Evidence for Groundwater Seepage Impacts on the Central Bitterroot River, Western Montana

Abstract

Channel instability on the Bitterroot River poses a threat to floodplain property between the towns of Hamilton and Stevensville, Montana. This stretch of river exhibits a complex multithread channel pattern characterized by a braided main stem within a network of narrower anastomosing channels covering a floodplain up to 5 km in width. While substantial channel migration and shifting are thought to have occurred in the area, historic changes are undocumented and the processes at work are poorly defined. This investigation employed a comparative analysis of channel features digitized from three sets of aerial photographs spanning a 50-year period to document and quantify historic channel changes of the Bitterroot River. Analysis in a geographic information system indicated that the main braided channel (the braid belt) widened substantially during the first half of the study period. However, lateral channel migration was limited and the positions of bifurcation nodes have remained constant. Evidence is presented suggesting that both the river's multi-channel configuration and tendency for rapid bank erosion are related to groundwater discharge. Groundwater seepage through the channel banks appears to be an important component of bank erosion and channel widening in the study area. Small discontinuous channels on the floodplain appear to be surface expressions of a groundwater discharge network. These observations suggest that groundwater interaction can have large effects on channel and floodplain morphology, and that changes in groundwater flow are among the impacts to be considered when evaluating stream perturbation.