Results from the District of Muskoka’s 2025 Water Quality Monitoring Program show increasing chloride levels in lakes across the region.
According to the district, lakes that are near highways and commercial areas, where road salt is frequently applied, are the ones that present the highest levels of chloride.
The results show that in 2025, 28 per cent of sampled sites recorded chloride levels above 10 milligrams per litre, a threshold that can harm sensitive aquatic life.
Additionally, 72 per cent of sites had chloride levels above their 10-year average.
Officials said the results also show “an ongoing concern” with calcium levels declining in Muskoka’s lakes.
In 2025, 31 per cent of sampled sites had calcium levels below two milligrams per litre, which can harm sensitive aquatic life. Over the past decade, calcium levels declined at 94 per cent of sampled sites.
The District’s long-term data confirms the trend, with 80 of 194 monitored sites recording declining calcium concentrations between 2001 and 2025.
Although the decline appears to be slowing, officials said many lakes remain below healthy calcium levels.
Officials said low calcium levels combined with high chloride concentrations can be particularly harmful to sensitive aquatic life, making ongoing monitoring important for understanding changes in Muskoka’s lakes.
The annual program helps identify water quality concerns and highlights the importance of preserving natural shoreline buffers and reducing road salt use to protect lake water quality and aquatic life.
“Each year, our monitoring program gives us valuable insight into the health of Muskoka’s lakes and helps us make informed decisions about where action is needed most,” said David Zilkey, Manager, Watershed Programs. “The 2025 results continue some encouraging trends, while also reinforcing the importance of advancing stewardship across the watershed.”
Officials said lake water sampling for 2026 is complete, with results expected later this year. They’ll be included in the 2027 annual water quality report.
