San Gregorio Creek Sediment TMDL


San Gregorio Creek is in western San Mateo County and drains a 52 square mile area from the Santa Cruz Mountains to the Pacific Ocean. It supports many beneficial uses including water supply, recreation and freshwater habitat for fish and other aquatic species. In 1998, the San Francisco Bay Regional Water Quality Control Board identified San Gregorio Creek as impaired due to excessive sedimentation under Section 303(d) of the Federal Clean Water Act. The 303(d) listing was made in response to concerns regarding adverse impacts to habitat for steelhead trout, coho salmon, and other threatened species whose populations have declined in recent decades.          

Photo showing El Corte de Madera Creek in the San Gregorio Creek watershed. Photo shows the creek flowing by moss covered rocks.

El Corte de Madera Creek in the San Gregorio Creek watershed

San Gregorio Creek is impaired by accelerated rates of erosion and sedimentation resulting from natural geologic and climatic processes augmented by human land use practices. Land uses that have contributed to the sediment impairment include historical logging and sawmills, road construction, poor grazing and agricultural practices, residential development, instream structures, and water diversions.

photo of San Gregorio Creek at the bottom of the watershed at the Highway 1 Bridge

 San Gregorio Creek at Highway 1 Bridge

These land uses contributed to one or more of the following effects: reach-scale channel incision, disconnection of the creek from its floodplain, reductions in large woody debris and channel complexity; reduced base flow in summer and fall; and, increases in fine sediment supply leading to deposition on the channel bed and filling of deep pools.





A Water Quality Improvement Plan (Plan) is being prepared to restore water quality by improving sediment and habitat conditions in San Gregorio Creek. The Plan examines the water quality problem, identifies sources of sediment contamination, and specifies actions to improve water quality. The Plan calls for voluntary actions, to be implemented where they are expected to achieve water quality objectives. We are pursuing this non-TMDL alternative approach to resolve excessive sedimentation impairment in San Gregorio Creek because some parties have already begun voluntary implementation efforts and the Water Board has regulatory authorities to require data collection and implementation actions included in the Plan if necessary. If the Plan is successful, it will lead to removing the San Gregorio Creek sediment impairment from the Section 303(d) list. If the Plan is not successful, then Water Board staff would develop a TMDL, which is a cleanup plan that can be implemented through permits and orders the Water Board issues.

Regional Water Board Approval Process

Our current schedule calls for distributing the Draft Water Quality Improvement Plan for a three-week public comment period in July and bringing the Plan to the San Francisco Water Board for approval in the September or October meeting. Please check back to this webpage for schedule updates.

Please join our mailing list!

Sign up to receive information by clicking the “Subscribe” button at the upper right of the screen, clicking on “Coastal San Mateo County” under the “All Basin Planning/TMDL Projects by Area” banner, and providing your name/email address.

For more information contact:
René Leclerc, P.E.
San Francisco Bay Regional Water Quality Control Board
1515 Clay Street, Suite 1400
Oakland, CA 94612
Phone: 510-622-2410
Fax: 510-622-2460