Serial Discontinuity and the Geomorphology of Tributary-Mainstem Interactions on the Green River

Robert P. Thompson

The Serial Discontinuity Concept predicts dams to have large impacts on a river directly downstream, with conditions gradually naturalizing with distance downstream, largely due to unregulated tributary inputs. Geomorphic structures caused by tributary-mainstem interactions should also follow this pattern of being most altered near a dam, with these alterations decreasing in magnitude as one travels downstream. General tributary-mainstem theory, as described in the Network Dynamics Hypothesis by Benda et al. (2004a), makes predictions about the effect of tributaries on variation and disturbance, describes general characteristics of confluence geomorphology, and predicts the likelihood of a tributary causing observable geomorphic effects based on factors such as basin size ratio, basin shape, confluence angle, and the type of tributary input. In the Green River system downstream of the Flaming Gorge Dam, debris flows are the most significant tributary input, creating numerous debris fans in several canyons, and greatly modifying the river hydraulics and deposition with the creation of fan-eddy complexes and associated habitat. Reduction of the frequency and magnitude of peak flows by the Flaming Gorge Dam has led to decreased reworking of debris fans and to channel narrowing on portions of fan-eddy complexes. These dam related impacts should decrease in magnitude and severity with distance downstream from the dam, with the largest difference occurring below the Yampa confluence, where the flows should be much closer to their natural state.