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From the Executive Summary:

The closed basins area, which includes parts of Day, Marshall, Roberts, Codington, and Clark Counties, in northeastern South Dakota, was declared a Federal Disaster Area in 1990. Under the sponsorship of Region 8 of the Federal Emergency management Agency, an interagency study team was formed to conduct an interdisciplinary scientific investigation of the flooding n the closed basins area. Agencies represented include Federal Emergency Management Agency, U.S. Geological Survey, U.S. Army Corp of Engineers, South Dakota State University, Natural Resources Conservation Service, South Dakota Department of Environment and Natural Resources, and the South Dakota Geological Survey. Task 2 focuses on predictions of lake-level and corresponding surface-area, and water-volume changes of the 10 major lakes in the Waubay Lakes Chain closed basin using the water-balance model, developed by the U.S. Geological Survey, as part of task 4. Northeastern South Dakota is situated in a unique geographic setting, the result of glaciation which ended about 10,000 years ago. The area is dominated by the Coteau des Prairies a flatiron shaped topographic high, rising up to 400 feet above the James River Basin to the west, and 900 feet above the Minnesota River - Red River Lowland to the east. Drainage on the Coteau is poorly developed, resulting in internal drainage only, producing areas called closed basins. The closed basins area consists of several internally drained basins that are not connected to each other, or to either the James River or Big Sioux drainages, under normal climatic conditons. The closed basins area contains thousands of temporary and permanent ponds (potholes and lakes. Streams may connect two or three of the potholes, but typically flow only during periods of high water levels in the ponds. All of the lakes and ponds in the closed basins area have experienced water level increases and corresponding surface-water inundation, but the greatest flooding has been within the Waubay Lakes Chain closed basin. Beginning in 1983, a period of above normal precipitation began whose duration and magnitude has not been observed previously in the modem instrumental record. The precipitation event, coupled with be ow normal evaporation, has resulted in a water surplus within the closed basins area. This et increase in available water has resulted in flooding which has not been observed in the moden record. During the 1990' s, many potholes have filled to capacity and are either spilling into other ponds or lakes, or have coalesced into larger surface water bodies. (See more in text)






South Dakota State University


Prepared for Day County Risk Assessment Study Federal Emergency Management Agency