The Corps Water Management System (CWMS) includes four interrelated models to assist with water management for the basin:

- GeoHMS (Geospatial Hydrologic Modeling Extension)
- ResSIM (Reservoir System Simulation)
- RAS (River Analysis System)
- FIA (Flood Impact Analysis)

The Big Muddy River basin is located in Southern Illinois, covering approximately 2,360 square miles. The headwaters are located within Marion County near the town of Kell, Illinois. The basin flows in a southwestern direction towards the outlet at the confluence with the Mississippi River in Jackson County, approximately 4 miles south of the town of Grand Tower, Illinois. The topography of the basin is characterized by hilly upland terrain and broad, almost flat lowlands along the principal streams. Maximum topographic relief varies from approximately 620 feet near the headwaters to 350 feet at the confluence with the Mississippi River. The major tributaries to Rend Lake are Casey Fork, Rayse Creek, and the Big Muddy River. Downstream of the dam, the major tributaries are the Middle Fork of the Big Muddy River, Little Muddy River, Crab Orchard Creek, and Beaucoup Creek. Soils in the basin were predominantly deposited by the succession of continental glaciers that advanced and retreated across the area during the Great Ice Age. These sediments fall into three major categories: till, lacustrine deposits, and outwash sediments. Loess soils can also be found within the basin. In general, the soils in the basin are rich in organic matter and help explain the major land use categories: agriculture and forested areas. The climate for the basin is considered moderate and is characterized by hot summers and cool winters. The basin lies within the transitional zone between the humid continental climate type and the humid subtropical climate type, and the area experiences four distinct seasons. Average annual rainfall is approximately 45 inches across the basin, and typically the maximum precipitation occurs in the spring (April, May, and June) and again in late November.