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The Clark Canyon Reservoir occupies the northern Red Rock Valley in Beaverhead County, southwestern Montana. The region is a part of the northern Basin and Range province of western North America. It is characterized by a series of north-south trending extensional basins that have served as sediment traps since their formation. The Red Rock Valley is drained by the Red Rock River, which flows from Lower Red Rock Lake north to the Clark Canyon Reservoir at its confluence with the Beaverhead River. This river system has been active in the Red Rock Valley since the Miocene epoch. This study characterizes gravels of the Miocene-Pliocene Sixmile Creek Formation, Quaternary alluvial fans, and Quaternary fluvial terraces in the area of the northern Clark Canyon Reservoir. Nearly all cobbles present within the Sixmile Creek Formation and Quaternary fluvial terraces are well rounded, indicating extensive fluvial transport. Cobbles in the Quaternary fan deposits were recycled from the Sixmile Creek Formation. The lithologies of randomly selected clasts in exposed Sixmile Creek Formation and reworked alluvial fan gravels were dominated by quartzites that were likely derived from the Proterozoic Belt Supergroup and Cambrian Flathead Formation and basalt and rhyolite likely derived from the underlying Eocene Dillon volcanics. Undisturbed Sixmile Creek Formation fluvial gravels form deposits at least 100 m thick. The Quaternary fluvial terrace gravels had smaller clast sizes and were polymictic, containing minor gneissic pebbles. A study of cobble orientations in undisturbed sedimentary deposits indicated paleoflow directions in the depositing fluvial systems based on the dynamics of bed armor formation in active stream channels. Cobbles in the Sixmile Creek Formation indicated fluvial flow to the SW in Miocene-Pliocene time. In situ outcrops were rare for the alluvial fan deposits, but a single site indicated down-slope flow to the west. The Quaternary fluvial terrace gravel was