Beavers - Friend or Foe?

Many have very strong feelings about beavers (Castor canadensis), on both sides of the issue. I know plenty of people that think the only good beaver is a dead beaver - not a sentiment that I agree with. On the other side of things, there are others that hold beavers in an esteemed position that I often times do not agree with. I do not have such strong feelings and I think "friend or foe" depends upon quite a number of factors. Those factors seem to be where it is (stream gradient, location in the watershed, etc.), age of the dam, number of dams along the stream / in the watershed, etc. Like most ecological questions, the answer is, "It depends" upon a lot of things.

Image source: Steve from Washington, D.C., USA - American Beaver

My hopes for this post is to provide what the research tells us about the effects of beavers on trout streams. Much of the research will be from the Great Lakes region where Johnson-Bice et al. (2018) wrote a comprehensive review of their impacts on salmonids in the western Great Lakes region. And the proceedings of the Wild Trout Symposium XIII was recently published and Matt Mitro's beaver study (2023) is another source I will draw heavily from.

While I will dig deeper below, the quick stories are that: 1) that beavers have a negative effects on trout and trout streams by warming the water and preventing movement of sediments and fishes, and 2) that they increase the diversity of flora and fauna and beaver dams help produce larger trout, and 3) for some salmonids, the ponds their dams create are critical to early life stages of those fishes. I believe these things to be true at different times and in different places. Previously, I wrote that the answer to most every ecological question is, "it depends" and I firmly believe that. The effects of any organism, management action, etc. is time and place dependent.

As Mitro (2023) writes about in his recent article, beaver management on trout streams in Wisconsin has largely been shaped by one study in one watershed, that of Avery (2002), that may not be applicable to the rest of the state. Avery's final sentence in the abstract is, "This study quantitatively validates the removal of beaver and beaver dams as on of Wisconsin's most cost effective trout habitat management tools". There is little wonder why "the only good beaver is a dead beaver" is believed by many trout anlgers (and probably more than a few biologists). However, this is one study from one location in Wisconsin and the transferability of this to other places is often questionable / unreliable. There are likely conditions under which beavers have positive effects, and other times and places where they are detrimental to trout streams.

A beaver dam that is in the process of being removed from a Driftless Area stream.

There are a few general trends in most beaver studies. The first is that their effects are largely a function of stream gradient. Many of the locations where beavers are heralded as an important part of the watershed are in high gradient streams (Colleen and Gibson 2000), this includes in the Great Lakes Region (Johnson-Bice et al. 2018). Their effects tend to be more negative in lower gradient streams. Many of the studies that show beavers to be critical to trout come from high gradient mountain streams of the Western United States, often with anadromous fishes like Coho Salmon (Oncorhynchus kisutch) which are dependent upon off-channel habitats in their early life history. In lower gradient streams, their effects on trout are most likely to be a negative. In higher gradient streams, beaver dams are less likely to remain in place long enough to be problematic.

Another beaver dam being removed from a Driftless Area stream

Beaver dams provide depth, which is lacking in some streams. Depth tends to hold larger fishes and may provide coldwater refugia in summer and warmer water refugia in winter. Without question, the slack water of beaver impoundments can help produce larger trout. There is a positive relationship between water temperature and fish species diversity in coldwater streams. That is, as trout streams warm, they host more fish species as few fishes can survive in cold temperatures. Dams also provide more habitat for frogs and salamanders and access to these prey sources can increase trout growth, particularly for larger trout. It is tough for trout to grow very large, say greater than 14 inches, on a diet of insects and scuds (see a post on bioenergetics).

Negative Consequences

There are two general negative consequences that are typically cited by those calling for beaver and beaver dam removal. First, beaver dams cause an increase in stream temperatures by slowing the water which allows it to warm. Likewise, in the winter, dams can create colder water areas in streams, generally not a good thing for winter trout streams. Coupled with this is that slower water deposits more sediments and this becomes particularly problematic when improved stream reaches are dammed by beavers. I totally understand this angler concern as much time, effort, and money goes into improving, restoring, or whatever you want to call what we do to streams. Second, dams can slow or stop the movement of trout which has a number of negative effects. This is most critical during spawning season. Certainly both of these things can be true, but as I'll highlight below, they are not always true or at least they are not always problems.

Most beaver dams are more passable by fishes than most people think. I have seen a few presentations on how fishes in the Mississippi River navigate the locks and dams of the "big river". Similarly, tagging studies in Wisconsin trout streams have shown that beaver dams are at least somewhat permeable to trout. Similar to the studies on the Mississippi River, high water provides fishes with an opportunity to move upstream through dams. At worst, they seem to be semi-permeable barriers that under normal flows may be more significant barriers but at high flows, they are generally not barriers. This is, of course, a generalization, but generalizations hold up most of the time. And the number of dams likely matters too - like it does for man-made dams in the Western United States. Salmonids may find their way around a dam but as the number of dams increases, the likelihood that fish can reach headwaters to spawn greatly decreases. And it just takes one dam that is a more significant barrier to stop upstream movement.

