Description
The river rises from several short headstreams in southern McLean County that arise from a glacial moraine southeast of Bloomington-Normal, Illinois. Part of the moraine is publicly owned as the Moraine View State Recreation Area. The river's course forms a large arc through central Illinois, first flowing east into Champaign County, Illinois, south through Mahomet, then west through Monticello and Decatur, then turning northwest to flow along the north side of Springfield. It receives Salt Creek at 40°7′33.24″N 89°49′30.36″W / 40.1259°N 89.8251°W / 40.1259; -89.8251, approximately 25 miles (40 km) north-northwest of Springfield; then the river turns west, forming the southern boundary of Mason County with Menard and Cass counties. It joins the Illinois River from the east just north of Beardstown.
The Sangamon is impounded in Decatur to form Lake Decatur, constructed in 1920–1922 to provide a water supply for Decatur. This lake, formed by damming the main stem of the river, with no control over upstream land uses, has had major problems with siltation and agricultural pollution. The lake often has excessive nitrate levels from agricultural runoff. Many times the city was forced to warn people not to allow babies to consume water in Decatur because of "blue baby syndrome", Methemoglobinemia. Decatur has now installed nitrate treatment to avoid this problem.
The upper Sangamon, between Mahomet and Monticello, runs along the face of a terminal moraine within the Lake Michigan Glacial Lobe, which ranges in age from 28,000 to 12,000 BP. During the glacial Woodfordian Substage (middle Wisconsin Stage), ice of the Lake Michigan Glacial Lobe advanced rapidly, leaving a terminal moraine parallel to the modern Sangamon River. The ice stagnated and melted behind this moraine, without the meltwater overtopping the terminal moraine. Along this stretch, the eastern part of the watershed of the Sangamon therefore consists of short creeks, two to three miles (3–5 km) in length, that drain the face of the moraine. This forms an asymmetric watershed typical of rivers formed along the face of a terminal moraine.
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