The Sangamon River basin is the largest of the Illinois River tributaries and drains 5419 square miles, with headwaters in McLean County near Ellsworth, flowing east into Champaign County and then southwest (Upper Sangamon River Area Assessment vol. 2, 1999). The Sangamon flows ~248 miles from Ellsworth to the Illinois River at Beardstown (Lower Sangamon River Area Assessment, vol. 2, 2000). The basin includes portions of Cass, Champaign, Christian, DeWitt, Ford, Logan, Macon, Macoupin, Mason, McLean, Menard, Montgomery, Morgan, Moultrie, Piatt, Sangamon, Shelby, and Tazewell Counties (Illinois State Planning Commission, 1938). The majority of the basin is in the Grand Prairie Natural Division (Page et al, 1992). The Sangamon was historically dammed near Decatur, beginning in 1878 with a wooden structure, replaced by concrete and with the addition of hydraulic gates controlling levels of Lake Decatur which provides the city's water supply (Upper Sangamon River Area Assessment, vol. 2, 1999). The Oakley Dam project initially proposed in the 1940s for flood control, water supply, and recreation would have inundated Allerton Park and due to strenuous opposition was not built (Marlin, 1971).