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Black Hills Hydrology Study

Study Area

The study area includes the topographically defined Black Hills and adjacent areas as shown in figure 1. The boundaries of the study area were modified slightly since publication of the original plan of study (Driscoll, 1992).

The Black Hills are a dome-shaped uplift of Laramide age, about 125 miles long and 60 miles wide (Feldman and Heimlich, 1980). Altitudes range from about 7,200 feet at the higher peaks to about 3,000 feet in the surrounding plains, resulting in an orographically induced microclimate characterized by generally greater precipitation and lower temperatures at the higher altitudes. The overall climate of the area is continental, which is characterized generally by low precipitation amounts, hot summers, cold winters, and extreme variations in both precipitation and temperatures (Johnson, 1933). Long-term trends in precipitation for water years 1931-98 for the study area are shown in figure 2; a water year is the 12-month period, October 1 through September 30, and is designated by the calendar year in which it ends. Average annual precipitation for water years 1931-98 in the study area is 18.61 inches, and has ranged from 10.22 inches in water year 1936 to 27.39 inches in water year 1995 (Driscoll, Hamade, and Kenner, 2000). The average annual temperature is 43.9 degrees Fahrenheit, and ranges from 47.6 degrees at Hot Springs to approximately 37 degrees near Deerfield Reservoir (U.S. Department of Commerce, 1990).

The oldest geologic units in the stratigraphic sequence are the Precambrian igneous and metamorphic rocks (fig. 3). The Precambrian rocks are exposed in the central core of the Black Hills, extending from near Lead to south of Custer (fig. 4). Surrounding the Precambrian core is a layered series of sedimentary rocks including limestones, sandstones, and shales that are exposed in roughly concentric rings around the uplifted flanks of the Black Hills (DeWitt and others, 1989). The outcrops of the hydrogeologic units and locations of numerous structural features (anticlines, synclines, domes, faults, and monoclines) are shown in figure 4. The bedrock sedimentary formations typically dip away from the uplifted Black Hills at angles that approach or exceed 10 degrees near the outcrops, and decrease with distance from the uplift (fig. 5).

Many of the sedimentary formations commonly are used as aquifers, both within and beyond the study area. Recharge to these aquifers is from infiltration of precipitation on the outcrops and from stream infiltration along the flanks of the Black Hills (Peter, 1985; Kyllonen and Peter, 1987; Greene, 1993). Within the Paleozoic section, the Deadwood Formation, Madison Limestone, Minnelusa Formation, and Minnekahta Limestone commonly are used as aquifers. These aquifers are collectively confined by the underlying Precambrian rocks and the overlying Spearfish Formation. Individually the aquifers are separated by minor confining layers, or by relatively impermeable layers within the individual formations. Leakance between the aquifers is extremely variable (Peter, 1985; Greene, 1993). Within the Mesozoic section, the Inyan Kara Group, which includes the Lakota and Fall River Formations, is used extensively as an aquifer, with various other units locally used to lesser degrees. Up to 4,300 ft of Cretaceous shales act as the upper confining layer to the Mesozoic aquifer units (Kyllonen and Peter, 1987).

Artesian conditions generally exist within the aforementioned aquifers, where an upper confining layer is present. This means that water in a well will rise above the top of the aquifer in which it is completed. If the water level, or potentiometric surface, is above the land surface, a flowing well will result. Similarly, artesian springs that originate from confined aquifers are common around the periphery of the Black Hills. The hydrogeologic setting of the Black Hills area is schematically illustrated in figure 6. Figure 6. Schematic of hydrogeologic setting Streamflow within the study area is affected by both topography and geology. The base flow of most Black Hills streams originates in the higher elevations, where increased precipitation and lower temperatures result in excess water being available for springflow and streamflow. Numerous streams have significant headwater springs originating from the Paleozoic formations on the western side of the study area (figs. 1 and 3). Most Black Hills streams generally lose all or part of their flow as they cross exposures of the Madison Limestone and Minnelusa Formation (Rahn and Gries, 1973). The Madison Limestone is a cavernous limestone and dolomite that is 300 to 650 ft thick (Rahn and Gries, 1973). Karst features of the Madison Limestone, including sinkholes, collapse features, solution cavities, and caves are responsible for the Madison's capacity to accept streamflow recharge. Recharge from streamflow loss also can occur to the Minnelusa Formation. The Minnelusa Formation consists of three units: an upper sandstone, up to 200 ft thick; a middle dolomite, sandstone, and shale with anhydrite in the subsurface, 200 to 300 ft thick; and lower sandstones and dolomites with basal shale, 0 to 300 ft thick (Rahn and Gries, 1973). Large springs occur in many locations downgradient from the streamflow-loss zones, most commonly within or near the outcrop of the Spearfish Formation, providing an important source of base flow to many streams beyond the periphery of the Black Hills (Rahn and Gries, 1973).

Photograph by J. Foster Sawyer, South Dakota Department of Environment and Natural Resources Large bedrock spring along Box Elder Creek