Long-term Geomorphic Effects of Dams on Rivers in the Central Valley of California: A Comprehensive and Comparative Approach

The rivers of the Central Valley of California pose an unusual challenge and opportunity for the field of river restoration. The main challenge is that there are sixteen major dammed tributaries arranged in parallel, some of which have been significantly altered by hydraulic mining, dams, and gravel mining over the last 150 years. Dams of different sizes reduce flows to varying extents and likely create different downstream geomorphic effects. Partly because of these difficulties, much of the restoration work currently is being done river by river, with little reference to conditions or ongoing restoration efforts on neighboring rivers, and without being pieced together coherently as a parts of the larger-scale restoration of Central Valley rivers. Partly because of these challenges, however, there is an opportunity for the field of river restoration to learn much from the Central Valley about the effects of dams on gravel-bedded rivers and their restoration. For example, there has been a significant amount of geomorphic and ecological research compiled over the last twenty years that is available to be mined. In addition, some of the best hydrologic data exist in the Valley, stretching back over 100 years in many cases and allowing for excellent pre- and post-dam comparisons. A critical need in the Central Valley is a geomorphic framework within which to understand and prioritize individual restoration projects and the relative similarity between rivers. Using data from previous reports, hydrologic gages, and field work conducted during 2005-2006, a first-cut framework based on magnitude-frequency comparisons appears to work well when comparing the effects of different dams in the Central Valley. This is a work in progress that hopes to benefit future large-scale river restoration projects, particularly in the Central Valley, and also smaller-scale restoration efforts on individual rivers.