Water Stewardship Information Sources

Citation Dessouki, TCE. 2010. Water quality assessment of the Cowichan and Koksilah Rivers. BC Ministry of Environment.
Organization Ministry of Environment
URL http://www.env.gov.bc.ca/wat/wq/pdf/cowichan-koksilah-assessment.pdf
Abstract/Description or Keywords The Cowichan and Koksilah rivers are both located on the south-eastern coast of Vancouver
Island, B.C. Both watersheds are adjacent to one another, originate in the Vancouver Island
Ranges and share very similar weather patterns. The Cowichan River watershed, which
includes Cowichan Lake, drains an area of 939 km2 while the Koksilah River watershed drains
an area of 302 km2
.
There is currently a federal-provincial water quality monitoring station located on each of the
rivers near their outlets to Cowichan Bay. Both sites have been monitored as federal-provincial
stations since 1999. Water quality objectives were established for both rivers in 1989 to protect
for various water uses including domestic and industrial uses and aquatic life. While permitted
discharges occur only in the Cowichan River, both rivers are impacted by forestry, agriculture,
on-site sewage disposal and urbanization.
Although both sites have been monitored since at least the 1980s, only the previous 10 years of
data have been analysed for the purposes of this report as the more recent, routinely collected
data is more suitable for status and trends assessment.
CONCLUSIONS
 Water quality in the Cowichan and Koksilah rivers was typically good.
 Aluminum, copper and iron seasonally exceeded relevant guidelines and objectives, but
these exceedences were generally related to increased seasonal flow and not considered
a water quality concern.
 Cadmium rarely exceeded the CCME guideline for the protection of aquatic life and
exceedences coincided with turbidity in most cases. It is recommended that dissolved
cadmium be measured in addition to total to ensure that exceedences are related to
suspended sediment and not to increases in dissolved portions of the analyte. Fecal Coliform levels continue to be an issue in both the Cowichan and Koksilah rivers,
and levels were significantly increasing in the Cowichan River over the sample period at
around 1 CFU per 100mL per year. Fecal coliform measurements exceeded the objectives
established for the Koksilah River. It is recommended that the shellfish harvesting
activities in Cowichan Bay be considered if the water quality objectives for the Cowichan
and Koksilah rivers are updated in the future.
 Manganese seasonally exceeded the aesthetic drinking water objective.
 Dissolved oxygen exceeded the seasonal objectives established for the two rivers. Water
temperatures also generally exceeded guidelines, although the magnitude and duration
of these exceedences were not measured. Increases in these parameters may impact
fisheries, especially if air temperatures and river flows change.
 Dissolved phosphorus concentrations in the Cowichan River were negatively correlated
with flow and tended to increase during the summer months.
 There was a statistically significant increase in turbidity in the Koksilah River over the
sample period although the rate of increase was not very large. This may result in
increased loadings of associated metals and nutrients due to increases in suspended
sediment.
 Since both watersheds are adjacent to one another, they share very similar weather
patterns. As a result, there was a high degree of correlation in flow between the two
rivers. This resulted in a high degree of correlation in flow-associated parameters in both
rivers (i.e. various total metals) and flow in one river generally correlated well with
certain metal concentrations in the other river (coherence).
 The Koksilah River appears to be more greatly influenced by groundwater, where as the
Cowichan River is influenced by Cowichan Lake, a large headwater lake.
Information Type report
Regional Watershed Vancouver Island South
Sub-watershed if known Cowichan, Koksilah
Aquifer #
Comments
Project status complete
Contact Name
Contact Email