Map-based tests on controls of anabranch river character on the lower Yellowstone River

Abstract

Processes responsible for the formation of anabranched river systems are poorly understood. The Yellowstone River, Montana, is a major unimpounded river with over 800 km of anabranched channel. There are from one to seven channels per reach. Current literature identifies at least seven variables that may influence anabranch river character: tributary junctions, bedrock lithology, vegetation, sediment pulses, debris/ice jams, water loss from stream (to groundwater or irrigation withdrawal) and tectonism. Two variables, tributary junctions, bedrock lithology (measured by relative bedrock hardness and valley width), were tested on the Yellowstone River using aerial infrared photographs, and geologic and topographic maps. Relationships between anabranch character and tributary junction, valley width, and bedrock lithology were analyzed at 0.5-1km intervals along the lower 770 km reach. A weak relationship is found between tributary junctions and anabranches on the lower Yellowstone River (R2=0.38). Increased anabranching occurs downstream from tributary confluences with high tributary stream power relative to Yellowstone River stream power (omega t / omega Y >1). Valley width is correlated with bedrock geology at a coarse scale. Neither valley width, nor bedrock resistance to erosion, appeared to have a perceptible control on number of anabranches (R 2=0.0014).