| Abstract/Description or Keywords |
The impact of fine sediments on salmonid egg incubation habitat in the Cowichan River is an ongoing concern among members of the Cowichan Stewardship Roundtable. High inputs of fine sediments, typically from excessive bank erosion, may infiltrate the interstices of the riverbed spawning gravel and reduce egg-to-fry survivals. Although sediment inputs from naturally eroding banks are apparent along the mainstem, several sites stand out as having larger areas of fine sediments and which are or have the potential to be significant sources of fine sediments to the river. For example, Stoltz Bluff has been identified as the most significant single source of fine sediments to the river (LGL and KWL 2005; KWL 2005). Rehabilitation measures were constructed at Stoltz Bluff in 2006 to stabilize the toe of the Bluff thereby reducing sediment inputs from this chronic erosion site. Significant erosion sites have also been identified further upstream above Skutz Falls. An assessment of these erosion sites was completed in 2008 (Gaboury et al. 2008). In the assessment, the composition of the eroding banks, the mechanism of bank failure, the relative rate of bank erosion, and the existing or potential risk to downstream fish habitat was investigated and evaluated. The recommendations from the report provided the basis for focused assessments that have followed in 2009. The recommendations included: 1. Investigate rehabilitation approaches that are less intrusive and less costly near Sites 5 & 7 2. Implement two activities at Site 9: a. characterize the unstable bluff to obtain a better understanding of the scale and potential impact of the hazard; b. explore rehabilitation opportunities for river re-alignment by moving the main river channel to the east (left bank) and away from the toe of the bluff; 3. Investigate the feasibility, cost and benefits of re-aligning the river near Site 8. This memorandum addresses the recommendations 1, 2b and 3 from Gaboury et al. (2008) and presents the results of assessments that focused on identifying opportunities for implementing rehabilitation measures at sites where the existing or potential risk of negatively affecting fish and fish habitat was considered high. The memo also presents conceptual rehabilitation designs to address these high risk sites in the Block 51 area. For this project, the area under consideration is restricted to the Cowichan River mainstem and between Site 5 and Site 9 (Broadway Run). This project was co-funded by Living Rivers-Georgia Basin/Vancouver Island (2006-11), a program of the BC Living Rivers Trust Fund. |
