Flood Bypasses in California

Restoring floodplains protects our cities, jobs, and industry. Flood bypasses divert floodwaters away from urban areas and into less developed lands.

California’s state engineer, William Hammond Hall, first proposed a system of bypasses in a report to the state legislature in 1880. The resulting Yolo Bypass has reduced the flooding that once plagued Sacramento for 80 years by diverting water away from the city and into agricultural lands. Periodic inundation of the bypass also provides significant ecological benefits to salmon and other fish species. Landowners work together with agencies to ensure appropriate compensation and land management practices.

Flood bypasses are a critical part of a comprehensive flood risk reduction strategy and they can work at all scales. The City of Napa, California created a flood bypass through the city center to reduce damages and enhance the local environment.  The Morganza floodway prevented major flooding to Baton Rouge and New Orleans, Louisiana in the 2011 Mississippi River flood.

Benefits of flood bypasses include:

  • reducing flood risk by giving rivers more room to spread out
  • allowing flood waters to rise slowly, giving communities time to move to higher ground.  (Levee breaks on the other hand, on the other hand, rapidly flood communities.)
  • sustaining fish and wild life habitat and support commercial fisheries
  • contributing to a more reliable water supply, a benefit not granted by technical approach

California’s past is a way forward: we can save lives, communities, and billions of dollars by expanding the bypass system through the Central Valley.  The Yolo Bypass is a successful example of a flood risk management strategy that achieves multiple benefits, and there are opportunities to expand it and increase the benefits.

Learn more about American Rivers’ efforts to reduce the growing flood risk in the Central Valley by advocating for expanding the Yolo Bypass and creating a bypass on the Lower San Joaquin River.