South Dakota Water Science Center
USGS IN YOUR STATE
USGS Water Science Centers are located in each state.
Black Hills Hydrology Study| Introduction|| Study Area|| Objectives|| Products|| References|| Digital Map Products|
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).Figure 1. Area of investigation for the Black Hills Hydrology Study
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).Figure 2. Long-term trends in precipitation for water years 1931-98 for the study area
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).Figure 3. Stratigraphic column for the Black Hills
Figure 4. Distribution of hydrogeoloic units
Figure 5. Geologic cross section
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).