Short-term geomorphic response of a mountain stream channel to dam removal and a major flood

Abstract

Despite recent progress, the current understanding of the geomorphic channel responses to river restoration activities, such as dam removal, remains incomplete. Our study addressed this knowledge gap by examining the short-term (first two years) morphological response of a forested mountain channel in the inland Pacific Northwest to a dam removal-generated sediment pulse, which coincided with a considerable flood event. Even though the pulse was equivalent to decades of background sediment transport and the flood had a ≥10-year return interval, our analysis indicated generally limited changes in channel morphology and lateral activity. We attribute this stability to three factors. First, the predominantly fine pulse material was rapidly transferred through the entire length (6.5 km) of the high energy and partly confined channel with limited storage potential. The release of coarse material trapped behind the dam was restricted as part of sediment management. During the 2-year period, the limited coarse component of the pulse induced modest aggradation, extending only 1.5 km downstream of the dam. This highlights trade-offs between hazard management objectives and restoration of sediment-starved channels. Second, erosion during the flood was hindered by coarse and resistant channel boundaries, with banks reinforced by riparian root system and localized rip-rap, and, in the confined reaches, valley margin. Third, low storage of large wood contributed to the subdued morphological response. We observed that logjams, which constricted and deflected the flow, created a hot spot of erosion and deposition where upstream supply of coarse sediment was available. This sediment source was inadvertently generated where in-channel engineering works disrupted bed surface armor and stabilizing bed structures. We propose that the interactions between sediment supply from locally disturbed bed as a conditioning factor, logjam as a forcing element, and flood as a hydraulic trigger could be replicated in river restoration to improve channel complexity and dynamism.