Ambient Water Quality Assessment and Objectives for the Cowichan and Koksilah Rivers

This report assesses the water quality of the Cowichan and Koksilah rivers located on the south-eastern side of Vancouver Island, and sets water quality objectives to protect their uses for drinking, irrigation, industrial consumption, water contact recreation and aquatic life.

The Cowichan River, which originates at the south end of Cowichan Lake, is the most important river on Vancouver Island for recreational fisheries (steelhead, cutthroat, rainbow, brown trout and kokanee) and commercial fisheries (coho, chinook and chum). The river is also important for water contact recreation such as kayaking, canoing, etc. and as a source of water for domestic, irrigation and industrial uses (BC Forest Products Crofton Pulp Mill). The Village of Lake Cowichan obtains its water supply from Cowichan Lake while the City of Duncan obtains its water from wells adjacent to the Cowichan River downstream from the Island Highway.

The Koksilah River is also important for recreational and commercial fish species as well as recreational activities and domestic, irrigation and industrial water supplies. However, since the river does not have a large lake within its watershed to provide water storage, summer stream flows are approximately 95% lower than flows in the Cowichan River. Consequently, the value of the above uses in the Koksilah River are reduced relative to the Cowichan River.

There are three permitted discharges on the Cowichan River. The sewage discharge from the Village of Lake Cowichan and the discharge of ground water used in a Ministry of Environment fish hatchery have little of no impact on the water quality of the Cowichan River. The third permitted discharge is sewage from the Duncan/North Cowichan treatment plant. The effluent is not adequately diluted in the Cowichan River during the summer low flow period. High concentrations of nitrogen and phosphorus cause excessive periphyton growth for several kilometers downstream from the discharge.

Water quality objectives are set in the Cowichan River for a number of possible contaminants. These originate from a sawmill located at Youbou, a small abandoned base metal mine adjacent ot Cowichan Lake, future base metal mining in the upper Cowichan watershed, public wharves near the outlet of the lake and the sewage discharges to the river.

Water quality objectives are set in the Koksilah River to protect water uses from two sources of contamination: potential bacterial contamination from dairy farms near Dougan Lake and contaminants from the Cowichan Valley Regional District incinerator/landfill and gravel washing operations near Duncan.

Water quality objectives are set to protect the most sensitive water uses in both rivers. The objectives and a monitoring program for the Cowichan and Koksilah rivers are summarized in Table 1, Table 2, Table 3 and Table 4.

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