Water Resources Collections and Archives

San Gregorio Creek drains approximately 51 square miles, debouching into the Pacific Ocean approximately 40 miles south of San Francisco. The San Gregorio watershed historically supported populations of steelhead (Oncorhynchus mykiss) and coho salmon (O. kisutch). These federally listed species are still present in the watershed, however; their habitat has been significantly altered due to a variety of land-uses, such as logging, grazing, and residential development. Although a variety of factors have contributed to the overall degradation of salmonid habitat in the San Gregorio watershed, the Highway 84 transportation corridor arguably has had the greatest direct impact over the past sixty years due to its location immediately adjacent to, and often crossing, the watershed’s main stem.

Both regulatory agencies and local stakeholders have expressed interest in restoring habitat for salmon within the San Gregorio watershed. Understanding the overall quality and quantity of habitat within the watershed and how specific sites along the Highway 84 transportation corridor impact habitat and fish passage will assist in future efforts to restore and protect salmon populations. For this study, I collected baseline information at twelve sites where creeks are crossed by public roads as well as four additional locations within the public right-of-way where channel banks were significantly altered, and in some cases physically moved, as part of the Highway 84 road realignment project (1953-1954) or subsequent bank stabilization projects. I identified three sites where Highway 84 bridge crossings impede fish passage during periods of low flow and one site that completely blocks fish passage. All bank stabilization sites observed show evidence of lateral erosion and contributions of fine-grained sediment directly to the active creek channel. Some of the materials utilized for bank stabilization have also created unanticipated hazards. While this study did not assess private infrastructure within active creek channels, it identifies locations where future habitat enhancement efforts and road maintenance activities on public roadways may have the greatest impact to salmon populations within the watershed.