The approach taken in the determination of a suitable minimum flow to be maintained in a stream channel for the preservation of trout populations has centered on three types of physical stream characteristics and the changes observed in them between various discharge levels, based on the average daily flow (ADF) over the period of record: hydrologic parameters; surface area and its composition, based on water depth and velocity; and available trout cover. Portions of Douglas Creek and Hog Park Creek, relatively small streams (average daily flows approximately 30 cubic feet per second) located in the North Platte River drainage of southeastern Wyoming, were intensively investigated in the summer and fall of 1972 at 200%, 100%, 50%, 25%, and 12.5% ADF. (Modified author abstract)