Course
The Whetstone is formed by the confluence of its North and South forks 4 miles (6 km) northeast of Milbank, South Dakota and flows generally eastward across the Coteau des Prairies in northern Grant County, past Big Stone City. It enters the Minnesota River at Ortonville, Minnesota, about 0.25 mi (0.4 km) after crossing the state line and just downstream of the Minnesota's emergence from Big Stone Lake.
Upstream of Big Stone City, the river collects two tributaries, the North Fork Whetstone River and the South Fork Whetstone River, each of which rises on the Coteau. The North Fork flows southeastward from Roberts County and passes near the town of Wilmot. The South Fork flows westward in Grant County and passes the town of Milbank.
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