Quamichan Lake Watershed

In the summer, a warm surface layer develops in the upper 3m of Quamichan Lake. The surface water contains high levels of dissolved oxygen, while the denser and cooler bottom layer becomes isolated from the surface and loses oxygen. The dissolved oxygen at depth is consumed by bacteria like E. coli that eats detritus. This creates extremely low oxygen levels in the deep lake. These layers have little mixing due to their differences in water temperature and density.

When the oxygen at depth is completely depleted, changes in chemistry in the lakebed result in phosphorus being released into the water column and making its way slowly to surface waters where it can fertilize algae production. This phosphorus is estimated to contribute 90-95% of the nutrients causing cyanobacteria blooms in the summer. Aquatic scientists use the term ‘internal loading’ to refer to places like Quamichan Lake where the majority of nutrients feeding algae blooms in a lake originate in the lakes’ own sediment instead of external sources. The phosphorus in the lakebed has accumulated over the past 150 years of runoff from deforested, farmed, and developed lands in the Quamichan Lake watershed combined with the lake having originally been in a relatively high nutrient state before intensive land development. 

Phosphorus and nitrogen are essential nutrients for plants and algae. Cyanobacteria, however, have are adapted to bloom when there is more phosphorus than nitrogen. Large amounts of phosphorus, released from the lakebed in the summer and fall, have thus led to cyanobacteria blooms in Quamichan Lake. Reducing the release of phosphorus from the lakebed should therefore reduce the intensity of summer/fall cyanobacteria blooms. 

• At least four dogs died after swimming in Quamichan Lake during a cyanobacteria bloom. Certain types of cyanobacteria can produce toxins which are harmful to humans, pets, livestock, and wildlife if ingested.
• In response, the Mayor assembled a Quamichan Lake Water Quality Task Force, composed of stakeholders and lake experts, to develop a list of actionable recommendations with priority on establishing a comprehensive water quality monitoring program.

A consultant monitored water quality throughout the summer and prepared a preliminary assessment. The report was presented to Council and had four major recommendations:

• Development of a multi-year plan for implementing a phosphorus mitigation and water quality 
  monitoring program,
• Commitment of significant financial and human resources to mitigation and monitoring work, 
• Partnering with federal and provincial agencies to offset financial and personnel costs, and
• Development of a program to control phosphorus input to the lake by working with the agricultural community.

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Following the assessment report, staff was directed by Council to establish a comprehensive water quality monitoring program for the lake and tributaries. This baseline is essential to understand lake dynamics, monitor changes overtime, and design remediation systems to improve water quality. The monitoring program is ongoing.

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A consultant conducted a study which involved extracting sediment cores from the lake to assess the applicability of phosphorus binding treatments. The study concluded that the vast majority of phosphorus causing cyanobacteria blooms came from the lake sediment. This conclusion substantiates the findings of the 2019 study. 

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• A consultant conducted a Sediment Oxygen Demand study on the lake sediment to assess the potential effectiveness of a hypolimnetic aeration system. The study concludes that, in addition to aeration, metal amendments will be needed to augment phosphorus remediation. 

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• Staff updated the bathymetry (depth profile of the lake), initially done in 1972. This is essential for calculating the volume of water that needs to be aerated. 

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• Staff presented to Council to provide an update on the status of water quality and seek direction on conducting an aeration trial in Quamichan Lake. Council voted to support the project. 
• Staff received approval from the Provincial and Federal Governments to conduct the aeration trial with accompanying nutrient binders.
• Staff presented to the Cowichan Valley Regional District Committee of the Whole to seek funding for $200,000 from the Drinking Water and Watershed Protection Program budget to be allocated to the aeration trial.

Staff and consultants will be conducting an aeration trial in Quamichan Lake from April to September 2026. To learn more about the aeration trial, visit the Connect North Cowichan Aeration Trial webpage.

According to Island Health, cyanobacteria is present in many urban lakes on Vancouver Island and the Gulf Islands. The abundance and species mix of cyanobacteria in a lake can vary dramatically depending on the environmental conditions present in and around a lake.

Island Health is responsible for advising the public as to whether or not a particular waterbody is safe for use by the public. Those who are recreating on or around Quamichan Lake should be aware of Island Health’s advisories relating to cyanobacteria.

The bottom of Quamichan Lake is owned by a private landowner, and water resources in BC are currently under the mandate of the Ministry of Forests, Lands and Natural Resource Operations. As such, North Cowichan has no jurisdiction on the lake. Despite this, the municipality understands the importance of this lake to residents and First Nations. People use Quamichan Lake for various forms of recreation in addition to it being an important resource for local wildlife. Council has, therefore, directed staff to research options to improve water quality conditions with the intent of attracting support from higher levels of government.

North Cowichan is sampling the lake; however, the presence or absence of toxins can vary at any given location or time so the testing methods available are not a reliable means of determining whether or the water is harmful at a particular location or time. Furthermore, weather, boat traffic, and other environmental factors can move algae and toxins around the lake unpredictably. For this reason, North Cowichan has been advised by Island Health to place permanent signage at access points around the Quamichan Lake in order to advise the public of the ongoing potential for exposure to toxins. 

Fish kills can occur in shallow, low elevation lakes and are usually caused by high temperature or low dissolved oxygen. Either of these two factors can lead to fish mortality and, because they often occur at the same time of the year, they can magnify each other’s negative effects. Neither of these pose an increased risk to human health when safely recreating on the lake. Such fish kills have been observed in Quamichan Lake at least twice in the last decade.