You can renovate a cottage. You cannot renovate a lake. In Ontario, much of the information you need is public and free.
Water clarity and Secchi depth
Clarity is measured with a simple tool called a Secchi disk. It is a flat black and white disk on a rope. You lower it into the water and note the depth at which it disappears. That depth, in metres, is the Secchi depth. A bigger number means clearer water.
Clarity is useful, but it is not the whole story. Some healthy lakes are naturally tea-coloured from the wetlands and forest around them. Some lakes have become clearer because invasive zebra mussels filter the water, which is not a sign of good health. Look at how a lake’s clarity has changed over many years, not at a single reading.
Phosphorus
Phosphorus is the nutrient that feeds algae and weeds. It reaches lakes from septic systems, fertilizer, eroding shorelines and runoff. More phosphorus generally means more algae, greener water and less oxygen in deep water for fish such as lake trout.
Results are reported as total phosphorus in micrograms per litre. Lake scientists use it to sort lakes into three broad groups.
| Total phosphorus | Term | What it usually means |
|---|---|---|
| Under 10 micrograms per litre | Oligotrophic | Low nutrients, clear water, little algae |
| 10 to 20 micrograms per litre | Mesotrophic | Moderate nutrients, some weeds and algae |
| Over 20 micrograms per litre | Eutrophic | High nutrients, more weeds and a higher chance of algae blooms |
None of these is a pass or fail. A shallow, productive lake can be excellent for fishing. Match the kind of lake to how you plan to use the water.
The Lake Partner Program
Most of Ontario’s long-term data on cottage lakes comes from the Lake Partner Program. It is a volunteer monitoring program coordinated by the Ontario Ministry of the Environment, Conservation and Parks from its Dorset Environmental Science Centre, in partnership with the Federation of Ontario Cottagers’ Associations. It has run since 1996.
Volunteers collect water samples and take Secchi readings on hundreds of inland lakes. Lakes on the Canadian Shield are sampled once in spring. Lakes off the Shield are sampled monthly from May to October. The ministry’s lab in Dorset tests the samples.
The program reports on:
- Total phosphorus
- Water clarity, from the Secchi readings
- Calcium, which many small lake animals need and which has been falling in some Shield lakes
- Chloride, which mostly comes from road salt
- Sulphate, a more recent addition
The results are published in the Ontario Data Catalogue under the name Ontario Lake Partner and are updated yearly. The cottagers’ federation also hosts a map-based view of the data. Check the township as well as the lake name, since many Ontario lakes share a name.
Not every lake is in the program. If yours is missing, ask the lake association about its own testing.
Blue-green algae advisories
Blue-green algae are bacteria that occur naturally in lakes. In warm, calm, nutrient-rich water they can multiply into a bloom that looks like green pea soup or spilled paint. Some blooms produce toxins that can make people and pets sick. Ontario says blooms are most common in late summer and early fall.
The province advises people to assume toxins are present and to stay out of the water during a bloom. Boiling does not remove the toxins, and home treatment systems may not either. That matters if the cottage draws its drinking water from the lake.
Local public health units issue the advisories and post them online, and many keep a record of past years. Suspected blooms are reported to the province’s Spills Action Centre.
One confirmed bloom does not mean a lake is ruined, and Ontario notes that a lake with one bloom may not have another. A pattern of blooms year after year is a different matter.
Invasive species
Zebra mussels, spiny water flea, Eurasian water-milfoil and other invaders change how a lake looks and how it can be used. Mussels cut bare feet and clog water intakes. Milfoil forms thick mats that make swimming and boating hard.
Ask the lake association which species are present. Since January 1, 2022, Ontario law has required boaters to pull drain plugs and remove plants, animals and algae before moving a boat over land.
Lake associations and lake plans
A lake or cottagers’ association is the best single source on a lake. Many publish water quality reports, newsletters and fish stocking records. Some lakes also have a lake plan, a document written by the community that sets out the lake’s condition, its problems and what residents want protected.
Motor restrictions
Boating rules are federal. Across Ontario, power boats are limited to 10 km/h within 30 metres of shore, with some exceptions such as pulling a skier straight out from the dock. On top of that, federal regulations list individual lakes with their own limits. Some ban all motors, some allow electric motors only, some cap engine power or speed, and some limit the hours for water skiing and wake surfing.
Check whether the lake is listed before you buy a boat or a cottage. A motor ban is a selling point if you paddle and a problem if you want to wakeboard.
Where to find the data
- The Ontario Data Catalogue, for Lake Partner Program results
- The local public health unit, for algae advisories and beach postings
- The lake association’s website, for reports and a lake plan
- The township and the conservation authority, for shoreline rules and watershed report cards
- Transport Canada’s vessel operation restrictions, for boating limits
Then go and look. Visit in August when the water is warmest. Check the shoreline for weeds and scum, and ask neighbours what has changed.
For more on shorelines and water systems, read Waterfront 101 and septic, wells and water, or browse cottages by lake.