Michigan Trout Unlimited
MICHIGAN Trout
Groundwater Influence
10
by Jacob Lemon,TU Monitoring and Community Science Manager
Summer 2022
On a chilly day in March 2022, Trout Unlimited and U.S. Forest Service staff took off on a float trip down the Au Sable River downstream of Mio, Michigan. They were there not to chuck streamers for brown trout but to launch a drone outfitted with a thermal imaging camera that detects variations in temperature of different parts of the river. The team was looking for warm spots that indicated warmer groundwater entering the cold river. These warm spots become cold in the summer, as groundwater temperature remains comparatively stable year-round. TU and the USFS hope to use these data to inform the placement of in-stream habitat enhancements.
With groundwater resources being so crucial to our coldwater fisheries in Michigan, it is imperative that we both protect and understand them. That is why the Michigan Council of Trout Unlimited has put great effort toward safeguarding groundwater resources by supporting regulations that ensure their protection. Bettering our understanding of groundwater will also lead to more effective conservation planning. The temperature of groundwater is less impacted by increasing average air temperatures and thus is an
Stream temperature is the most important factor influencing coldwater fish distribution, such as brook and brown trout. Here in Michigan, the cold water that allows these fish to thrive is primarily the result of groundwater influence. As precipitation hits the land surface, some amount will soak into the ground. A portion of this water will continue downward, recharging groundwater aquifers. Soil is a poor conductor of heat, and if you dig down far enough, you’ll find that soil remains a pretty constant temperature year-round. The slow-moving water is cooled when warmer surface water soaks into the ground and reaches these cooler conditions. In the winter, colder water will meet warmer conditions underground. Days, weeks, months, or years later, this water may be discharged into a stream through seeps, springs, or upwellings at roughly 40-50 degrees Fahrenheit. When streamflow comprises a greater amount of groundwater relative to water that enters the stream from the surface, you’ll find more stable temperature conditions — cooler in the summer and warmer in the winter — than streams with less groundwater influence. This supports conditions that enable trout to access cooler water during the warm summer months and warmer conditions in the winter.
USGS map showing modeled groundwater yield for Michigan streams. Purple and dark blue streams have the highest yield. A quick look shows the overlap between high groundwater yield and some of Michigan’s best trout streams.
