Lower San Joaquin River Flood Bypass

In order to meet the pressing need for expanded conveyance capacity on the lower San Joaquin River, and reduce the risks of catastrophically flooding thousands of residents in San Joaquin County’s urban communities, American Rivers is working with local project partners to propose a flood bypass on the Lower San Joaquin River.

A Lower San Joaquin River Flood bypass would route floodwaters out of a highly constrained urbanizing reach of the San Joaquin River system between Mossdale and Stockton, and away from densely developed communities in Manteca, Lathrop, and Stockton. Floodwaters would instead flow into larger channels through a rural area of the Delta Primary zone where urban development is prohibited.

The bypass would:

  • Substantially reduce flood stages on the main stem San Joaquin River between Mossdale and Stockton.
  • Reduce the probability of catastrophic urban flooding and loss of life in the communities of Lathrop, Manteca, Stockton, and unincorporated San Joaquin County.
  • Substantially increase flood conveyance capacity through a constrained reach of the San Joaquin River floodway.
  • Provide floodplain and riparian habitat for a variety of sensitive species including riparian brush rabbit, giant garter snake, Sacramento splittail, and juvenile Chinook salmon.
  • Preserve agricultural land and protect it from uncontrolled flooding
  • Provide flexibility in how upstream reservoirs are managed so that operators may more effectively maximize water supply without subjecting downstream communities to increased flood risk

A flood bypass as described above is the only opportunity for expanding conveyance capacity through the lower San Joaquin River. Facing the consequences of climate change, with higher seas, bigger floods, rapidly declining Delta ecosystems, and increasing pressure to urbanize agricultural land, it has never been more important to expand conveyance capacity on the San Joaquin River to reduce flood risk, improve ecological function, and preserve agricultural land.

The Lower San Joaquin River Bypass proposal is a collaborative effort between American Rivers and other local and national agencies, including the South Delta Water Agency, the River Islands Development Company, Reclamation District 2062, Reclamation District 17, San Joaquin Resource Conservation District, the American Lands Conservancy, and the Natural Resources Defense Council.

The bypass will be developed and managed locally by the South Delta Channel and Levee Maintenance Authority. The University of the Pacific has agreed to provide technical review to improve the planning and public outreach processes.