Dam removals and downstream bar-pool morphology

Graduation date: 2012Bar-pool morphology in rivers can provide vital habitat to aquatic species, notably salmonids, which require gravel riffles to bury eggs and pools for hydraulic and cold-water refuge. In some cases, the erosion and subsequent deposition of coarse sediment downstream of a dam removal can modify habitat by inundating bar-pool structure, filling pools, and homogenizing the channel bed. To assess extents and durations of geomorphic impacts of dam removals, project managers may devote significant resources to detailed one-dimensional numerical modeling studies and requisite data collection, but the manner in which these width-averaged sediment inundation depths are expressed laterally and interact with pre-existing morphology is not considered. We address the uncertainty associated with the lateral impacts of sediment pulses by presenting and evaluating a conceptual framework for both\ud visualizing dam removal data and explicating hypotheses describing the response of cross-channel bed relief to changing sediment thicknesses. Within this conceptual framework, we examine, using data from two dam removals in Oregon, the hypothesis whereby bed relief in a cross-section is formed and initially increased by sediment deposition and subsequently diminished during continued deposition. Results suggest that the conceptual framework is a useful means for visualizing dam removal data and effectively captured the different behaviors and changing morphologies observed in the cross-sectional data at each field site. Although there was some data that the hypothesis did not describe sufficiently, the confirmation of the general trends of the hypothesis in much of the data make it a good starting point for estimating the range of responses to dam removal of alluvial channels that exhibit bar-pool morphology and developing more physically detailed and site specific hypotheses