Areal variability of low flows in a basin of diverse geologic units.

Large-scale areal variations in low-flow characteristics of streamflow are caused primarily by differences in geology in the 576-square-mile Swatara Creek basin just east of Harrisburg, Pennsylvania. Although the low-flow index (the average annual minimum daily flow) of small streams in the basin ranges from 0 to slightly more than 1.0 cubic foot per second per square mile (csm), the flows within each geologic zone show marked similarity, except where affected by mining, diversion, or other activity of man. These natural average minimum flows are lowest in the area underlain by the Martinsburg Shale and range from 0.01 to 0.10 csm. Moderate average minimum flows of 0.1 to 0.2 csm occur in the areas underlain by sandstone, conglomerate, and shale in the northern mountainous part of the basin; greater average minimum flows of 0.3 to 0.4 csm occur in the zone in the southern part of the basin underlain by the Gettysburg Shale, which consists mostly of coarse-grained sandstone. The variability in the low-flow indices of streams is greatest in the area underlain by various limestones and dolomites, ranging from zero to over 1.0 csm. However, even within this zone of limestones and dolomites there is reasonable consistency to the low flows within each of the several formations. [ABSTRACT FROM AUTHOR]