The Big Sioux river rises in the Coteau des Prairies in the north-east and flows almost directly south for a distance of 300 m., in the lower part of its course forming the boundary between South Dakota and Iowa.
In the extreme north-east there is a range of low hills known as the Coteau des Prairies, which crosses the state in a S.S.E.
West of the James River Valley lies an elevated table-land, known as the Coteau du Missouri, which marks the waterparting between the James and the Missouri rivers, and has a general elevation of about 1800 ft.
The escarpment of the Coteau du Missouri is the dividing line, that portion to the N.
Of this ridge lies the third and highest plain within the state, the so-called Coteau du Missouri.
The ascent to the upper plain then becomes very gentle, though there is a rise of 400 or 50o ft., until it reaches the south-eastern portion of Sargent county and changes into the more abrupt Coteau des Prairies, a plateau about 2000 ft.
Is reached in the Coteau des Prairies.
South and west of the Coteau des Prairies lie vast stretches of plains, including the valleys of the Big Sioux and James rivers.