GRAVEL RESTORATION ON THE AMERICAN RIVER, CALIFORNIA- SUCCESS AND LONGEVITY OF ARTIFICIALLY CREATED SALMONID SPAWNING SITES AND IMPLICATIONS FOR MANAGEMENT DECISIONS
Project design is a complex process involving geologists, biologists, engineers and Federal, State and local regulators. Gravel restoration sites are designed to create spawning riffles where pools or glides existed, and focus on appropriate depth, velocity and substrate size for fall run Chinook salmon and steelhead. Pre- and post- project measurements show dramatic changes in river and stream bed conditions at the restoration sites. Grain size and sorting are engineered to remove armored surface layers and appropriate sized spawning gravel is added. Most sites show order of magnitude increases in permeability and 50% increases in dissolved oxygen content immediately after spawning gravel is added. Surface water depth is adjusted to range between 1.0 and 2.5 ft during spawning season, and surface water velocity is usually between 1.5 and 4.0 ft/s in the gravel addition sites. These ideal physical conditions lead to corresponding increases in spawning use. Sites with little or no spawning prior to restoration often hold hundreds of spawning pairs after restoration. These estimates are based on high resolution air photographs and photo enhancement techniques.
Difficult management decisions are related to the longevity of sites and the unpredictable nature of flows on the American River. Recent drought has resulted in low flows, but even under these mild flow conditions the sites are degrading. Dissolved oxygen content has dropped with time, grain size has increased and spawning gravel is mobile. Predictions of site longevity have huge error bars, but it appears that spawning sites will need to be replenished on a 15 to 20 year frequency, or more often if high flows cause complete bed mobilization.