Quantifying channel responses to the removal of the Glines Canyon Dam in the middle reach of the Elwha River, Washington
Bryon James Free
Four different study sites throughout the middle reach of the Elwha River were monitored before, during, and after the dam removal process over a period of two years from 2012-2014. The complexity of the river geometry was a major factor in the ability of the river to trap and accumulate the new influx of woody debris and sediment from the dam removal, which influenced the response of the river channel. The change that occurred was quantified by using repeat Terrestrial LiDAR (TLS), sediment distribution surveys, and large woody debris mapping techniques. The morphologic changes that occurred during this time were caused by multiple different geomorphic influences. The most notable was the initial sediment pulse that that inundated the downstream river channel in the first few months of the reservoir sediment release. In turn, it filled the riffles and pools throughout the entire middle reach of the river, and the subsequent deposition was channel geometry dependent. As the initial sediment wave dissipated and the river continued transport sediment from the Glines Canyon Dam, the channel geometry was still the major factor in woody debris collection and sediment deposition followed by river discharge. Woody debris anchored and accumulated on sediment bars throughout the entire middle reach; it became apparent that the more complex the channel system (i.e. multiple channels, vegetated islands, riffles and pools, or a sharp channel bend), the more likely the woody debris was to collect. Furthermore, as the woody debris deposited coalesced into log jams, it influenced the sediment deposition by armoring the banks of channels and creating areas of slow moving water. The combined deposition of sediment and woody debris caused areas of the channel to migrate, increasing the complexity of the river geometry. This study has provided some much-needed empirical data necessary to model future dam removal projects. It demonstrated that the use of TLS combined with surveys of large woody debris and sediment distribution can provide detailed information about the effects of the dam removal in different geomorphic settings.
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