Water Quality Assessment and Objectives for the Cowichan and Koksilah Rivers 2011

Water Quality Assessment and Objectives
for the Cowichan and Koksilah Rivers
First Update August 2011

This report summarizes the findings of the 2002, 2003 and 2008 water quality objectives attainment monitoring of the Cowichan and Koksilah Rivers, located near Duncan on Vancouver Island, British Columbia. The sampling program was based on the Cowichan-Koksilah Rivers Water Quality Assessment and Objectives report (McKean, 1989). At that time, forestry, agriculture, industrial operations and residential development were all occurring within the watershed, and there was a concern that these would influence water quality in the future. To date, these activities are still occurring and some are likely contributing to the water quality objectives exceedances observed in the Cowichan and Koksilah Rivers.

Less than half of the parameters sampled met the water quality objectives. Dissolved oxygen, non-filterable residue (NFR) and turbidity objectives were often exceeded in both rivers. A habitat improvement project in the Cowichan appears to have remedied the suspended sediment issue in this river, while the problem persists in the Koksilah River. Fecal coliform and Escherichia coli results exceeded the objectives on most occasions and at most sites, and have been attributed primarily to non-point sources. Chlorophyll a objectives were occasionally exceeded in the lower part of the Cowichan River.

The objectives for bacteriological parameters, NFR, turbidity and metals have been updated based on changes to the provincial guidelines. New temperature and total phosphorus objectives are also proposed. The objectives for total residual chlorine and copper-8-quinolinolate have been removed as they are no longer relevant.

Recommendations include identifying and addressing the sources of elevated turbidity and NFR in the Koksilah River, and using these parameters as a surrogate for other parameters of concern. In addition, a mitigation plan should be developed for bacteriological contamination and phosphorus inputs in both rivers. Finally, benthic invertebrate sampling should be incorporated into the Cowichan and Koksilah Rivers monitoring program.

The updated water quality objectives for the Cowichan and Koksilah Rivers are listed in the table on the following page.


Download the report here.

Document Date: 01 Aug 2011
Author: Nicole Obee