Nurturing forest health may ameliorate spring flooding: Muskoka Watershed Council
By Norman Yan |Saturday, March 28th, 2026
It’s quite likely that we have all been taught things in school that turn out to be wrong.
For example, in 1970, I took a fourth-year course on lakes at the University of Toronto from Frank Rigler, the “Dean” of Canadian limnology at the time. He taught us that Canadian Shield lakes were naturally a bit acidic.
That was wrong, but the widespread nature of acid rain wasn’t announced to the world until 1972. In the Muskoka region, it wasn’t until 1978 that acid rain was first reported by scientists. That understanding overturned past assumptions of what controlled the chemistry of Muskoka’s lakes, and there were broader implications for our ecosystems.
There may even be consequences for the understanding and management of spring floods, a clear concern in Muskoka at this time of year.
Floods cost Canadians almost $3 billion annually, virtually all of the damage occurring in the spring. Muskoka is not exempt. The 2019 flood damaged over 1,000 properties locally and led to a declaration of a state of emergency in Bracebridge. The causes of that flood are relatively well accepted — an unusually deep snowpack, a cold spring that delayed the melt, followed by heavy rains on the snow.
Of course, the snowpack melts every spring, but it would help to know how to reduce the chances that these melts lead to damaging floods. Clearly, some management of our behaviour could help. Reducing development in flood-prone areas will reduce financial damage, and planning around the reality of seasonal water level changes will help us all make better decisions about built infrastructure. However, nurturing our forests may also help.
Intact forests reduce flood risk. In the late spring, summer and fall, forests act as natural sponges, absorbing and storing rainwater, and slowing runoff, increasing infiltration and preventing erosion. That’s well and good, and widely understood. Perhaps surprisingly, intact forests can also reduce the threat of spring floods. Their canopies, especially conifer canopies, provide shade, slow snowmelt and increase water retention. They may also reduce ground freezing, increasing soil infiltration of meltwaters.
But forest health, not just cover and species composition, may also matter. There may be a link between the past blight of acid rain and the severity of spring floods, tied to the widespread forest calcium decline caused by acid rain, and the influence of this loss of calcium on tree transpiration.
For that story, we turn to a catchment-scale experiment done in the Hubbard Brook Experimental Forest in New Hampshire — a forested area, like Muskoka, that was impacted by acid rain. To understand how the acid-induced leaching of calcium from soils had influenced forest health, scientists at Hubbard Brook restored the lost soil calcium, adding 0.75 tonnes of calcium per hectare to an 11-hectare sugar maple stand. To their surprise, evapotranspiration rates increased by 18 to 25 per cent.
Replacing the lost calcium “woke the trees up.” They pumped more water from the soil and into the atmosphere through their leaves, leaving more room for infiltration of rainwater into the soil and reducing the risk of flooding. It turns out that all the prior assumptions of what “normal” stream flow was like were in error.
Normal stream flow in that forest was 18 to 25 per cent less than what had been observed in the years before the addition of calcium because, under conditions of calcium deficiency, the trees were transpiring less water than normal. Acid rain reduced tree health and less water was being transpired.
So nurturing our mixed forests in Muskoka just might help reduce the severity of spring floods by reducing peak meltwater spikes. Friends of the Muskoka Watershed runs a volunteer program to recycle firewood ash to restore soil calcium levels in our forests. Beyond nurturing our forests, the ASHMuskoka program might also be helping to ameliorate our spring floods. In an ecosystem, everything is connected to everything else and surprising links, in this case, tree health and spring floods, should be expected — another good reason to nurture our forests.
This article was first published by MuskokaRegion.com.
Norman Yan
This is article No. 13 in the current series, Nurturing Our Watersheds, from Muskoka Watershed Council. Its author is Norman Yan, aquatic environmental scientist, long-time Muskoka resident, and a director of Friends of the Muskoka Watershed. The series is edited by Peter Sale.