Wildlife resources in the Cache Creek drainage include the second largest wintering population of bald eagles in California, peregrine falcons, tule elk, river otters, foothill yellow legged frogs and other wildlife.  A total of 154 species of birds have been observed in the watershed (BLM web site, Bird Check list for Cache Creek).  Waters within the Cache Creek Watershed are typically warm and alkaline, consequently the predominant fish resources in Cache Creek are members of the minnow family (Cyprinidae) such as pikeminnow, hitch, and California roach as well as Sacramento sucker, catfish and largemouth bass.   The Cache Creek watershed has been identified as a dominant Coast Range source of mercury to the Sacramento River and the Delta (California Department of Fish and Game 1988 CVRWQCB, 1998). There are three inactive mercury mining districts in the upper Cache Creek Basin, the Clear Lake District, the Wilbur District and the Knoxville District, each of which has a number of abandoned and un-reclaimed or partially reclaimed mercury mines including one superfund site at Clear Lake.  Currently there is an interagency effort to evaluate these mines within each of the sub-basins for contributions to mercury loads borne by Cache Creek.

Methods: We collected invertebrates, fish, amphibians, and bird eggs from the Cache Creek watershed in the spring and early summer of 1997 and 1998.  In 1998, water quality measurements were taken at each site where fish were collected. Water quality parameters included temperature, pH, dissolved oxygen and electrical conductivity.  Water quality parameters were collected in 1998 concurrent with fish collections.

Results: Loads from Bear Creek are largely influenced by Sulphur Creek but other undocumented sources are suspected (Sulphur Creek for example on February 2, 1998, had mercury concentrations of 11,421 ng/L - 3 orders of magnitude EPA's aquatic life criteria).  Sulphur Creek drains the Wilbur mining district which includes the Elgin Mine, the Wide Awake Mine, Abbot Mine and Empire Mine. Bear Creek loads are typically a smaller part of the Cache Creek load but Bear Creek at least in some years may contribute more of the mercury load in early season storms (CVRWQCB, 1998) when the other two tributaries with reservoirs are in storage mode.  Concentrations of total mercury in water in all sub-basins while variable were generally high and almost always above 12 ng/L, with sometimes extremely high concentrations of several thousand ng/L found during storm events.  We found by sampling individual fish in 1998 that we were able to statistically distinguish Bear Creek from other sub-basins.  Whole body mercury concentrations in pikeminnows, suckers and roach were all significantly elevated in Bear Creek and the concentration of mercury in each of these species increased as sampling progressed downstream from above the confluence with sulphur creek to our furthest downstream sampling station on Bear Creek.  The pattern in invertebrates was the same as in fish. Mercury concentrations in tricopteran and anisopteran larvae increased in Bear Creek below Sulphur Creek.  Yellow legged frog comparisons in Bear Creek above and below the Sulphur Creek confluence also hinted at a downstream increase although a slightly larger sample size would be needed for statistical confirmation.

Discussion: While water monitoring by the Regional Water Quality Control Board has indicated Cache Creek tributaries within the North Fork and South Fork of Cache Creek sub-basins are responsible for the extraordinary exports of mercury loads to the Cache Creek Settling Basin and the Yolo Bypass during the winter, Bear Creek appears to experience the greatest mercury bioaccumulation hazard.  Bioaccumulation is enhanced in the Bear Creek sub-basin during the summer months and increases with distance downstream from Sulphur Creek.  This may imply additional sources to Bear Creek besides Sulphur Creek, or enhanced bioavailability of mercury in downstream waters, or both.  A Total Maximum Daily Load strategy that seeks to reduce biological hazards of mercury within the Cache Creek Basin should focus primarily upon the Bear Creek sub-basin.

Learn More by Reading the Full Report: Schwarzbach, S., Thompson, L., Adelsbach, T. , Cache Creek Mercury Investigation USFWS Final Report, USFWS, Div. of Env. Contaminants, Sacramento, CA. 2001.

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