Geologic controls on ground-water storage and base flow

Abstract In the mountains of northern Nevada many streams have a permanent and nearly uniform flow whereas the streams of adjacent drainage basins, similar in many respects, have a flow that declines very quickly after every surface runoff event. The purpose of this study was to investigate the geologic factors that cause the different hydrologic characteristics of these streams. Forty-three drainage basins in the mountains of northern Nevada, ranging in size from 0.90 to 15.12 square miles were chosen for analysis in this study. The geology of each basin is described in terms of quantitative geomorphology following the principles laid down by R. E. Horton and A. N. Strahler. Despite the fact that many of the basins are in ranges thought to be but recently uplifted, the composition of the stream systems conforms to Horton's laws of stream numbers and stream lengths. On the basis of general appearance the mountain ranges of the area may be readily divided into two groups corresponding to mature and youthful mountains of W. M. Davis' classification. On scatter diagrams of mean slope versus relief ratio, and mean slope plus relief ratio versus drainage density, the points for drainage basins from the two groups of mountains plot in different fields showing that quantitative geomorphology can be useful in describing these mountains. The hydrologic characteristics of the drainage basins are described by the time and volume factors as determined from the base-flow recession curves. The recession curves were found by plotting spot-flow measurements against time, and assuming no recharge to the ground-water reservoir after the snow had melted (June 1). Evapotranspiration losses from the ground-water reservoir are estimated from the recovery of the base-flow at the end of the growing season. This apparent evapotranspiration bears an approximate relationship to the length of channel lined with phreatophytes. Scatter diagrams of the hydrologic factors plotted against selected geologic factors indicate that the time factor varies directly with the average depth of the main channel and the drainage density and inversely with the relief ratio. Deeper channels are more likely to intersect the zone of permanent saturation and longer channels tend to give sustained flow. The influence of drainage density is opposite to what has been found in humid or plains areas. In arid areas of high relief the characteristics of the basin that influence ground-water discharge have more effect on the permanence of a stream than the characteristics that influence ground-water recharge. The volume factor of the recession curve, total potential discharge, varies directly with the average depth of the main channel and the total relief of the basin, and inversely with the maximum width of the basin. A deep channel acts as a more efficient drain and a narrow basin is most efficiently drained by the channel. A high relief ratio reflects high elevation of summits and consequently relatively high precipitation and in turn relatively high volumes of recharge and discharge. Basins larger than two square miles tend to behave hydrologically as though underlain by an isotropic, homogeneous rock medium. For these basins the large scale is sufficient to overcome the influence of the many local variations in lithology and structure of the rocks.