The Upper Sangamon River Basin (USRB) encompasses an area of approximately 2400 km2 in central Illinois, USA, with an average annual precipitation of 1020 mm (Figure 1). Outflow from the USRB drains into Lake Decatur, a man-made reservoir which was constructed in 1922, and is the main water source for the City of Decatur (population 73,000) and the Village of Mt. Zion (population 5,800). The USRB has undergone significant alterations from human activities. A study of the historic changes in the basin indicated that in 1820, the USRB was 90% prairie and 10% forest; with forested areas mainly found along the riparian zones. Wetlands used to occupy approximately 50% of the total land area prior to European settlement as opposed to the current 2%. Presently, almost 90% of the watershed area is under row crops, primarily corn and soybeans. The transformation of prairie and savannah into agricultural cropland occurred with the installation of ditches and tile drains that were used successfully to drain flat, poorly drained soils in the USRB, which significantly altered its hydrologic responses. As a consequence, the water entering Lake Decatur stems from groundwater pathways rather than from surface runoff, with drain tiles being the primary pathway for water and nutrient transport (Botero-Acosta, 2017).
Figure 1. The Upper Sangamon River Watershed (USRW) (Source: Botero-Acosta, A., M.L. Chu, A. Stumpf. 2018. Impacts of environmental stressors on the water resources of intensively managed hydrologic systems. Hydrol Process. 2018; 32:2947–2962. doi.org/10.1002/hyp.13244)