Surface-water hydrology of California coastal basins between San Francisco Bay and Eel River

Abstract

This report presents an analysis of the surface-water hydrology of the coastal basins of California that lie between the north shore of San Francisco Bay and the south boundary of the Eel River basin. Its purpose is t<?provide hydrologic information in convenient form for use in project planning by the California Department of Water Resources and other water agencies operating in the State. The report area, comprising about 5,000 square miles, lies wholly within the northern California Coast Ranges (physiographic section). Most of the streams are small and drain watersheds of less than 100 square miles. A notable exception, however, is the Russian River, which has a drainage area of almost 1,500 square miles. Precipitation is distinctly seasonal, and very little occurs from June through September. About 80 percent of the total precipitation falls during the 5 months November through March. Mean annual precipitation increases from south to north and is strongly influenced by the altitude, shape, and steepness of mountain slopes. Mean annual precipitation ranges from a low 20 inches in the Napa Valley to a high of 110 inches on the mountain divide of the Mattole River basin. Snow has an insignificant influence on the hydrology of the region. Average annual natural runoff from the region is about 5.5 million acre-feet, which is equivalent to about 21 inches from the entire region. Runoff, however, has an areal distribution similar to that of precipitation and ranges from about 5 inches in the south to about 85 inches in the north. About 80 percent of the runoff occurs during the 4 rainy months December through March. The rains of November, falling on rather dry ground, generally contribute little runoff. Flow in the summer and early fall is poorly sustained, and many of the smaller streams go dry. This seasonal distribution of runoff reflects not only the seasonal distribution of precipitation but also the influence exerted by the geologic characteristics of the California Coast Ranges. The low permeability of the soil and surficial rock and the limited capacity for subsurface storage impede infiltration, and as a result there is little lag between rainfall and runoff. Study of the runoff regimen indicates that, for any stream, there is a close relationship between the flow-duration curve and the frequency curves for low flows of various durations. Both are influenced by basin characteristics, and the relationship is maintained by the regional consistency of the seasonal pattern of precipitation. The recurrence intervals of low flows sustained for periods ranging from 1 day to 274 days may be derived from the flow-duration curve 1 2 SURFACE WATER, SAN FRANCISCO BAY TO EEL RIVER, CALIF. with considerable confidence. The characteristics of the flow-duration curve were found to be roughly related to mean discharge. Seven major floods have occurred in the region in the past 25 years. In many of the coastal basins south of the Russian River, six of the seven floods were of nearly equal magnitude. In the Russian River basin the flood of December 1964 was generally the maximum of these events, but in the coastal basins north and west of the Russian River the flood of December 1955 generally produced the greatest peak discharges. A flood-frequency study of the region indicates that the magnitude of floods of any given frequency can be related to size of drainage area and to mean annual basinwide precipitation. This precipitation is an excellent index of the relative magnitude of storms of any given frequency because the bulk of the precipitation occurs during several general storms each year, and the same number of general storms occur at all stations in any given year. The magnitude and frequency of high flows, for durations ranging from 1 day to 274 days, were analyzed by a method that closely paralleled that used in the flood-frequency study. Average discharges for each selected duration and frequency were correlated with drainage area and mean annual basinwide precipitation. Results were highly satisfactory because all correlations had coefficients of multiple correlation that were equal to or greater than 0.99.