choices for treating contaminants & exotics

Fig. 2 Suggested procedure to use for mitigating sedimentation hazards downstream from dams and reservoirs. Text
box lists steps in procedure; italic text explains product; diagram at right gives hypothetical example of product

(1) Mapping grain-size distribution within the reservoir and estimating the grain-size distributions of
sediment that will be mobilized through time. Downstream sediment transport and storage will be
governed by the balance between the transport capacity of the flow and the sediment volume and grain-
size distribution. Therefore, it is necessary to map grain-size distribution within the reservoir in three
dimensions, and to estimate how this sediment will be released through time. For example, a sediment
release drawing only on the downstream end of the reservoir may be mobilizing only the finest sediments,
which are readily transported in suspension. In contrast, a sediment release drawing on the entire
reservoir may be mobilizing progressively coarser sediments with time, so that downstream transport will
shift from suspended to bed load sediment. The grain-size distribution of sediment released from the
reservoir will partly determine the mode (wash, suspended, bed load) of downstream sediment transport,
and, thus, determine transport distance and type of sediment deposition and storage.
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(2) Mapping shear stress and sediment transport capacity as a function of discharge on the basis of
channel units for the length of the river likely to be affected. As previously noted, sediment deposition
and storage commonly occur on a site-specific basis, as in preferential filling of pools. Estimates of
downstream sediment dynamics following a reservoir sediment release are, thus, most effective if they
account for differences in transport capacity among channel units such as pools and riffles, rather than
using a cross-sectional or reach-scale average estimate for transport capacity. These estimates of channel-
unit transport capacity will be very dependent on discharge. Laterally-constricted pools, for example,
have uniformly low velocity flow during lower stages, and sediment in transport is likely to form an even
veneer across the pool (Wohl and Cenderelli, 2000). A central jet of high velocity and transport capacity,
and marginal eddies with low transport capacity, become increasingly pronounced within laterally-
constricted pools as stage increases (Thompson et al., 1998, 1999). These conditions produce substantial
sediment storage along the pool margins, but this sediment may be remobilized during the falling stage as
the central jet declines in strength and marginal sediment slumps into the pool thalweg (Wohl and
Cenderelli, 2000).
(3) Mapping potential depositional zones, and aquatic habitat and “acceptable losses,” along the
downstream channel, and comparing these volumes to the total sediment volume stored in the reservoir as
a means of estimating total transport capacity required to mobilize reservoir sediment delivered to the
channel. This step can be approached in terms of a balance between potential sediment storage capacity
and potential sediment supplied by reservoir release. If sediment supply is likely to exceed storage
capacity, then the discharge accompanying the sediment release must be sufficient to transport excess
sediment out of the river reach of concern. In many laterally confined channels, for example, pools
represent the primary sediment storage sites. Pools also contain critical aquatic habitat, in that some
minimum volume or depth of water during low flow is necessary to ensure fish survival. If this minimum
can be specified for a given river and fish population, available pool volume in excess of the minimum
may be regarded as temporary sediment storage, and, thus, an acceptable loss following reservoir
sediment release.
(4) Designing discharge and sediment release regime (magnitude, frequency, duration) to minimize
adverse downstream impacts. Designing the magnitude of a sediment release so as to minimize
downstream impacts primarily refers to minimizing downstream aggradation or channel change by
comparing available sediment storage volume to sediment supplied. The timing of the sediment release
must also take into account the flow regime in the downstream channel, and the lifecycles and resiliency
of downstream organisms. Flow regime is important in that it controls sediment transport following the
sediment release. The worst-case scenario would be a sediment release during declining flows, followed
by a prolonged period of very low flow. A much better scenario for enhancing downstream sediment
mobility would be to release sediment during the rising stage of flow, thus maximizing downstream
transport and re-distribution of the released sediment. The lifecycles of downstream aquatic and riparian
organisms may influence the timing of sediment releases in that some fish species spawn during autumn,
whereas others spawn during the spring. A sediment release during declining autumn flows would not
only maximize the duration of sediment storage along the river, but would also interfere with the flow of
oxygenated water past the fish eggs for a much longer period of embryo development. The resiliency of
organisms to a pulse of sediment transport or storage varies between types of organisms and between
species. A diverse community of macroinvertebrates is likely to lose both density and taxa richness
following a sediment release, but some species can recover within days, whereas other species require
more than a year to recover (Zuellig et al., 2002).
(5) Developing plans to remove, treat, contain, or track contaminants, and to restrict establishment of
exotic vegetation. Several case studies of mining sediments contaminated with heavy metals indicate that
downstream dispersal of these sediments creates long-term hazards for aquatic and riparian organisms
and human communities (Prokopovich, 1984; LaPerriere et al., 1985; Graf et al., 1991; Miller et al., 1999;
Stoughton and Marcus, 2000). Previous work also indicates that reservoir sediments may be contaminated
by adsorbed heavy metals, organochlorine compounds such as pesticides and PCBs, or excess nutrients
from agricultural runoff (Graf, 1990). The hazards posed by these contaminants make it critical to contain
or at least monitor the downstream dispersal of the contaminated sediments. Newly-created depositional
surfaces may also serve as germination sites for exotic riparian vegetation such as tamarisk (Tamarix
chinensis) or Russian-olive (Elaeagnus angustifolia). These species may out-compete native riparian
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vegetation (Olson and Knopf, 1986), reduce riparian habitat for native birds and other species (Ohmart et
al., 1977), and alter water and sediment movement along rivers and, thus, channel planform (Graf, 1978).
If the release of sediment from a reservoir is likely to provide new germination sites for such exotic
species, measures to minimize germination potential (such as timing of sediment release relative to plant
growth cycles, or actively seeding newly deposited surfaces with native species) may be necessary.

