Damming research: Study finds beavers might not be all bad for trout streams

DULUTH — For the past century or so of trout stream management in North America there’s been a general consensus that beavers, beaver dams and beaver ponds are bad for trout.

Beaver dams slow stream flows, warm the water and block fish from passing up and downstream, according to the mainstream view on the issue. So the general rule of thumb has been to trap the beavers and take out the dams from streams where you want trout to thrive.

But new research by several University of Minnesota Duluth scientists has found that beaver dams may actually help trout in some smaller streams.

The research, conducted on the upper reaches of Knife and French rivers on Minnesota’s North Shore of Lake Superior, found that beaver dams on small streams actually helped cool water downstream as well as store water to provide better flow during dry spells. The study even found that small fish can pass over small dams after heavy rains, so the dams may not be the insurmountable walls to fish after all.

The hydrology of beaver dams and streams “is complicated and sometimes contradictory,’’ said Karen Gran, a geomorphologist at UMD’s Swenson College of Science and Engineering and lead researcher on the project. “We have pretty compelling data that beaver dams were helping keep groundwater levels high and the stream discharge flowing and the temperature cooler’’ and that the practice of removing beaver dams on the upper Knife River “designed to help trout was actually detrimental to them.”

The research was funded with a grant from Minnesota Sea Grant.

Gran picked the Knife River after concerns were raised by a local group, Advocates For the Knife River Watershed, that the repeated removal of beaver and dams along the river was excessive. Over the past decade, the Minnesota Department of Natural Resources has overseen removal of 250 beaver and 170 dams on the Knife River, a major element in the ongoing effort to improve trout fishing on the once storied stream.

Gran and Josh Dumke, a fisheries ecologist with UMD’s Natural Resources Research Institute, led teams into the field in the summers of 2018 and 2019 to see what happened on the streams before and after beaver and their dams were removed.

Gran and Dumke are quick to point out that their research was limited in scope and time, and that more research should be conducted to see how far the findings might change over different years or may apply to other streams in other places. But it’s clear not all beaver dams may not be as bad as once thought.

“Removing beaver dams in these small catchments does not guarantee cooler temperatures and may even increase downstream temperatures,’’ Gran noted.

While surface water in beaver ponds is indeed warmer than the moving water, thanks to solar radiation, much of the water that leaves small beaver ponds, especially during low-flow periods such as summer or during droughts, comes out of the dam from below and is in fact cooler. Groundwater that upwells below the dam also is cooler than surface stream water.

The research, which looked at streams with beaver dams intact and streams after they dams were removed, also found that beaver dams and the ponds they create actually helped maintain stream flow during low-flow periods, such as late summer. When the dams were removed, the stream downstream from that point spent more time at very low flows compared to when the dams were in place, indicating beaver ponds may serve an important function in storing water for dry periods.

The research also found that smaller beaver dams on smaller streams sections also can be breached by fish. The study, which looked at dams up to 3 feet high, found minnows and small trout were able to move up and downstream of dams when rainstorms swelled the streams and pushed water over the top.

Dumke noted that 78% of the previous scientific writing on beaver dam impact on trout that the UMD researchers could find was in fact speculative and not based on actual field work, apparently leading to some false assumptions.

The DNR has been removing beaver and dams as part of a long-term effort to improve spawning habitat on the Knife River, which is by far the major North Shore reproduction area for rainbow trout. In fact, the Knife River has more miles of fish-swimmable stream than all other North Shore streams combined. (Most Minnesota North Shore streams are blocked to fish close to Lake Superior by barrier falls.) The Knife River also has been the subject of major restoration efforts to curb erosion and reduce sediment, another problem for trout.

While Gran’s team focused on stream flow levels and temperatures below dams, Dumke focused on tracking fish movement in streams with beaver dams, including small trout and minnows. Dumke’s team used electro-shocking devices to stun fish in the upper reaches of the stream, then carefully marked each fish and returned them to their ponds and riffles.

But that part of the project turned out to be more complicated than expected, with most of the 1,200 fish marked by researchers never recaptured. Apparently, the fish moved farther downstream, or upstream, than researchers were looking. It’s also possible some of the small fish were eaten by critters. Only 240 of the 1,200 marked fish were recaptured.

Of those, about 10% made it across low-head dams during periods of high water. (Researchers didn’t have the gear needed to look at bigger dams with deeper ponds.)

The small sample size has Dumke hoping to go back and do more work in more streams. “We documented eight species of fishes — two trout and six non-trout — passing active and inactive low-head beaver dams on Knife and French rivers,’’ Dumke said. “The number of fish passing each dam was significantly correlated to the amount of time water flowed over the tops of dams.”

Deserae Hendrickson, regional fisheries manager for the Minnesota Department of Natural Resources, said the recent UMD research revealed some interesting findings. But she said trout stream managers aren’t ready to welcome all beavers in all rivers just yet.

Hendrickson noted that the recent research focused on very small streams, essentially headwater tributaries to the Knife and French rivers, and that previous research clearly found beaver dams can hurt trout on larger streams.

“There’s been a lot of research that (shows) there is often lower oxygen below beaver dams that are being overtopped, at least those on bigger streams that run perennially or all the time, that oxygen levels below the dams can be too low for trout to survive,’’ Hendrickson noted. “And this (UMD) study looked at shorter dams, less than 3 feet, and we have seen a lot of evidence that bigger dams can be a real impediment for steelhead and coaster brook trout that are trying to migrate upstream, cutting off a lot of potential spawning habitat.”

In addition to the Knife River dams removed, the DNR also removed about 80 beaver and 40 dams on the Blackhoof River, the only other stream where the DNR conducts regular beaver control, Hendrickson noted.

Across northern Minnesota, biologists and others are seeing more beaver in more and more places as beaver trapping has declined due to low fur prices. Many former beaver trappers have quit and the DNR now must pay to have beavers removed.

“We’ve seen a pretty big increase in beaver across the landscape on our trout streams over the last 10 or 15 years. Certainly the most I’ve ever seen,’’ Hendrickson said. “It’s costing us more just to keep up.”

Still, Hendrickson said the UMD research has pushed fisheries managers to look at beaver dams differently in headwater streams, the uppermost portions where big fish tend not to go as often but which may be critical for young fish and to supply cool water downstream.

“We’ve certainly looked at what our (beaver) removal practices are because of this. … We have reduced some of our beaver removal on parts of the Knife, the upper areas, in part based on this information,’’ Hendrickson said. “I’d say that we’ve tweaked our beaver management as opposed to making any big changes … We now know there’s no one answer when you have beaver on a trout stream.”

UMD’s Gran says she hopes the research can be one tool for fisheries management and that people who love trout may realize one-size management does not fit all trout streams.

“We’re hoping that the data we’ve collected can lead to a more nuanced approach to trout management on the Knife River,’’ Gran said.

John Myers reports on the outdoors, environment and natural resources for the Duluth News Tribune. He can be reached at jmyers@duluthnews.com .