The Middle Cheyenne Spring Watershed (HUC 10120109) covers approximately 1,230 square miles in central South Dakota. The area is largely agricultural, with over 80% of the land used for farming or ranching. The hydrology of the watershed is dominated by the Cheyenne River, which flows through the eastern portion of the watershed. Surface water in the watershed is limited, with most streams and rivers only flowing during wet periods. Snowpack is important in the winter months, with snowmelt contributing to the surface water supply in the spring. Reservoirs in the watershed include Angostura Reservoir, which provides water for irrigation, recreation, and municipal use. Interesting climatic facts include the area's extreme temperature fluctuations, with temperatures ranging from -40°F in the winter to over 100°F in the summer. There has been a trend of decreasing snowpack in recent years, which may have implications for water supply in the watershed.