By CVC’s Adrienne Ockenden, Specialist, Watershed Monitoring

We’re in the dog days of summer. For many of us, when the weather heats up, we escape to cooler places – an air-conditioned building or maybe to the dock up at the cottage. But what are the options for fish that need cold water to survive? How do coldwater fish like Brook Trout beat the heat?

The key lies under the ground. It turns out that during the summer almost half of the water flowing through the Credit River and its tributaries comes from groundwater. Groundwater plays a role in moderating stream temperatures. For example, data collected from our Integrated Watershed Monitoring Program (IWMP) showed water temperatures in three of our stream monitoring sites in the headwaters (Erin and Caledon) stayed below 20 degrees Celsius throughout the summer last year. That was even during a straight week of daytime temperatures topping 30 degrees Celsius!

Streams with plenty of groundwater discharge act as refuges for coldwater fish during the summer. They’re also the places where we have some of the most abundant Brook Trout populations.

As the weather heats up, coldwater fish seek out these groundwater-rich areas. In a large river, refuges may be deep pools with groundwater upwellings or areas where small groundwater-fed streams join the river. During the peak of summer, streams run low and air temperatures are at their highest. Groundwater is critical in maintaining an abundant supply of water and supporting a healthy fish community.

We’re seeing a concerning trend in stream temperatures. Our monitoring data show increasing temperatures across the watershed overall, including at a number of our coldwater stream sites. With climate change, this trend is likely to continue. We’re also seeing a decline in the watershed’s Brook Trout populations, a trend that’s being mirrored in streams across the province. We need to protect coldwater habitat under the threats posed by climate change.

If you have property along a stream, plant native trees and shrubs along the streambank. Not only does the shade from plants help keep streams cool, their fallen leaves and branches also make good habitat for fish and other stream life. Besides the wealth of other ecosystem benefits that trees and shrubs provide, they also trap carbon to help fight climate change.

The long-term data we collect through IWMP are vital to understanding the conditions and trends in our watershed’s groundwater, stream, forest and wetland ecosystems. With this information we can make informed decisions on where to focus our efforts to protect, restore and enhance our local natural environment.

Learn more about IWMP.


Brook Trout are a coldwater species that seek out cool groundwater upwellings when the weather heats up.


We monitor water temperature at more than 90 stream sites across the watershed.

Comments (5)

  1. This data is so important, and few agencies have been willing to date to indicate real rises in temperatures and the implications for the ecological norms in local streams. As things warm, we are also getting opossums and other terrestrial fauna and flora moving north. Thank you, and please keep up the publicity.

  2. Also restrict the salt content from seeping into our streams. Educate snow removal drivers that they are destroying our environment by depositing large amts. in parking lots, roadways.

  3. Great PR piece! Do bold the plant trees and shrubs in it to help draw attention to this so that people can take action and help you do this!

    ACER would like to help with the planting and monitoring of plantings in riparian zones as we have in 13 other conservation areas.
    Alice http://www.acer-acre.ca

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