The original power company built a 75-foot high earthen dam on a branch of the East Fork of Shoal Creek in 1963. The lake filled by 1966 and now serves as the cooling lake for the coal-fired Coffeen Power Station. The power station has a generating capacity of 945 megawatts of electricity, with the first unit coming into operation in 1965 and the larger, second unit in 1972. The heated discharge affects 73 percent of the surface water. The cooling loop is 4.1 miles in length.
The oak-hickory forests surrounding Coffeen Lake are representative of the native cover found within the Southern Till Plain Natural Division of central and southern Illinois. Soils are of loess and till, rather light and a characteristic "clay pan" can be found. Pre-settlement vegetation was a mixture of 60 percent forest to 40 percent prairie and wetlands. A variety of trees, woodland and prairie plants cover the slopes of the stream valley. Visitors also may find a diverse wildlife community. Muskrats, turtles, herons and mussels are seen in or near the water. Red-tailed hawks, blue jays and dragonflies may be seen overhead. Bobwhite, coyote, white-tailed deer and black rat snake are common to the area.