Title: A brief hydrologic and geologic reconnaissance of Pinto Basin, Joshua Tree National Monument, Riverside County, California
Abstract: basin, in the north-central part of Riverside County, Calif., is a typical desert valley formed by downfaulting along several major fault zones. The valley is filled with alluvium, and ground water in the alluvium discharges as subsurface outflow through an alluvium-filled gap at the east end of the valley. Occasionally surface water from cloudburst floods also discharges in a wash through the gap at the east end of the valley. A northeastward extension of the major fault along the south side of the valley acts as a barrier to the discharge of ground water from the valley. The average ground-water gradient is less than 1 foot per mile across the main part of the valley above this barrier, but the water level drops abruptly across the fault. The ground-water storage capacity of the uppermost 100 feet of saturated alluvium beneath the central valley area is estimated to be about 230,000 acre-feet. All this water in storage occurs at depths greater than 95 feet below the land surface and cannot be reached by plants or animals. During 1959 virtually all the water pumped in the area was withdrawn from storage. However, the quantity of water pumped is small in relation to the total quantity in storage. Except for a small decline in head, no evidence indicates that the pumping will greatly impair the yield for many years or cause the water to deteriorate in quality. INTRODUCTION PURPOSE AND SCOPE OF THE INVESTIGATION This report presents the findings of a brief geologic and groundwater reconnaissance made by the U.S. Geological Survey in basin, Riverside Count}^, Calif. The investigation was made at the request of the National Park Service to determine if the existing pattern of ground-water pumping from will reduce the yield of wells or impair the quality of the ground water in basin, which lies almost wholly within the Joshua Tree National Monument. Specifically, this report presents: (1) a brief geologic reconnaissance of basin, with particular emphasis on the unconsolidated deposits and the geologic structure in the area of subsurface outflow at 537 538 HYDROLOGY OF THE PUBLIC DOMAIN the southeast end of the basin; (2) information on the occurrence and movement of ground water; (3) an approximation of the groundwater storage capacity of the water-bearing deposits in the eastcentral part of and the relation of the storage capacity to pumpage; and (4) data on wells, water-level records, drillers' logs of wells, pumpage from wells, and chemical analyses of ground water. Fieldwork was done intermittently between June 1955 and December 1959. This work has been carried on by the Long Beach, Calif., subdistrict office of the U.S. Geological Survey under the general supervision of J. F. Poland, G. F. Worts, Jr., and H. D. Wilson, Jr., successive district supervisors, Sarcamento, Calif. LOCATION OF THE AREA is in the Mojave Desert in the north-central part of Riverside County, Calif, (fig. 64). As usually applied, the term Pinto basin refers to the alluviated or valley-floor part of a drainage system that discharges to the south along a dry wash between the Eagle and Coxcomb Mountains. However, this investigation is concerned chiefly with the downstream part of the drainage area. Therefore, where the term Pinto basin is used in this report, it refers to the main valley area between 115°20/ and 115°42'30 west longitude. The area studied (pi. 28) is shown on the Pinkham Well, Eagle Tank, and Coxcomb Mountains quadrangles (scale 1:62,500) of the U.S. Army, Corps of Engineers. The area north of lat 34° N. is shown on the Amboy quadrangle (scale 1:250,000). However, this area is not critical to the present study and has not been included as part of plate 28. PBEVIOTJS INVESTIGATIONS AND ACKNOWLEDGMENTS The mountainous region south of was examined by E. C. Harder (1912) during the summer of 1909 in connection with a study of the iron-ore deposits of the Eagle Mountains. The report of that study contains a reconnaissance geologic map of the Eagle Mountains showing the types and extent of the consolidated rocks, but it does not show basin. As part of an anthropologic study, the major geologic units of the lower part of are briefly described in a report by Scharf (1935). That report contains several excellent panoramic photographs but no geologic map. As part of a general hydrologic study by the National Park Service, J. V. Lewis prepared a map, showing the extent of alluvial deposits and the locations of wells and springs (written communication, 1941), and a report discussing the water resources of Joshua Tree National Monument and vicinity (written communication, 1942).