Importance of Hydrology on Channel Evolution Following Dam Removal: Case Study and Conceptual Model

Abstract

A slow draining reservoir on the U.S. East Coast was monitored to identify the processes governing channel evolution upstream of a dam removal. Channel evolution was documented through cross section surveys, sediment size analysis, discharge measurements, and visual assessments of vegetative growth. The reservoir drained slowly, allowing for an analysis of channel evolution and identification of the morphometric parameters defining the path and time required for a channel to reach dynamic equilibrium. Channel evolution was a multidirectional process, and evolving channel reaches actively migrated laterally while alternating between aggradation and degradation. Channel formation was dominated primarily by the hydrologic regime at the time of dam removal and secondarily by the ability of vegetation to establish and stabilize the channel form. The importance of seasonal site hydrology over the minimum time required for channel evolution and the process through which the channel evolves indicates the complexity of channel formation within the first year following dam removal. Existing channel evolution models (CEMs) were modified and a new CEM that explicitly incorporates local hydrology and vegetative growth in the channel evolution process is presented. The modified CEM is applied to dam removals in Illinois, Virginia, and New Hampshire to illustrate its application beyond the immediate study area. In all cases, the seasonal flows were a dominant factor over the time frame and process of channel evolution. The CEM can be used to improve predictions of the length of time needed for a channel to evolve and the magnitude of channel change during the evolutionary process, such that it can be used to aid in planning and facilitating dam removals in similar regions.