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Report: Cowichan River: Summary of Historical Disturbances, Water Use Pressures and Streamflow Trends

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This report summarizes some of the major disturbances and water use pressures that the Cowichan River has faced over the last century, and presents a characterization of water and climate as well as associated trends for the area. The Cowichan River will require increased attention in the future to mitigate existing water stressors, habitat disturbances, continued water pressures and ever changing natural drivers.

Author:  Pike, R; Young, E; Goetz, J; Spittlehouse, D

Date Published:  May 2017

Report ID:  54179

Audience:  Government and Public

This study analysed trends from 1965-2015 for various surface and climate variables for the Cowichan basin. There were statistically significant trends in rising annual, January, June and July air temperatures. Trends in groundwater detail particular wells are in a state of decline, as are surface waters, particularly with decreases for July, August and September. Disturbances and water pressures in the Cowichan River basin have had a cumulative influence on the hydrologic regime of the river. Prior to the 1850's the Cowichan River was most likely in a natural state with limited amounts of disturbances along its riparian areas and within the stream channel. The first large-scale disturbance was a period of log driving from 1890 to 1908, for which explosives were used to remove rocks, hang-ups or other impediments to downstream log movement. This resulted in major changes in channel morphology of the Cowichan River. Also, for several decades up until the mid-1960's, dredging was performed as a flood control measure in the lower river floodplain, near Duncan. Materials dredged from the streambed likely cut off many side channels, thereby affecting access to important fish habitat. The installation of a weir at the outlet of Cowichan Lake in the late 1950;s was the most significant alteration of the flow regime. After weir construction summer low flows became less severe, almost by an order of magnitude. Weir control, however, has not been the only recent influence on streamflow; surface water diversions have increase substantially over the last 65 years as the number of surface water licences and groundwater wells increasing from 1954 to 2012. Also, there have been several inputs to the Cowichan River from sewage and aquaculture operations. Human pressures on groundwater resource use from industrial, aquaculture, agricultural and urban developments have also been increasing in the Cowichan basin. In general, groundwater levels in the lower Cowichan basin display a seasonal fluctuation that is in a synchronous pattern with surface water discharge. No statistically significant trends for chances in precipitation. No snow data was available for analysis.

Report Type
  Meteorological - Rain or Snow - Rain
  Region - Vancouver Island
  Watershed Groups - 920 - Vancouver Island (East) Rivers
  Water Information - Groundwater
  Water Information - Hydrometric / Hydrology

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