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The evolution of southwest Montana’s Big Hole and Beaverhead River drainage basins is determined from topographic map evidence related to mountain passes crossing what are today high altitude drainage divides including North America’s east-west Continental Divide. Map evidence, such as orientations of valleys leading away from mountain passes (and saddles) and barbed tributaries found along the downstream drainage routes, is used to reconstruct flow directions of streams and rivers that once crossed the present-day high mountain divides. Large south-oriented anastomosing complexes of diverging and converging channels are interpreted to have eroded what are today closely spaced passes and saddles now notched into high mountain ridges. Water in those south-oriented channels is interpreted to have flowed across emerging mountains and subsiding basins. Headward erosion of deeper southeast-oriented valleys, assisted by crustal warping, concentrated south-oriented water in fewer and deeper valleys as the water flowed from southwest Montana into what are today Idaho and the Snake River drainage basin. Headward erosion of the Big Hole River valley between the emerging Anaconda and Pioneer Mountains, also assisted by crustal warping, reversed all Big Hole Basin drainage so as to create the north-, east-, and south-oriented Big Hole River drainage route. A final and even more major reversal of flow in the present-day north-oriented Montana Missouri River valley, with the assistance of additional crustal warping, next ended all remaining flow to Idaho and the Snake River drainage basin and reversed and captured all drainage in the present-day north-oriented Big Hole, Beaverhead, and Red Rock River drainage basins. The observed map evidence indicates that prior to the final flow reversal events, large volumes of south-oriented water flowed across southwest Montana’s Big Hole and Beaverhead River drainage basins.

Origin of Mountain Passes across Continental Divide Segments Surrounding the Southwest Montana Big Hole and Beaverhead River Drainage Basins, USA