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OOJ074 HYDROLOGIC RECONNAISSANCE OF THE YAMPA RIVER DURING LOW FLOW, DINOSAUR NATIONAL MONUMENT, NORTHWESTERN COLORADO By Timothy Doak Steele, Dennis A. Wentz, and James W. Warner ABSTRACT A hydrologic reconnaissance of a 74-kilometer reach of the Yampa River in Dinosaur National Monument was made during low flow in mid-August 1976. Stream discharge, which was measured along this reach every 16 to 24 kilometers, ranged from 9.4 to 10.6 cubic meters per second. Variations in streamflow were explained, in part, by underflow, loss to ground water, and evaporation. Specific conductance was measured about every 2 kilometers and indicated a downstream increase on the order of 11 to 12 percent for the reach. Except for mercury, bottom-sediment trace-element concentrations in the study reach were less than maximum concentrations determined during August-September 1976 for bottom sediments at unperturbed sites upstream in the Yampa River basin. At one of five sampling sites, the mercury concentration in bottom sediments exceeded the maximum measured upstream level. INTRODUCTION Effects of regional economic development on water resources of the Upper Colorado River Basin, which extends upstream from Lees Ferry, Ariz., have been of great concern to downstream water users (Iorns and others, 1965; Upper Colorado River Commission, 1975; Weatherford and Jacoby, 1975). This concern has intensified in recent years, as the development of energy resources in the basin has progressed. The U.S. Geological Survey currently (1977) is undertaking a detailed evaluation of the water resources of a subbasin of the Upper Colorado River-- the Yampa River basin, Colorado and Wyoming, upstream from Dinosaur National Monument (Steele, Bauer, Wentz, and Warner, 1976; Steele, James, Bauer, and others, 1976; U.S. Geological Survey, 1976). The primary objective of the study is to assess water-related consequences of increasing coal-resource and associated economic development in the Yampa River basin. It is not anticipated that energy-resource development will occur in Dinosaur National Monument. However, as authorized in Public Law 90-542 and amended under Publ ic Law 93-621, the lower Yampa River in the monument (fig. 1) is being evaluated by the U.S. Department of the Interior for possible inclusion in the