Sarah Dunn, GSA 2022 Science Communication Intern

American beaver. Photo from Wikimedia Commons.

The Advocates for the Knife River Watershed came to Dr. Karen Gran with a problem. As a non-profit dedicated to promoting a healthy watershed, the community group had concerns about the Minnesota Department of Natural Resources’ (MNDNR) beaver management strategy. The MNDNR was trapping beavers and removing their dams throughout the Knife River watershed located on the North Shore of Lake Superior.

Steelhead trout are a management priority in the watershed, and the MNDNR hoped that removing barriers such as beaver dams would help the fish population. But the Advocates for the Knife River Watershed were not so sure. As Gran explains, “The people living within the watershed had seen that rivers were getting lower and lower… in some cases going completely dry. That’s not so good for fish or anything that lives in the river.” The group suspected that beaver dam removal reduced summertime stream flows in the river. Both the Advocates and MNDNR sought to promote a healthy ecosystem, but they needed Gran’s research to begin to understand how beaver dams function on the Knife River. Gran, a professor at University of Minnesota Duluth, studies how rivers respond to change, so she was well-equipped to investigate this question.

The waterways that crisscross Minnesota and drain into Lake Superior are impounded by an abundance of beaver dams. Gran describes the Knife River watershed as “beaver heaven,” adding that “there’s beavers everywhere and these beavers will build seven ponds in a row.” Beavers use sticks, stones, and mud to construct dams across streams. The resulting ponds provide the large rodents refuge from predators. Since the fur trade decimated beaver populations across North America, they have made a slow recovery, leaving dams in their wake.

Steelhead trout, prized by anglers, also inhabit the Knife River watershed. The trout were introduced in 1895 and now constitute a naturalized, self-sustaining population. As part of their lifecycle, the trout migrate up rivers to spawn. The unique geology of the Knife River watershed means that it is the only river on the North Shore of Lake Superior accessible to steelhead. The same continental glaciation that carved Lake Superior left behind steep waterfalls at the mouths of majority of the rivers. The falls form natural barriers preventing fish from moving upstream, but not on the Knife River. Though the river remains physically accessible to fish, Gran characterizes the trout habitat as “marginal, right at the cusp of being too warm.” If stream flow gets too low, water temperature becomes too high, which can be stressful or even lethal for trout. Thus, low-flow hydrology is of particular interest for the steelhead trout cold water fishery of the Knife River watershed.

Gran devised a study funded by a Minnesota Sea Grant to quantitatively determine the impacts of beaver dam removal on low-flow hydrology and water temperature. She utilized a Before After Control Impact (BACI) design where, with the help of collaborators, she monitored four pairs of dams in the Knife River watershed over two years. The team measured stream flow below each dam and installed temperature sensors to record water temperatures both above and below the dams. They used this data to determine the amount of time each stream spent below low-flow thresholds and at stressful or lethal temperatures. They also measured groundwater levels above and below each dam to understand how dams alter water flow through the subsurface. During the first year, all the dams remained untouched, which allowed the researchers to establish ratios between each pair of dams. In the second year, the MNDNR removed one dam of each pair. The research team then compared the difference in ratios between the two years to determine dam removal impacts.

“Our ultimate conclusion was that removing these dams can make low flows even lower and warm temperatures up,” explains Gran. These results may seem counterintuitive if we imagine beaver dams as similar to human dams, which are designed to minimize leakage. However, beaver dams are inherently leaky. Water seeps through gaps in the dam like pasta water spouting out of a colander. Gran’s temperature sensors show that it is the cool, deep water rather than warm surface water getting through. Additionally, her groundwater monitoring revealed that the dams are “not just holding the water back” because water moves through the subsurface. The ponds and the groundwater infiltration increase the storage capacity of the watershed, meaning that water stays on the landscape longer. Similar to a sponge, the water is released slowly throughout dry months rather than gushing straight downstream during the wet season. “They [the streams] do run dry and the only place the fish can live is in the beaver ponds” says Gran, adding, “There was a mud puddle at one of our sites, and they pulled out a 12-inch trout. It was there only because there was still water present.” The research on the Knife River illuminates the complex ways beaver dams function, some of which may be beneficial to steelhead trout. Her team hopes to conduct more research into how beaver dams impact trout movement during spawning season. Gran is careful to emphasize that she is not pitting beaver against trout, instead reiterating her goal: “I’m trying to provide information so that people can make informed management decisions.” Her study, motivated by citizens’ questions, contributed to a collaborative effort to better understand and manage a unique ecosystem.