LONG-TERM GEOMORPHIC EFFECTS OF DAMS IN THE CENTRAL VALLEY OF CALIFORNIA: A COMPREHENSIVE AND COMPARATIVE APPROACH

Large dams alter hydrology to varying degrees and completely halt sediment supply to downstream river channels. This alteration of physical processes results in different channel geomorphic responses and end channel trajectories. In the Central Valley of California, fourteen of the sixteen major tributaries are impounded by large dams, defined here as larger than 8 meters high, but the degree to which these dams have altered downstream channel geomorphology relative to one another is unknown. We hypothesize that dams that do not reduce larger flows but are able to trap all bedload will cause the most incision in the downstream bed, whereas dams that are able to greatly reduce larger flows will “freeze” or slow the channel response downstream. Both incision and “freezing” effects have negative environmental effects on downstream aquatic ecosystems, which need to be approached with different restoration techniques. To test our hypothesis, we have evaluated dam-related changes in flow from hydrologic gauges on major Central Valley rivers, investigated channel morphologic adjustment using survey records of bed elevation at the gauges and from older channel topographic surveys. Preliminary results suggest that dam-affected rivers in the Central Valley are behaving differently, partially due to different operations and sizes of the dams, as well as downstream tributary influences. These results have important implications for river restoration, suggesting that blind importation of restoration techniques that have been successful on other rivers will not necessarily solve issues on a target river without first understanding the underlying principles driving geomorphic change.