Citation | Northwest Hydraulic Consultants Ltd. 2012. Cheakamus River Water Use Plan, Monitoring Channel Morphology in Cheakamus River, Implementation Year 5, Study Period: April 2008-April 2012. Prepared for BC Hydro. |
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Organization | BC Hydro |
URL | https://www.bchydro.com/content/dam/BCHydro/customer-portal/documents/corporate/environment-sustainability/water-use-planning/lower-mainland/cmsmon-8-yr5-2014-05-07.pdf |
Abstract/Description or Keywords | Following implementation of a new WUP flow regime in February 2006, the Cheakamus WUP Consultative Committee expressed concern regarding the potential effects on salmonid habitat quality and availability from physical changes. To address these concerns, baseline and post-‐WUP monitoring was recommended to examine the effects of more frequent, higher flows that would mobilize and distribute sediment introduced by large floods, debris flows and mass wasting. The monitoring program also examined seasonal minimum flow releases from Daisy Lake Dam and access to side channel habitats. Cheakamus River extends 25 km from Daisy Lake Dam to the confluence of Squamish River. The channel can be broadly characterized by four distinct sections: Rubble Creek landslide deposits, a bedrock canyon, a broad alluvial section and the Cheakamus River fan. These sections were divided into 13 distinct reaches above the Cheakamus fan that reflect differences in slope, morphologic characteristics, sediment supply, and discharge. Within each reach, a baseline morphologic map was prepared from 1:5,000 colour orthophoto mosaics collected during low water conditions in April, 2008. The river was re-‐flown at the same scale in March, 2012 and the mapping exercise was repeated to illustrate the types and magnitude of channel changes during the past four years. The wetted low flow channel dominates the areal fraction of most reaches, and is representative of the available overwinter habitat for salmonids. This low water wetted channel area has remained constant (averaging 59% of the total channel area) since 2008 indicating no substantive changes to the channel characteristics since implementation of the WUP. Unvegetated bars are the second most common morphologic feature in most reaches, averaging 28% of total channel area in 2008 and 23% in 2012. Together, unvegetated bars combined with the low flow wetted channel represented roughly 90% or more of total reach area in 9 of 12 reaches in 2008. The dominance of bar and wetted channel areas declined to 7 of 12 reaches in 2012. The loss of open bar surface area is not due to erosion or degradation, but to the establishment and growth of vegetation on previously exposed bar areas. The increase in the mean area of young vegetation on formerly bare bars was statistically significant. There was also an increase in the mean area of young and mature islands – although not significant – which supports the observed trend that the channel is becoming increasingly stable. Unperturbed, the expansion and maturation of vegetation is expected increase bank strength, increase channel depths and reduce channel complexity, resulting in a loss of natural side channel habitat. Large sediment inputs from Rubble Creek or other tributaries could reverse or modify the established trend and help mitigate the effect of sediment supply reductions from the construction of the Daisy Lake Reservoir. The low flow wetted channel was further divided into three (3) hydraulic mesohabitat units: pools, riffles, and rapids. Riffle habitats were dominant in mainly unconfined reaches, and rapids dominant in steeper, confined channel sections. Between 2008 and 2012 the distribution of pools, riffles and rapids remained relatively stable along the study reach. The variability in the size of the mapped units also remained the same (p <0.05). Overall the Cheakamus River maintained a consistent morphology and distribution of habitat and channel units from 2008 to 2012. During the same period relatively few sediment supplying events occurred in the watershed. To better understand the linkages between the habitat and fish productivity the long-‐term study design should attempt to inventory overall mesohabitat proportions in key alluvial reaches of the Cheakamus River, as this was also undertaken by the Fisheries Technical Committee prior to the WUP. These reaches should be selected based on concurrent fish use, populations and habitat quality studies undertaken as monitors under the WUP to allow some comparative analyses prior to review of the WUP. Additional work could be undertaken to inventory and monitor natural side channels on the alluvial reaches of the river, and document their change through time as it relates to flood history and geomorphic changes in the river. Side channels have a high utilization by salmonids in the Cheakamus River, and are key habitats for some life stages and species. However, since these are side channels are located a ways downstream of the Daisy Lake Reservoir, separating project effects from the influence of unsteady tributary sediment and debris inputs will be difficult. Additional data should be collected on sediment quality as it relates to substrate use by rearing and spawning salmonids. Changes to the distribution and quality of spawning substrates within the Cheakamus River area especially critical to chum and pink salmon, and are influenced by tributary and lateral supply and mobilization due to high flows. Further mapping and sampling should be completed, correlated to documented spawning sites, to provide direct data on spawning substrate conditions and how it changes over time. In summary, Based on 2008 and 2012 air photos the overall morphology of the Cheakamus River has been shown to be reasonably stable, but a more detailed set of data looking at substrate quality and quantity at key biologically important sites is missing. Such data should be collected to characterize both the general substrate conditions that will effect habitat use by parr and fry, as well as spawning substrate surveys that characterize the availability and quality of the substrate. Inputs from tributaries also need to be qualitatively monitored to assess the temporal trends in supply and explain the observed channel changes. |
Information Type | report |
Regional Watershed | Howe Sound & Sunshine Coast |
Sub-watershed if known | Cheakamus River |
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Comments | |
Project status | complete |
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