Status of the Lower Sacramento Valley Flood-Control System within the Context of Its Natural Geomorphic Setting

Abstract

The Sacramento River’s flood-control system was conceived as a series of weirs and bypasses that routes floods out of the leveed main channel into natural floodways engineered to drain directly into the bay delta. The system, superimposed on a natural geomorphic setting consisting of geologic, sedimentary, and tectonic controls, still relies on weirs and bypasses to keep low-lying communities dry during floods. However, the Sacramento Valley bypass system exhibits widespread evidence of impairment by sedimentation, especially at prehistoric loci of alluvial splays. Episodic flooding in the basin delivers large volumes of sediment that accumulate throughout the flood bypasses, especially from legacy tailings fans that originated in the hydraulic mining era. In addition to decreasing flow capacity, these deposits promote colonization of vegetation, which, in turn, increases roughness and decreases flood conveyance. We document three bypass regions that are affected by natural geomorphic controls, where consistent sedimentation occurred prior to and since bypass construction. Deposits forming at the entrance to Colusa and Yolo Bypasses increase stage thresholds for flows entering the floodway, exacerbating flood risk in the main channel downstream of the entrance. Deposits forming in downstream reaches of bypasses such as Colusa Bypass affect flood conveyance, potentially causing backwater effects that could limit diversion of flood discharge into the bypass system. Systematic bypass deposition tends to occur in locations where local backwater effects are imposed by river confluences. One particularly acute choke point for sediment occurs at the confluence of Sutter Bypass, Feather River, and Sacramento Rivers, where the ancestral Cache Creek fan compresses valley drainage. These factors and concerns generated by the Katrina disaster have motivated sediment removal from the inlets to bypasses and levee repair along main channels, but it is unclear how effective these measures will be in the coming decades, especially within the context of flood conveyance throughout the entire system and regional climate change.