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of the paleoflood investigations, however, provide much better physically based information on low-probability floods than has been available previously, substantially improving estimates of the magnitude and frequency of large floods in these basins and reducing associated uncertainty.

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The Black Hills Hydrology Study is a long-term investigation (10 years) that was initiated in 1990 as a cooperative effort between the U.S. Geological Survey, the South Dakota Department of Environment and Natural Resources, and the West Dakota Water Development District (which represents local and county cooperators) . The study is designed to assess the quantity, quality, and distribution of surface and ground waters in the Black Hills area of South Dakota.The study will investigate regional hydrogeologic characteristics of selected aquifer units in the recharge area near the Black Hills, with an emphasis on quantifying interactions between surface water and ground water. The study is not intended or designed to address various site-specific problems that are not of regional concern. The study will be divided into two phases because of the large study area and complex hydrologic system.Phase I will consist primarily of data collection and compilation necessary to describe precipitation amounts, streamflow rates, ground-water levels, and water-quality characteristics of the study area. Phase II will concentrate on: quantifying relations among precipitation, streamflow volume, and aquifer response for three diverse stream-aquifer systems; describing the significance on the hydrologic system of four areally extensive aquifer units (the Madison Limestone, Minnelusa Formation, Deadwood Formation, and Minnekahta Limestone); and possible development of regional ground-water flow models.

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