The Lac qui Parle River watershed is underlain by thick water-bearing sections of glacial drift and Cretaceous rocks. Drainage is from the Coteau des Praries, a plateau in the southwest, to the Lac qui Parle reservoir, about 800 feet lower than the plateau. The term "watershed" as used in this report refers to that part of the drainage basin (767 square miles) within Minnesota. The total area of the drainage basin, including South Dakota, is 1110 square miles. Most waters from the watershed are of good quality.