Approximately 7,000 m
of clay-to-gravel-sized sediment were released from Halligan Reservoir into
the North Fork Cache la Poudre River in late September 1996. The sediment was released at the end of
the annual snowmelt hydrograph peak, as the reservoir was being drawn down for the winter. Discharge
was approximately 4 m
/s during the sediment release, but this was immediately decreased to
approximately 0.06 m
/s following the release. As a result, reservoir sediment accumulated along the
channel for more than 8 km downstream, and approximately 4,000 fish were killed. The descriptions that
follow come largely from Wohl and Cenderelli (2000), Rathburn (2001), Rathburn and Wohl (2001), and
Rathburn and Wohl (in press).
The North Fork is a bedrock-controlled pool-riffle channel flowing through a deep canyon. Channel
substrate is bedrock, or cobble-boulder sized sediment. The coarser bed material is not mobilized during
normal snowmelt years. Pools occur where bedrock outcrops laterally constrict the channel. Sediment
released from the reservoir accumulated preferentially in the pools as a function of distance downstream
from the dam. At 0.5 km downstream, pools up to 3.5 m deep were completely filled; at 3.2 km
downstream, pools were half-filled. Infilling sediment became progressively finer-grained downstream.
Sediment also formed a thin but continuous veneer over the riffle and run sections of the streambed, and
infiltrated the coarse sediment to a depth of 6 cm. The net effect of the sediment deposition was to reduce
the undulations in bed topography associated with the pools and riffles, and create a more uniform, plane-
bed channel that maximized sediment transport.
The onset of the snowmelt hydrograph in February 1997 initiated re-mobilization of the reservoir
sediment. By September 1997, eighty to ninety percent of the sediment stored in pools had been re-
mobilized and transported downstream. The remaining sediment has not subsequently been removed from
the pool margins, where it is effectively stabilized by riparian vegetation, or shielded from erosion by the
presence of flow separation. Re-mobilization of reservoir sediment began in February 1997 with a flush
of suspended sediment transport that lasted only a few days. The timing and duration of bed load
transport varied with distance downstream. At 0.5 km downstream from the dam, bed load peaked during
the first 10 days of the 40-day snowmelt peak. At 3.2 km downstream, bed load transport had a later and
more sustained peak. These differences occurred because of the storage created by pools, which acted as a
series of sediment sources and sinks. Bed load sediment was temporarily stored in and re-mobilized from
each pool, so that upstream portions of the channel became depleted of reservoir sediment earlier in the
snowmelt hydrograph, while downstream portions were still receiving sediment re-mobilized from
upstream pools. The magnitude of discharge, as this influenced the strength of marginal circulation and
eddy storage in the pools, and the duration of discharge, as this influenced the progressive downstream
movement of bed load from pool to pool, were both critical controls on sediment re-mobilization and
transport from the portion of the North Fork affected by the reservoir sediment release.
The sediment transport models HEC-6 and GSTARS 2.0, and the two-dimensional flow model RMA2,
were used to model pool sediment dynamics along the North Fork. HEC-6 is a one-dimensional model
that predicts scour and deposition within rivers and reservoirs (U.S. Army Corps of Engineers, 1998a). In
river applications, HEC-6 simulates uniform changes in river bed elevation, over the entire width of the
channel, caused by erosion and deposition over time under subcritical flow. The model has no provisions
for simulating lateral channel changes, such as meander migration, or lateral changes in bed slope. The
