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Description
The Upper Sweetwater River Basin is defined as the area lying upstream of the Sweetwater River gage near Descanso, California. A field study, involving 30 springs, was conducted in the 45.5 square mile basin between June 5 and September 14, 1973. The study involved making temperature and discharge measurements, as well as descriptions of the local geology. The area of the topographic drainage basin for each spring was measured, and the discharge rates were computed in gallons per acre per minute. Water samples from 20 of these springs were analyzed for Na, K, Ca and Mg. After filtering, the samples were run on a Varian Techtron model AA-4 atomic absorption spectrophotometer. The results were recorded in parts per million. Precipitation and gage flow records were obtained, and soil moisture data were examined in an attempt to compute a water budget for water years 1972 and 1973. It was found that there are two basic types of springs in the area. They are termed soil zone springs and hard rock springs. Soil zone springs involve groundwater flow in the soil zone, usually along the soil-hard rock interface. Hard rock springs, on the other hand, involve flow through permeable channels in the hard rock. The discharges of all of the soil zone springs were observed to decline as the dry season progressed. The highest average discharge per acre for the soil zone variety was found to be .11 gallons per acre per minute during the time of the field study. The highest average for a hard rock spring was found to be .30 gallons per acre per minute. This difference may be due to the fact that hard rock springs tend to intercept water from outside their topographic drainage boundaries. Not all of the hard rock springs declined in discharge as the dry season progressed. The water sample analyses showed that there was a correlation between the Na, Mg, and K contents and the rock type surrounding the spring. Water samples from gabbros tended to be higher in Mg , and those from granodiorites tended to be higher in K and Na. Water budget data indicated that for water year 1972, almost all of the 16.46 inches of precipitation was trapped in the upper soil horizons where it was depleted by evapotranspiration. Only 0.5% of the precipitation reached the gage near Descanso. Data for water year 1973 indicated that a significant percentage of' the 41.96 inches of precipitation reached the water table. Approximately 7% of the precipitation reached the Descanso gage.
