Maintaining and Restoring Sediment Load Downstream of Dams
G Mathias Kondolf1
1

University of California Berkeley, Professor of Environmental Planning
Dept Landscape Architecture & Environmental Planning
E-mail:kondolf@berkeley.edu

Dams interrupt the natural continuity of sediment movement through river systems,
creating problems not only within the reservoir (lost capacity and function due tosedimentation), but also, downstream, where sediment-starved flows can erode the channel
bed and banks, accelerate erosion of coasts and deltas, and result in loss of instream habitats.
Whether the reach below a dam is sediment starved or in sediment surplus depends on thebalance of the sediment supply and post-dam sediment transport capacity. Below large dams
that significantly reduce high flows, the river may not have sufficient energy to movetributary-supplied sediment (or its coarsest fractions), resulting in buildup of tributary deltas
and/or general channel aggradation. However, more commonly, the transport capacity isgreater than the sediment available, and the resulting ‘hungry water’ causes incision and bedcoarsening in the channels downstream (at least until the sediment deficit is compensated bytributary input and/or sediment derived from erosion of bed and banks).
Sediment routing around or through dams (by sediment bypass channels or sluicing
through outlets in the dam) can mitigate this effect by restoring at least part of the naturalsediment load. Where feasible, these approaches are sustainable in that they use the river’s
energy to supply the natural sediment load to the downstream reach, and they contribute toreservoir sustainability by reducing the rate of sediment accumulation. More commonly,
coarse sediments are mechanically placed in the river downstream of dams to partiallymitigate the sediment starvation, requiring ongoing maintenance, and usually not reducing
reservoir sedimentation. The vast majority of these gravel augmentation projects have been
implemented to restore habitat, especially spawning habitat for anadromous salmonids, but
the largest single gravel augmentation project is to prevent bed incision downstream, toprevent undermining of infrastructure: on the Rhine River below the Barrage Iffezheim.
In northern California, gravels have been added below dams on over 20 rivers, totalingover 400,000 m3. These projects have all been undertaken to improve habitat for salmon andtrout in downstream channels, and with one exception, the gravels used have come not fromsediment deltas in the reservoirs themselves, but other sources, including tailings left from
historical gold dredging. The early projects were mostly construction of spawning riffles,
often by placing lines of boulders across the channel and back-filling with gravel suitably
sized for spawning by salmon or trout. As these projects failed or required maintenance overtime, an alternative approach has become more common: to inject gravel for redistribution byflows and ultimately deposition in riffles for spawning. The largest amounts of gravel havebeen added to the Sacramento River mainstem, its tributary Clear Creek, and the Trinity River,most of whose flow is diverted to the Sacramento. Both Clear Creek and the Trinity haveundergone extensive augmentation of gravels downstream of dams, modifications to the flow
regime, and restoration projects to improve bank and floodplain connectivity. The Trinity
River has arguably the most comprehensive program of sediment augmentations, deliberatehigh flows released to restore natural processes of sediment transport and deposition, and to alesser extent, physical manipulations, which have resulted in increased channel complexityand habitat for anadromous salmonids. Detailed study of gravel bars on the Trinity indicatesthat the bars consisting of fresh gravel deposits (derived from gravel injections upstream) had-9-

greater hyporheic flow, and consequently better modulated water temperature and retained
suspended particulate organic matter than mechanically-constructed bars.
The Mekong River basin, Southeast Asia, is undergoing extensive dam construction,with 140 major dams built, under construction, or planned for the mainsteam river and its
tributaries. These dams are virtually all to produce electricity, which will mostly be exported
to neighboring markets in Thailand, Vietnam, and China. If these dams are all built as
currently planned, application of the 3W model indicates that they will trap 96% of the natural
sediment supply to the Mekong Delta. The Delta is a highly vulnerable landscape, alreadyexperiencing subsidence, both natural and accelerated from groundwater pumping, andaccelerated coastal erosion. Sediment supply to the Delta has already been reduced byextensive sand and gravel mining, mostly in Vietnam and Cambodia (some used locally inconstruction and to fill wetlands, some exported to markets such as Singapore). Thus, cutting
off the supply of sediment from upstream can be seen as an existential threat to the Delta.
However, if planned dams can be rethought and redesigned, in many cases it may be possible
to implement sustainable sediment management approaches, such as sediment bypasses orpassing sediment through dams. Thus, it may be possible to reduce the sediment trapping andthereby allow more of the natural sediment load to reach the Delta. A related impact of dams
is blocking the migration of fish to their spawning areas in tributaries upstream. By relocating
and redesigning dams, in some cases barriers to fish migration can be avoided, as currently
being explored by Cambodian and Laotian government staff working with internationalexperts.
Looking at the reservoir itself, accumulation of sediments creates a host of problems,from suspended sediments abrading turbine blades, to interfering with intake structures, tooutright filling of reservoirs with sediment, displacing water storage volume. Sedimentproblems are typically evident long before the reservoir completely fills with sediment, but
completely filled reservoirs are the most obvious manifestation of this problem. Many dams
suffer from structural stability problems to begin with, and accumulating a wedge of sediment
against their upstream side only exacerbates the problems. As illustrated by Barlin Dam on
the Dahan River, Taiwan, such sediment-filled dams can release enormous pulses of waterand sediment in a matter of hours. In California, an analysis of sediment yields bygeomorphic region, coupled with application of the 3W model, predicted where in the state
reservoirs would be likely to fill. Small water supply reservoirs are more vulnerable to fillingwith sediment by virtue of their small initial capacity, and those located in the Coast Ranges
are most vulnerable by virtue of high sediment yields there.
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Ock, G., D. Gaeuman, J. McSloy, and G.M. Kondolf (2015), Ecological functions of restored
gravel bars, the Trinity River, California. Ecological Engineering 83:49-60.
http://dx.doi.org/10.1016/j.ecoleng.2015.06.005-11-