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Physical and ecological organization in a large, gravel-bed river and response to disturbance

University of British Columbia

Along the 80-km gravel reach between Hope and Mission, Fraser River has a wandering morphology that includes secondary channels, gravel bars, and islands that together support a diverse and productive ecosystem. The wandering morphology is produced, in part, by annual sediment deposition within the reach. Sediment deposition, however, is perceived as a threat to flood security and gravel mining is proposed as a profitable solution to flood risk. This thesis presents a hierarchical habitat classification for the gravel reach, which provided a spatial framework to, first, examine habitat associations of benthic invertebrates and fish and, second, evaluate the physical and ecological responses to habitat disturbance by gravel mining. At the highest level of the classification, the river is divided into 5 sub-reaches (104 m scale) that vary in morphological expression and sediment gradational tendency. The intermediate level specifies gravel bar units (103 m scale) each consisting of a riffle, gravel bar, and adjacent pool. Nested within gravel bars are physically distinct habitat units, which represent the finest level of the classification (101- 102 m scale). Results demonstrated that the assemblages of invertebrates and fish associated with habitats are moderately distinct and differentiated along a hydraulic gradient corresponding to velocity. However, the congruence between habitat structure and the structure of aquatic communities was weakened by large spatial and temporal variability in the distribution and abundances of many species. This variability was predicted to afford the invertebrate and fish community resilience to physical disturbance. The second component of this study examined the effect of disturbance by gravel mining from an exposed bar in 2000. Physical changes immediately after mining were substantial as the pre existing cobble surface was replaced by loose gravel and sand. Two subsequent freshets transformed the site into a topographically complex area with similar substrate texture as before mining, although sediment replenishment to the site was negligible. Habitat availability at flows <3000 m3 s-1 increased and habitat quality was comparable with reference sites. A third above-average freshet replenished 31% of the removal volume and restored average bar surface elevation to within 9 cm of the pre-scalped surface. Physical changes elicited a significant reduction in invertebrate density immediately after mining, however, the impact lasted less than one freshet cycle. No change in fish density as a result of mining was found, although statistical power to detect an impact was low and fish sampling was carried out at flows <5700 m3 s-1. These results support the expectation that the invertebrate and fish community in the gravel reach has resilience to disturbance from a single gravel removal provided that site recovery by way of sediment transport and replenishment occurs.

Text

2011-10-14

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http://hdl.handle.net/2429/37993

Geography

University of British Columbia

Graduate