Big Stone Lake on the western edge of the county, and Lake Traverse, just north of it, lie in the valley channeled by the River Warren which, toward the close of the last glacial epoch, flowed from glacial Lake Agassiz. The part of the ancient watercourse between these two lakes, a distance of five miles, is called Brown’s Valley. (This valley forms the lowest point of a continental divide.) The waters of Lake Traverse reach Hudson Bay through the Bois de Sioux River, the Red River of the North, Lake Winnipeg, and the Nelson River. The drainage from Lake Traverse reaches the Gulf of Mexico through the Minnesota and Mississippi Rivers.

The greater part of Big Stone County is a rolling prairie of till plain with a clay loam soil dotted with several small lakes. A morainic belt in the northwestern part of the county leads southward near Beardsley to Big Stone Village, where it continues on the South Dakota side of the lake re-entering Minnesota in the southern part of Big Stone County is mainly to the Minnesota River except in the northeast corner of the county where it tends toward the Mustinka River and through the stream to the Red River of the North.

The natural drainage lines are imperfect and are supplemented by large ditches that have been dug along highways. Part of the Minnesota Valley in the county is too wet for cultivation. Ledges of granite are found in the valley not far from Big Stone Lake. There are also many boulders.

Big Stone Lake is 26 miles long and from one to one and a half miles wide. Although called a lake, it is a widened part of the Minnesota River. There were more than 50 lakes in the county in the early eighties but many have dried up, and recent maps show only a few. The largest is Artichoke and Toqua. The Minnesota River, rising in the coteau of western South Dakota, enters the State of Minnesota near Browns Valley village, flows into Big Stone Lake at its northern end, and out of the lake at the southern extremity.

The original boundaries of Big Stone County, as defined by the Legislature in 1862 were practically the same as a present, with the exception that townships 123 and 124, range 44, were included. Their townships were placed in Stevens County in 1868. The same act provided that the remaining area of the two northern tiers of townships in Big Stone County should be included in Traverse County. However, this would have left Big Stone with less than the 400 square miles that the Constitution requires. Therefore, the Stevens County provision went into effect, but the Traverse County provision was ignored. All uncertainty was cleared up by a legislative act of 1876 specifically outlining the boundaries. This act was amended in 1893 to correct a minor technical error in the description of the northwest corner.

An act approved by the Legislature in 1937, effective January 1, 1938, annexed to Big Stone County all the land situated in sections 16 and 21, township 121, range 46, in Lac Qui Parle County. When Minnesota was admitted as a State in 1858, iron monuments were placed at the head of Big Stone Lake and Lake Traverse to mark that part of the western boundary of the State.