upstream from its mouth; its reservoir filled with silt and mud soon after the dam's completion. The hydroelectric facility was later decommissioned but reactivated in 1984. The Blue Earth River's largest tributaries are the Le Sueur River, which it collects upstream of its mouth; and the Watonwan River, which it collects upstream of its mouth. The two rivers drain 31% and 24% of the Blue Earth's watershed, respectively. Tributaries of the river in its upper course include the East Branch Blue Earth River, long, which rises in southwestern Freeborn County and flows westwardly through Faribault County to the city of Blue Earth; and Elm Creek, long, which rises in northeastern Jackson County and flows eastwardly through Martin County into northeastern Faribault County, where it joins the Blue Earth River near Winnebago. The Blue Earth River flows in most of its course through till plains and the plain of a former glacial lake. The drain of the glacial lake, Union Slough, drains in two directions, south into the East Fork of the Des Moines River and north, as Union Slough, into the West Branch of the Blue Earth River. The lower (northern) part of the river's watershed was historically covered by the Big Woods, a tract of hardwoods that has since been largely converted to agricultural use; segments of the woodland extended southward along the riparian corridors of the Blue Earth River and its tributaries. In its lower course below Rapidan Dam, the river flows through a wooded gorge in the valley of the Minnesota River; this section is a popular canoeing route. According to the Minnesota Pollution Control Agency, approximately 84% of the Blue Earth River's watershed is used for agricultural cultivation, primarily that of corn and soybeans. The river is one of the most polluted in Minnesota, with elevated levels of sediment, bacteria, nitrates, phosphorus, mercury, PCBs, and pesticides, contributed in part by runoff in the watershed. Fecal coliforms, contributed by manure fertilizers, livestock waste, substandard septic tanks and outdated sewer systems, are often present in the river at levels considered by the state government to be unsafe for swimming. Flow rate The United States Geological Survey operates a stream gauge on the Blue Earth River below Rapidan Dam in Rapidan Township, downstream of the mouth of the Watonwan River and upstream of the mouth of the Le Sueur River, and upstream from the river's mouth. The annual mean