The Lake of the Woods is an important natural resource for Canada and the United States. It is:

  • a world class fishery
  • a significant tourism destination
  • a drinking water source for more than 700,000 people in Ontario, Minnesota and Manitoba

Environmental challenges

High levels of nutrients, like phosphorus, in the lake cause extensive growth of blue-green algae. This can be toxic to fish, birds, animals and people. At some times of the year algal blooms can cover up to 80% of the lake’s surface. This can cause problems for recreational use of the water. It can also cause taste and odour problems in drinking water while destroying lake trout habitat.


Most of the nutrients in the Lake of the Woods (up to 75%) come from the Rainy River. In the past, pulp and paper companies and municipal sewage were the biggest sources of nutrients to the lake. But stricter regulations introduced in the 1970s and 1980s have since controlled these sources.

Today, most nutrients enter the Rainy River through natural sources like rainwater, air and sediment. They can also come from farming, faulty septic systems and gardening or lawn care practices.The nutrients problem in the Lake of the Woods is complicated. Some of the sources are not well understood, and difficult to measure. Some sources can be controlled, and others can’t. Scientists are trying to understand the role of invasive species and climate change and how they may be making the problem worse.

What Ontario is doing

Ontario is one of many government departments in Canada and the United States working to understand and develop action plans to deal with the nutrient problem in the Lake of the Woods watershed through science, compliance and collaboration. Ontario is a:

Promoting compliance

Ontario is promoting compliance with our environmental laws by inspecting communal sewage facilities in the Lake of the Woods watershed. We also respond to pollution incidents, spills and public complaints to protect against further decline in water quality.

Lake of the Woods Watershed Stewardship Strategy

The Ministry of Environment and Climate Change developed a Lake of the Woods Watershed Stewardship Strategy to provide guidance and:

  • promote compliance with environmental laws
  • build and enhance partnerships
  • provide outreach and education
  • support international cooperation and data sharing

Building scientific knowledge

Through Ontario’s Lake Partner Program, local volunteers check water quality at more than 100 collection stations in the Lake of the Woods watershed. Water samples are used to monitor and assess nutrient levels in the lake.

Ontario conducts annual water quality monitoring from the Canadian rivers and streams that flow into the Lake of the Woods watershed and Rainy River. The information helps fill gaps in our understanding of the sources and quantities of nutrients coming into the watershed. It also shows changes in water quality and algae in the lake over time.

Next, Ontario plans to strengthen international partnerships related to water quality research and management.


The lake drains into the Winnipeg River and all waters flow north as part of the Arctic Watershed, eventually draining to Hudson’s Bay. About 66% of the lake is in Ontario. The Rainy River is the largest tributary to the lake, and contributes more than 70% of the inflow.

The entire Lake of the Woods and Rainy River watershed is the size of New Brunswick (72,000  square km). There are more than 25 First Nations and tribes within or near the watershed area.