Establishing a context for river rehabilitation, North Fork Gunnison River, Colorado

Initial river rehabilitation efforts along the North Fork Gunnison River in Colorado focused on the use of in-stream structures and channel stabilization to create a single-thread channel with pools along a braided river. These efforts were based on the assumption that the river's braided planform results primarily from land use during the past century. In order to establish a context for further rehabilitation, we evaluated the possibility that the river might be braided as a result of processes independent of land use. We estimated volume, grain-size distribution, and lithology of sediment sources along the river corridor and evaluated the planform stability of the river during the past century using historical sources, aerial photographs covering 1939-1997, and comparison of bankfull discharge and gradient in the study area to values published for braided and meandering rivers. Our results indicate that the North Fork Gunnison River has been primarily braided in its lower reaches during the past few hundred years, although the channel planform tends toward a single-thread channel during decades of lower precipitation and discharge. Although land use is not the primary cause of braiding along the North Fork Gunnison River, it has decreased channel stability, and rehabilitation efforts should be designed to reduce these effects. Our results illustrate the importance of planning river rehabilitation measures within a historical context that accounts for both catchment-scale and reach-scale controls on channel processes and planform.