Hydrologic reconnaissance of Point Reyes National Seashore Area, California

Dale, R; Rantz, S

This report summarizes the results of a hydrologic reconnaissance of the Point Reyes National Seashore area, the primary purpose of which . was to appraise potential sources of water supply at park sites where visitor accommodations are proposed. Point Reyes National Seashore is a peninsular area on the California coast about 50 miles north of San Francisco. Inverness Ridge, with peaks about 1,400 feet above sea level, forms the eastern boundary of the seashore. Marine terraces, extending from sea level to altitudes of about 400 feet, form the remainder of the area. The general climate is strongly influenced by the proximity of the Pacific Ocean, but marked local differences in temperature, wind velocity, and precipitation result from the topography of che peninsula. Precipitation is distinctly seasonal and more than 75 percent of the annual (October 1 through September 30) total occurs In the 5 months, November through March. Mean annual precipitation over the peninsula ranges from 20 to 40 inches, whereas mean annual runoff is estimated to range from 6 to 16 inches. About 90 percent of the annual runoff occurs in the 5 months, December through April, when water-supply requirements are minimum. A streamflow reconnaissance of the area, consisting of a series of low-flow discharge measurements at each of nine streams, was made in 1964 and 1965. From these measurements, the low-flow characteristics of the streams were derived and an evaluation was made of their adequacy as sources of surface-water supply for sites where visitor accommodations are proposed in the national seashore. Results of the study showed that summer streamflow generally is inadequate at beach sites north of the latitude of the western tip of the peninsula. The development of a surface-water supply is feasible at the other proposed park sites. The principal geologic features, with reference to a -round-water supply, consist of a basement complex overlain by marine sandstone and by shale and raudstone, which, in turn, are overlain by terrace and dune sand, alluvial sand and gravel, and tidal-marsh clay and silt. Wells completed in basement complex, shale and mudstone, and clay and silt yield about 1 gpm (gallon per minute) ; in sandstone and in terrace and dune sand, about 10 gpm; and in sand and gravel up to 60 gpm. Small quantities of water (less than 5 gpm) may be developed at most of the sites proposed for visitor accommodations, but quantities in excess of 25 gpm will be very difficult to develop. Except for the iron content, the water quality of potential sources of supply generally appears to be satisfactory, according to U.S. Public Health Service standards. Analysis for iron was not made but field evidence suggests that a high-iron content in the water will be a problem throughout the seashore area. Because this investigation was of the reconnaissance type, the quantitative results obtained are estimates. However, they provide a basis for the preliminary planning of park developments. Detailed studies are in progress (1965) to define hydrologic conditions more adequately at proposed park sites.