governing equations in HEC-6 include the energy equation, and conservation of mass for water and
sediment. The momentum equation is not included in HEC-6, so environments with rapid fluctuations
between subcritical and supercritical flow are inappropriate for modeling. In addition, HEC-6 assumes
that sediment supply and demand are satisfied within each reach at each time step, and the model takes
into account the effects of sediment gradation. HEC-6 is one of the most widely used and economical,
commercially available sediment transport models.
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Three model components comprise HEC-6, and require specification by the user. These include a
geometric component consisting of surveyed channel cross sections; a hydrologic component of discharge
at the upstream boundary, represented as a series of steady, uniform flows; and a sediment component,
including inflowing sediment load, sediment rating curve, and the gradation of bed material. Several
default options allow the user to select recommended input settings, should some of the input data be
unavailable, or should the user be unsure of which option to choose. In other cases, input settings offer
several choices, such as selecting one of fourteen sediment transport equations. The output of interest in
this application of HEC-6 is the average or uniform bed change at a given channel cross section.
GSTARS 2.0 is a quasi-two-dimensional model that utilizes a stream tube approach to accommodate
differential scour and deposition over the width of a cross section (Yang et al., 1998). Stream tubes are
conceptual tube-like surfaces whose walls are defined by streamlines, imaginary lines which show the
direction and magnitude of velocity as the tangent at every point along the line, at each instant in time. In
GSTARS, hydraulic parameters and sediment routing computations are made for each stream tube,
allowing the position and width of each stream tube to change. In this way, vertical and lateral variations
in cross sectional elevation can be simulated. The governing equations are largely similar between HEC-6
and GSTARS, except that GSTARS incorporates the momentum equation in backwater computations
when the flow regime changes from subcritical to supercritical or vice versa. Input for GSTARS is similar
to HEC-6, but offers a broader range of options with very few default choices built into the model.
RMA2 is a two-dimensional finite element model that computes water-surface elevations and depth-
averaged horizontal velocity components for free-surface turbulent flow (Donnell et al., 1997). RMA2 is
a numerical model that divides the flow domain into discrete but not necessarily uniform elements. The
model uses iterative numerical approximation techniques to approach a convergent solution to the
nonlinear mathematical expressions that describe two-dimensional flow. Numerical models of this type
are based on a vertically-integrated form of the full three-dimensional Reynolds-averaged Navier-Stokes
equations for turbulent flow. Input includes channel geometry used to build the finite element mesh;
material properties and boundary conditions (eddy viscosity and roughness coefficient) for each element;
and water-surface elevations calculated using HEC-RAS (U.S. Army Corps of Engineers, 1998b).
Comparison of model results to field data collected during the 1997 snowmelt hydrograph indicate that
HEC-6 yielded the closest agreement between predicted and measured changes in pool elevation as a
function of discharge magnitude and duration (Table 1). Greater than 50 percent of the actual scour and
deposition within the three pools investigated was modeled using HEC-6. RMA2 improved delineation of
flow hydraulics in areas of flow separation and recirculation within the pools, but failed to represent the
simultaneous aggradation and degradation measured in the pools.

