Gravel Augmentation Below Dams: California Experiences

Most dams block all coarse sediment traveling downstream, such that reaches downstream are commonly typically depleted of gravel, causing a variety of effects such as incision, bank erosion, coarsening of the bed material, and reduction of salmonid spawning habitiat. To compensate for this reduction in coarse sediment supply, gravel has been artificially added below dams, using techniques such as high flow stock piling, high flow direct injection, artificial riffle construction, riffle supplementation, and construction of side channel or artificial spawning channels. In the Trinity and Sacramento-San Joaquin River systems of northern California, loss of suitable salmonid spawning gravels below dams has motivated augmentation of over 320,000 m3 of gravel in 73 separate projects on 19 rivers since 1978, mostly since 1990. Of the 67 projects for which adequate data were available, 48 involved adding less than 7,500 m3 each. Costs reported for 57 of the projects totaled nearly $8,753,000, but these figures generally did not include the cost of staff time involved in planning, design, and oversight. Despite the magnitude of this experimental intervention, fewer than half of the projects were monitored, and of those few had monitored sufficient parameters pre- and post- project to evaluate project performance. Performance of these projects to date has been mixed: in many cases the imported gravels have promptly washed out, some channel forms created have been unnatural and not heavily used by salmon. In all cases, the volumes of gravel artificially added have been only a small percentage of the annual coarse sediment deficit.