4 minute read
Groundwater Influence
by Jacob Lemon, TU Monitoring and Community Science Manager
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.
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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.
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
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.
essential source of resiliency to adverse impacts of climate change on coldwater fisheries. Simply put, streams with greater groundwater influence are likely to experience less warming than streams with little groundwater.
So, we need to understand the distribution and magnitude of groundwater influence to target restoration and protection efforts toward areas likely to persist as coldwater fisheries under future conditions. TU has been evaluating methods for identifying groundwater input and its influence on stream temperature.
Using paired air and water temperature sensors is an emerging tool for evaluating groundwater influence. At a basic level, by comparing stream temperature data to air temperature data collected nearby, we can understand the relative impact of air temperature on stream temperature versus groundwater and other factors. By deploying networks of paired air and water temperature sensors, we can evaluate groundwater’s relative influence and better understand the stream’s resiliency to increased air temperature.
New methods using these data even allow us to get a sense of the depth of groundwater sources in an area. This is an important consideration, with deeper groundwater less influenced by the air temperature. Starting in 2022, TU staff in Michigan has joined others in Wisconsin, Idaho, Oregon, and California in a pilot project to test these methods and their application. With sensor networks deployed in parts of the White River watershed, we’ll evaluate these approaches to better support the broader implementation of these efforts by TU chapters, staff, and partners around the country.
TU in Michigan is also evaluating thermal infrared cameras (TIR) to map stream temperature in fine scale and identify specific groundwater discharges and thermal refugia. Thermal refugia are areas that maintain a temperature suitable to coldwater organisms, even during the warmer summer months. Often fish seek out these refugia when ambient stream temperatures get too warm. When TIR cameras are mounted to an unmanned aerial system (UAS), or drone, thermal images can be captured showing where groundwater seeps or cold tributaries are creating areas of cold water within a river or stream.
TU staff in Michigan is working with the US Forest Service Geospatial Technology and Applications Center and Northern Research Station to dial in the best practices for acquiring, processing, and using these images. Our goal is to enable mapping of thermal refugia and groundwater discharges along rivers to aid in siting of in-stream habitat enhancement as well as land protection, stream reconnection efforts, research efforts, etc.
TU is a science-driven organization, and we strive to use the best available tools to support conservation planning. In the end, we are working to focus our resources on efforts that will have the maximum benefit to coldwater ecosystems and where those benefits will persist into the future. In Michigan and elsewhere, groundwater is a crucial piece of the puzzle and one we need to understand better.
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