MAP-BASED TESTS OF ANABRANCH CONTROLS ON THE LOWER YELLOWSTONE RIVER, MONTANA

Processes responsible for the formation of anabranching river systems are poorly understood. The Yellowstone River, Montana, is a major undammed and extensively anabranched river. There are over 800 km of anabranched channel, ranging from one to 11 channels per reach. Current literature, identifies at least seven variables that may influence anabranch river character: tributary junction, bedrock, channel slope, vegetation, sediment pulse, debris/ice jam, and tectonism. Several of these variables have been analyzed for the lower Yellowstone River using aerial infrared photographs, topographic maps and field surveys. Specifically, the relationships between anabranch character and tributary junction, bedrock geology, valley width and channel slope were analyzed at 0.5-1km intervals along the lower 770km reach. The literature suggests a relationship between tributary junctions and number of anabranches. No such relationship is found on the lower Yellowstone River. The literature suggests that a change from resistant to erosive substrate leads to an increase in the number of anabranches. Valley width is highly correlated with bedrock geology at a coarse scale and neither appear to have a perceptible control on number of anabranches; however, more detailed stratigraphic analysis is planned. Similarly, initial analysis of channel slope on 1:24,000 topographic maps with 20ft contours shows no correlation with anabranch character, however more detailed analyses are underway.