Base-flow discharge and chemical-quality measurements were made at 233 selected sites on streams during October-November 1977, August-September 1978, and October 1978 to provide data on the interaction between surfacewater and ground-water systems in the northern Great Plains area of Montana and Wyoming. The tabulated data provide an areally broad data base of concurrent base-flow conditions.Streamflow gains or losses were computed for stream reaches not significantly affected by irrigation.On October 17, 1978, the change in flow of the upper Powder River between Sussex and Arvada, Wyoming, was a loss of 14 cubic feet per second. On the same date, the change in flow of the lower Powder River between Arvada, Wyoming, and Moorhead, Montana, was a gain of 6 cubic feet per second. Except for August-September 1978, major subbasins showed little significant differences in water discharge, chemical character, or dissolved-solids concentrations.

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This report documents the results of a cost-effectiveness study of the streamflow-gaging program in Wyoming. One phase of evaluating :.he costeffectiveness considers the use of alternative methods to simulate streamflow records. Regression analysis or hydrologic flow-routing techniques were considered for 24 combinations of stations from a 139-station network operated in 1984 to investigate suitability of techniques for simulating streamflow records. Only one station was determined to have sufficient accuracy in the regression analysis to consider discontinuance of the gage. The evaluation of the gaging-station network, which included the use of associated uncertainty in streamflow records, is limited to the nonwinter operation of the 47 stations operated by the Riverton Field Office of the U.S. Geological Survey. The current (1987) travel routes and measurement frequencies require a budget of $264,000 and result in an average standard error in streamflow records of 13.2 percent. Changes in routes and station visits, using the same budget, could optimally reduce the standard error by 1.6 percent.

Introduction 1 Methods of investigation 1 Location-numbering system 2 Step-backwater surveys and rating development Site descriptions 2 Lodgepole Creek 2 Raven Creek 6 Sand Creek 6 Stage-discharge relation development 6 Summary 6 Reference cited 6 Metric conversion 7 FIGURES Contents III

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