San Pedro Creek, in Pacifica, California, has been subjected for the last three years to continuous water removals from the headwaters of one of the three main forks. This water, withdrawn at rates ranging from 0.56 to 1.12 cubic feet per second, has been used to supplement the city water supply. A study was conducted to determine the effects of this restricted streamflow on the creek and its local steelhead trout population. Weekly measurements were made of streamflow, temperature, and water quality characteristics. A fish population sample was taken, and the aerobic zone of the substrate was determined. Data for these measurements is presented. It appears that streamflow artificially reduced below a certain critical level could damage the trout habitat by lowering the dissolved oxygen and reducing the current in the spawning areas. Calcium and magnesium hardness might be reduced, while soluble orthophosphate content may increase. Additional management problems, resulting from the local residential development, are also discussed.
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