In summer, Quebec's rivers warm to levels at which Atlantic salmon, accustomed to cool water, find themselves under threat. Just a few extra degrees turn their usual environment into an unfavorable one: the fish grow weak, feed poorly and cannot reproduce normally.
One way to hold on to the cool is to give the banks back to the trees. Canopies over the water cast shade and shield the river's surface from the sun, so riverside forest works as a natural air conditioner for the stream. But where exactly should trees be planted for the effect to be noticeable? That question was investigated on the Dartmouth River on the Gaspé Peninsula, which is known for its Atlantic salmon population and for recreational fishing.
The study was carried out at the Eau Terre Environnement Research Centre at INRS with funding from the Atlantic Salmon Conservation Foundation. The scientists used LiDAR, a remote sensing technology that builds a three-dimensional model of the terrain and vegetation. The analysis was performed at a resolution of one metre, which made it possible to pinpoint the stretches of riverbed most exposed to the sun. They then modelled how much solar energy reaches the river's surface and compared the results with water temperature data.
The picture that emerged was clear: wide stretches with sparse vegetation receive more solar energy, whereas steep banks and dense thickets protect the water naturally. To assess vulnerability more precisely, the researchers introduced a "specific radiation" indicator — the amount of solar energy per a given volume of water. It is this indicator that helps identify the zones most prone to warming.
The team then modelled the restoration of a 30-metre-wide strip of trees with canopies no lower than 12 metres. The calculation was applied only to degraded stretches where logging or the loss of riparian vegetation had deprived the river of natural shade. On certain sections, the influx of solar energy dropped by almost 40 percent. Such an effect helps keep the water cooler precisely where salmon most need thermal refuges.
There is no need to plant trees everywhere — what matters more is choosing where they will deliver the greatest return. On the Dartmouth, the priority stretches are concentrated mainly in the upper reaches, as well as on certain tributaries and at the points where it flows into other, heavily sun-exposed rivers. It is here that a modest planting effort can noticeably change the temperature regime.
The scale of such work is already visible in Scotland, where foundations and fisheries protection organizations plant trees along rivers by the millions. According to Marine Scotland's assessment, only about 35 percent of Scottish rivers have sufficient tree cover to obtain a cooling effect — which means the potential for work is enormous. Another thing matters too: trees need time. According to Scottish environmental authorities, plantings may need around 20 years before they grow enough to provide real shade.
Restoring riverside forests is not a panacea but a slow yet real adaptation measure that can be started today. It softens the impact of hot summers and preserves the familiar environment for salmon, which matters both for the ecosystem and for local communities.
Targeted planting of trees where the river heats up most gives the most noticeable return on every effort invested and helps salmon survive the hot summer.
