No abstract

The low-flow characteristics of a stream largely govern the type and the economics of its utilization. The magnitude, duration, and frequency of low flows included in this report are used both to determine whether a water-utilization project can be operated without storage and, if not, to estimate the amount of storage required to provide the minimum flows needed. When direct runoff from precipitation ceases, the flow of streams is governed by the volume of water in ground storage and by the rate at which the ground water discharges into the stream. The character and distribution of the geologic formations within stream basins influence the quality and quantity of the low flows of streams. Manmade changes to the land and to the stream systems probably have altered the regimen of flow of many streams. Heavy pumping of ground water near the streams may have lowered the water table, caused low flows to diminish or cease, and permitted the stream to recharge other aquifers with water derived from an adjacent aquifer. Limited low-flow data, in cubic feet per second per square mile, for 23 daily-record gaging stations and 37 partial-record stations are summarized for ready comparison. The summary gives the minimum average 7-day and 30-day discharges that may be expected to recur at 2-and 10-year intervals and the flow at the 90-and 95-percent duration points. More detailed data on the magnitude and frequency of low flows and flow duration, in cubic feet per second, are given for the 23 dailyrecord gaging stations. The 7-day low flows at the 2-year recurrence interval expressed on a per-square-mile basis, are used to demonstrate areal variations of low flow in this area. These indices range from 0 to 0.49 cubic foot per second per square mile. Drafts that may be made from specified amounts of storage with a chance of deficiency once in 10 and 20 years on a longterm average are related to the median annual 7-day low flow to permit preliminary estimates to be made of the storage required to supplement natural low flows. Chemical analyses of surface-water samples collected at 12 sites during low-flow periods show the dissolved solids to range from 90 to 333 ppm (parts per million) ; the hardness to range from 57 to 275 pprn; and the iron content to range from 0.00 to 0.08 ppm. The surface waters in the study area generally are suitable for some uses with little or no treatment, but for municipal and industrial supplies, the waters would require softening, coagulation, filtration, and pH adjustment for corrosion control. The results of the study suggest fields for further investigation to define additional causative features of the hydrologic' systems and to determine the effect that manmade changes to the stream systems may have upon the low flows of the streams and the groundwater systems.