San Francisco Estuary and Watershed Science

Interest is great in projects that would restore Central Valley steelhead (Oncorhynchus mykiss) and Central Valley Chinook salmon (Oncorhynchus tshawytscha) to California drainages where they have historically existed and where there is good quality habitat upstream of instream barriers. The Calaveras River has garnered renewed attention for its potential to support these anadromous fish. I evaluated migration opportunity in the Calaveras River, and whether these salmonids could have been present in the river historically, by comparing historical anecdotal and documented observations of Chinook salmon and steelhead to recorded flows in the river and Mormon Slough, the primary migration corridors. Collected data show that these fish used the river before New Hogan Dam was constructed in 1964. Three different Central Valley Chinook salmon runs, including fall-, late-fall- and spring-run salmon, and steelhead may have used the river before the construction of New Hogan Dam. Fall and possibly winter run and steelhead used the river after dam construction. The timing and amount of flows in the Calaveras River, both before and after the construction of New Hogan Dam, provided ample opportunity for salmonids to migrate up the river in the fall, winter, and spring seasons when they were observed. Flows less than 2.8 m3/s (100 ft3/s) can attract fish into the lower river channel and this was likely the case in the past, as well. Even in dry years of the past, flows in the river exceeded 5.6 m3/s (200 ft3/s), enough for fish to migrate and spawn. Today, instream barriers and river regulation, which reduced the number of high flow events, has led to fewer opportunities for salmon to enter the river and move upstream to spawning areas even though upstream spawning conditions are still adequate. Improving migration conditions would allow salmonids to utilize upstream spawning areas once again.