The Positive Aspects of Beavers

Beaver dams increase retention time and productivity of trout streams. This is generally a greater positive in lower productivity streams - productivity is certainly not an issue in Driftless Area streams. Many trout streams are largely devoid of the larger, more energy dense foods required for larger trout to be able to maintain weight or even continue to grow. The bioenergetics of large trout in cold streams rarely works out in their favor; beaver dams are capable of altering this. As dams warm the water and provide low flow habitats; minnows, small suckers, and amphibians find more suitable habitats. These bigger mouthfuls help grow larger trout. Beaver dams can grow larger fish - something most anglers are in favor of.

Part of the equation seems to be the length of time that beaver dams are in place. Relatively new beaver dams often demonstrate positive aspects - in particular, they increase productivity by leaching nutrients from recently flooded lands and increasing water temperatures (only true for streams with temperatures below the thermal preferences of trout). Dams also provide an area for sediment deposition. This means that the best spawning locations may be below dams because the stream below them is free of fine sediments (much the same as tailwaters below human-made dams).

Beaver Management

I know some of you are going to think that this is "crazy talk" but it is not just about the trout. There are, in fact, other things to consider other than just trout. Beavers create habitat for waterfowl, amphibians, turtles, wetland plants, insects, and a host of other species. The trout angler that is also a duck hunter is likely more conflicted about beavers than are most trout anglers.

I hope you were not looking for a definitive answer about friend or foe. In my opinion, the answer is largely that it depends. I am certainly not the first to have a similar observation - Matt Mitro's studies about the effects of beavers, to date, have shown mixed results. Similarly, Niles et al. (2013), a graduate school colleague of mine, found the that effects of beaver dam removals on trout were mixed and varied over time in West Virginia. And the Johnson-Bice et al. (2018) article similarly shows that the effects of beavers are a mixed-bag.

From my experiences, beaver are largely detrimental on most Driftless Area streams mostly because they cover up a lot of habitat with fine sediments. This is part of the legacy of cultural sediments in our area. Under "normal" conditions, we would not have the many feet of easily transported sediments in the valley bottoms which is really the issue. We can talk about beavers as a natural part of the Driftless landscape but little about our stream valleys is natural - including the fact that we have eliminated wolves, one of their main predators, from much of the landscape.

• Driftless Stream Corridors - From Past to Present

• What Did the Driftless Look Like?

• The Re-Birth of the Driftless

Link to 2024 Trout Habitat Workshop talk (in case it does not load)

Below is an AI created annotated bibliography, ChatGPT using the "deep research" module, on the effects of beavers on Midwestern trout streams. Before you turn-up your nose at AI, give it a look and judge the usefulness of ChatGPT and other generative AI. While I have plenty of reservations about AI - finding articles and summarizing them is one of the applications I have certainly come around to myself.

Effects of Beavers on Midwest Trout Streams_ Annotated Bibliography.pdf

Download PDF • 79KB

Links

• Wisconsin Public Radio - Wisconsin wildlife officials say controlling the state’s beaver population is key to healthy trout streams. But some conservation advocates are pushing back

• Superior Bio-Conservancy - Midwest Beaver Summit 2023

• Wisconsin DNR - Beaver Damage

Literature Cited / References / Reading List

Avery, E. (2002). Fish community and habitat responses in a northern Wisconsin brook trout stream 18 years after beaver dam removal. Fish Research Section, Bureau of Research, Wisconsin Department of Natural Resources.

Collen, P., & Gibson, R. J. (2000). The general ecology of beavers (Castor spp.), as related to their influence on stream ecosystems and riparian habitats, and the subsequent effects on fish–a review. Reviews in Fish Biology and Fisheries, 10, 439-461.

Johnson‐Bice, S. M., Renik, K. M., Windels, S. K., & Hafs, A. W. (2018). A review of beaver–salmonid relationships and history of management actions in the Western Great Lakes (USA) region. North American Journal of Fisheries Management, 38(6), 1203-1225.

Larsen, A., Larsen, J. R., & Lane, S. N. (2021). Dam builders and their works: Beaver influences on the structure and function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems. Earth-Science Reviews, 218, 103623.

Lokteff, R. L., Roper, B. B., & Wheaton, J. M. (2013). Do beaver dams impede the movement of trout?. Transactions of the American Fisheries Society, 142(4), 1114-1125.

Lyons, J., Thimble, S. W., & Paine, L. K. (2000). Grass versus trees: managing riparian areas to benefit streams of Central North America 1. JAWRA Journal of the American Water Resources Association, 36(4), 919-930.

Majerova, M., Neilson, B. T., & Roper, B. B. (2020). Beaver dam influences on streamflow hydraulic properties and thermal regimes. Science of the Total Environment, 718, 134853.

McRae, G., & Edwards, C. J. (1994). Thermal characteristics of Wisconsin headwater streams occupied by beaver: implications for brook trout habitat. Transactions of the American Fisheries Society, 123(4), 641-656.

Mitro, M. (2023). The influence of beaver dams on coldwater habitat and trout in Wisconsin streams. Conference: Wild Trout Symposium XIII. West Yellowstone, MT.

Niles, J. M., Hartman, K. J., & Keyser, P. (2013). Short-term effects of beaver dam removal on brook trout in an Appalachian headwater stream. Northeastern Naturalist, 20(3), 540-551.