Table 1 Comparison of numerical models based on certain criteria by which potential users might evaluate
the models for applicability to a given sediment release situation. Accuracy based on experience for
simulation of the 1997 snowmelt hydrograph sediment release event of the North Fork Poudre River

Data requirements
very high
>50% accuracy, pool-
wide trend
13-90% accuracy, no
pool-wide trends
replicated low-velocity areas
well, general flow field for
low and high discharge
cross section based,
default options, sediment
transport model
cross section based, semi
sediment transport model
nodal hydraulic parameters,
visual display of output
purely one-dimensional,
limited transport formulae
few default options, not
suited for stratified beds
hydraulic model only,
outstrips calibration data,
calibration data difficult to
Predictive ability for
North Fork Poudre
Predictive utility for



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The application of these models indicates the limitations on producing accurate, quantitative
descriptions of sediment dynamics within a reach of river affected by reservoir sedimentation. Referring
back to the five-step procedure outlined earlier, the accuracy and effectiveness of steps such as mapping
shear stress and sediment transport capacity (step 2), mapping potential depositional volume (step 3), or
designing a discharge regime (step 4) would be based largely on the nature of the simplifying
assumptions used in the procedure. For example, laterally-constricted pools were the key channel unit
determining long-term sediment storage and re-mobilization along the North Fork. These pools have
strong zones of flow separation and associated strong cross-pool gradients in shear stress, sediment
transport capacity, and storage volume. Use of a cross-sectional average value for these variables, as in
HEC-6, might be expected to produce results that are too coarse to be useful (Fig. 3). However, a program
such as GSTARS 2.0, which uses a stream-tube approach that allows for differential erosion and
deposition across a cross section, does not accommodate large differences in grain sizes of bed sediment
over short distances, such as between riffles comprised of boulders, and adjacent pools of fine sand.
These limitations compromised the model’s effectiveness when applied to a river such as the North Fork.


uniform scour/deposition
Quasi two-dimensional
differential scour/deposition


Fig. 3 Schematic cross sections illustrating (A) differences in model characteristics between HEC-6 and GSTARS
2.0. The solid black pattern indicates scour along the bed of the channel, and the stippled pattern indicates
deposition (after Rathburn and Wohl, 2001, Figure 4) and (B) measured changes in pool geometry following
reservoir sediment release into the North Fork Poudre River, Colorado. The solid line indicates the pre-release,
bedrock channel boundaries; the dashed line indicates the extent of reservoir sediment deposited in the pool
initially; the light stipple indicates the sediment remaining in the pool six months later, after moderately high
discharge; the dark stipple indicates sediment in the pool one year later, after high discharges (after Wohl and
Cenderelli, 2000, Fig. 5)

Different limitations compromised each of the three models applied to the North Fork. Such limitations
are likely to compromise attempts to model sediment dynamics in many channels downstream from dams,
which commonly have sediment transport characteristics that cannot be simulated using the sediment
models applied along the North Fork. Such characteristics include large spatial differences in bed grain
size; strongly three-dimensional flow and associated differential scour and deposition across a cross
section; temporal changes in sediment supply and bed-material grain-size distribution; and the presence
of spatially-discontinuous portions of immobile bed material (e.g., boulder riffles) (Fig. 4).

The five-step procedure outlined earlier is an ideal. The ability to mitigate sediment hazards
downstream from dams using this procedure will depend on (1) the spatial and temporal resolution at
which field measurements and modeling are undertaken for a given reach of river, and (2) the accuracy
with which individual processes of hydraulics and sediment transport can be described. The first
limitation is one of time and cost; the second limitation depends on quantitative understanding of
processes. Significant progress in mitigating downstream sediment hazards will probably depend on
advances in understanding and simulating processes in rivers subjected to reservoir sediment releases.
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Compound pool

Fig. 4 Schematic illustration of characteristics which limit the ability of most models to simulate movement of fine
sediment along channels downstream from reservoirs. Such characteristics include large spatial differences in
bed grain size; strongly three-dimensional flow and associated differential scour and deposition across a cross
section; temporal changes in sediment supply and bed-material grain-size distribution; and the presence of
spatially discontinuous portions of immobile bed material

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