Summer/Fall 2006

UW Water Resources Research

Keep It Cool
Prized Wisconsin trout streams need steady supply of groundwater

By Kathleen Schmitt

Wisconsin is known as a Midwestern mecca for trout anglers, boasting over 10,000 miles of trout streams throughout the state. These streams are gifts of nature, but they’ve been nurtured for decades with management and restoration. In the past, the focus has been to stabilize shorelines and reconstruct spawning habitat.

Today, researchers are looking upstream and underground to safeguard these prized coldwater streams.

As a hydrogeologist with the Wisconsin Geological & Natural History Survey, Steve Gaffield examined how changes in land use could impact the temperature of trout streams. Trout—especially brook trout, the only type native to Wisconsin—need cold water, and “in Wisconsin, groundwater is what keeps streams cold,” he said.

Groundwater enters streams through cracks and crevices in rock. As the cold water moves downstream, its temperature is affected by weather, the width of the stream bed, and the amount of shade provided by shoreline plants.

Gaffield and his colleagues developed several models that use these factors to predict how stream temperature is affected by different types of land use. As subdivisions continue to trickle out into rural areas, buildings and paved surfaces block rainfall from soaking into the soil and replenishing (or “recharging”) the groundwater supply.

Using the models, they found that building a large development in a watershed—such as a suburb with pavement and other impervious surfaces that block groundwater recharge—had about the same effect on stream temperature as installing a high-capacity well. They also simulated smaller, widespread residential development—characterized by large lots in rural areas—and found that it produced no appreciable change in stream temperature.

Gaffield cautioned that existing models of groundwater recharge may not be sophisticated enough to tease out the true impacts of developing agricultural land. For example, developments that use best management practices to allow rainwater to soak into the ground will have far less impact than those that do not. Also, the greatest effect on stream temperature in any area with paved surfaces may occur during storms, when large amounts of stormwater run off hot, paved surfaces and enters streams.

Montgomery Associates, a Madison consultancy where Gaffield is now a senior hydrologist, is proposing to use methods from his trout stream research and other studies to help the city of Verona plan future growth in a way that protects water and ecological resources. This study is in the proposal stage and will be considered in the city budget process this fall. Gaffield expects other communities will be interested, too.

“We anticipate an increasing requirement for these types of evaluations to be done as local municipalities apply for the necessary approvals to grow, and this type of temperature analysis will be important to evaluate development near the numerous coldwater streams in the area,” he said.

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