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Salmonid distribution in relation to stream temperatures in Fortune Creek, British Columbia : the influence of surface water and groundwater interactions McGrath, Elinor Olivia


There is concern over the impacts of increased water temperature on salmonid populations in British Columbia’s (BC) Southern Interior. Groundwater influx may moderate stream temperatures and provides thermal refuge for salmonids. The objective of this study was to establish quantitative linkages among groundwater, water temperatures and juvenile coho salmon (Oncorhynchus kisutch Walbaum), Chinook salmon (Oncorhynchus tshawytscha Walbaum) and rainbow trout (Oncorhynchus mykiss Walbaum) in Fortune Creek, a small regulated stream in BC’s Southern Interior. In addition, this study quantified whether groundwater influx could provide thermal relief to salmonids during periods of heat stress. Salmonid enumeration and field data collection were carried out between July 2008 and June 2009. Generalized Linear Mixed Models (Poisson and logistic regression) were used to establish linkages between juvenile salmonids and physical and chemical fish habitat indicators. Thermal modeling of vertical groundwater fluxes in the streambed was conducted using the software VS2DH, which simulates fluid and conductive-convective heat transport. The relative influence of groundwater and stream discharge on stream temperatures was assessed using a conceptual energy balance approach. Statistical modeling confirmed that the distribution of juvenile salmonids was consistently related to water temperatures. Salmonids avoided reaches where maximum daily water temperatures exceeded 22°C, which was exceeded at most of the sites studied. Relationships to water chemistry were less consistent and existed almost exclusively in combination with water temperature variables. While groundwater flux to the stream was widespread, estimated upward fluxes were low (10-7 m/s) and were primarily constrained by low streambed hydraulic conductivities. The influence of advective cooling on stream temperatures in unshaded reaches was too small to provide significant thermal relief for salmonids. However, groundwater influx provided up to 88% of baseflows during the summer low flow season and was critical in maintaining flows in the creek. Conceptual energy balance modeling confirmed that low flows in Fortune Creek limit juvenile salmonid rearing by causing water temperatures to escalate. Recommendations for maintaining suitable water temperatures for juvenile salmonids include re-vegetation of the banks to provide shade and strategically supplementing flows to increase discharge at times of high water temperature potential.

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