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UBC Theses and Dissertations

Channel adjustments and salmonid habitat alteration due to emergency dredging in Creighton Creek Hettrich, Lisa


Creighton Creek, near Lumby, British Columbia, is a small, snow-melt driven, fish-bearing stream that has had its lower reaches confined due to agricultural, residential, and industrial development. Over the past four decades, the channel bed has aggraded and the channel geometry has become tortuous and multi-threaded causing habitat degradation and increased flood frequency. Therefore, in May 2017, emergency orders allowed instream dredging operations to take place without significant planning. The availability of pre-dredge data on channel characteristics provided a unique opportunity to quantify the geomorphic adjustments caused by the emergency dredging operations. An extensive field data set and hydraulic models were used to quantify the spatial and temporal changes within the study reach across three time periods including: (1) the 2015-2016 pre-treatment period; (2) the 2017 immediate post-treatment period; and (3) the 2018 recovery period, which includes the channel response after one freshet. During the 2018 freshet, large volumes of material infilled the dredged zone, returning the streambed to its initial grade. Study observations suggest that the stream was not supply-limited and that the material deposited in the dredged zone was from upstream sources. A reduction in sediment transport potential within the affected zone immediately after dredging was consistent with general coarsening of the stream bed substrate. After recovery, there was a degradation of salmonid habitat in the dredged zone because of grain-size coarsening. The research suggests that human modifications to stream channels should only be undertaken with full understanding of the broader (and long-term) geomorphic and hydrologic context of the reach. Emergency dredging operations, as occurred in Creighton Creek, may have relatively minor long-term impacts on stream characteristics as reflected in the longitudinal profile or in various average flow parameters. However, this is only the case if there is ample sediment supply that enables the system to return to its pre-dredging state. Despite the stream re-achieving grade quickly, there may be impacts on fisheries habitat quality, especially as reflected in substrate size distributions. These observations from this study can be used to inform management decisions, emergency response procedures, and improve salmonid habitat in systems like Creighton Creek.

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