A Deeper Look into Beavers and Trout Streams in Michigan

by Dr. Bryan Burroughs, Michigan TU Executive Director

A perennial topic of interest to trout anglers, trout managers, and wildlife managers is the role of beavers and their dams in trout streams. This topic has been around longer than any of us reading this article, yet there routinely remains many subtopics that seem to go unresolved. The purpose of this article is to review what we know and share insights.

Regional Context Matters

In Michigan, there is much-published research on the role of beavers on rivers. However, most of this was conducted in trout streams in the eastern or western United States. The Midwest and Michigan specifically are fundamentally different from those areas for numerous reasons that are critical in considering how beavers affect trout streams.

In the more mountainous east and west, the geology is different, as is the topography and the resulting hydrology of trout streams. In those areas, bedrock and large elevation drops result in trout stream hydrology heavily influenced by runoff. Snowpack melt, much of which does not make it into deep groundwater aquifers, results in high spring flows followed by lower summer base flows. In contrast, Michigan has relatively low elevation drops and deep deposits of sand and glacial till where precipitation feeds into deep aquifers, delivering water to streams in a more stable flow regime. Because of lower stream slopes or gradients and the geology, our streams are dominated by sand and smaller gravels compared to high gradient cobble and boulder streams seen in the eastern and western United States.

The effects of beaver dams in the east and west can generally be viewed as dams that create patches of slow water and low gradient habitats within large areas opposite those conditions. Hence, they can serve to create unique habitat diversity. From a hydrology perspective, these small impoundments and created wetlands can absorb and store some of the high peak runoff flows and deliver those floodwaters more slowly, acting as small aquifers of groundwater. This is a summary of many dynamics, but essentially, beaver dams in these contexts create unique and valuable habitats and conditions that are often rare without the beavers. Thus, much of the published research from those areas document positive aspects of beavers to trout streams.

In Michigan, most of our trout streams are stable groundwater-fed streams, flowing across low slopes of terrain through relatively finer sediments. Here, beaver dams generally exacerbate already predominant conditions. They can further slow water where it was already flowing slowly. This can lead to additional sedimentation issues where sedimentation is a challenge already. Also, beaver dams can increase water temperature, and although they may help with flood attenuation, flows may already be relatively stable. The types of habitat diversity we often seek to add or maintain are unique sections of the stream with higher slopes and abundant gravel and cobble. Hence, beaver dams often move us further away from the habitat portfolio we seek to create, even though in other regions of the country, beavers help create sought-after habitat diversity.

One interesting benefit of beaver dams widely recognized is that they create a small area of deep water. The deep water behind a beaver dam often serves as a unique holding habitat that fosters larger than average trout, especially brook trout. Almost any ardent brook trout fisherman will take the time to fish a beaver dam pond. However, beaver dams, like all dams, eventually fill in with sediment. After years, deep water becomes shallow warm water with a dark bottom of organic sediments, and the benefit of deep water is lost. Additionally, the warming effect of the beaver dam is a detriment to trout downstream. Thus, a beaver dam has a life cycle that will provide diminishing returns within a few years of being built.

Local Site Characteristics Determine Benefits/Detriments of Beaver Dams

In general, Michigan beaver dams pose some concerns for trout streams. Even when a beaver pond can make fishing better, those benefits can be very temporary and quickly turn negative. But, for each trout stream with beaver dams that we evaluated, the specific site characteristics mattered in determining how concerned we should be about taking action. Thus, if a trout stream is very cold with an average July mean temperature of 60 degrees, one or more beaver dams that cumulatively warm water temperatures by two degrees would not cause significant impacts. The resulting temperatures would still be ideally cold, and the slight bump in temperatures might improve trout growth rates.

In contrast, if a stream is already temperature marginal, with a July mean temperature of 68 degrees, the addition of beaver dams warming the water by two degrees might mean that downstream of the beaver dams may become

largely unsuitable and the entire population of trout could be significantly diminished.

The location of the beaver dam also matters. Suppose a stream has a modest slope and good gravel areas throughout, and the beaver dam is built in a low land area, low river valley slope, or swampy area. In that case, the beaver dam may not significantly alter the physical habitats in the stream. In contrast, where a beaver colony floods an area of higher stream slopes, which served as a relatively rare higher gradient trout spawning or nursery habitat, the dam’s impacts can be very significant and negative.

Cumulative impacts of multiple beaver dams also matter. If a trout stream only has a couple of beaver dams spread over several miles, the impacts may not be severe. However, if multiple beaver colonies have created largescale dam complexes, the cumulative effects may indeed be significant.

Beaver Dam Complexes

In recent years, Michigan TU has encountered several situations that have compelled action regarding beavers, namely long-established beaver colonies and multiple dam complexes. Michigan has an estimated 70,000 miles of streams, of which over 30,000 miles are trout streams. Over recent decades, trout angling has shifted away from small tributary streams. Fewer anglers are fishing miles of these small streams with fewer “eyes” on these waters. During the same period, recreational beaver trapping has also declined.

These factors have created situations where extensive beaver colonies have developed, creating large complexes of dams on some small trout streams largely “out of sight, out of mind.” TU has increasingly adopted watershed perspectives to identify projects of value and looking to understand how to enhance or protect trout fisheries for the long haul. Even if we may not actively fish many small trout streams, we are analyzing their function as critical habitat, including as thermal refuges and as spawning, nursery, and over-wintering sites.

When beaver complexes are identified, we need to assess their impacts. If dam complexes exist in important thermal refuge areas or impact essential spawning areas, we decide to take action. In contrast, if they occur on a warm or cool water tributary or don’t impact an otherwise valuable trout habitat, we may take no action. In cases where action is appropriate, quite a bit of work needs to be done. First, typically, a willing and effective trapper must be hired, and DNR Wildlife Division permits obtained to remove the beavers. Removing their dams without removing the beavers just doesn’t work. After a year or two of beaver population control, we must get permits and make plans to remove the beaver dams.

There is a method to remove a series of beaver dams. Over time the ponding of water results in fine sediments accumulating upstream of the dam. If an individual dam is breached gradually or only breached at the normal stream’s width, it is possible to leave much of the accumulated sediment in place. The stream will cut or erode a new channel downward through the deposit, releasing some sediment downstream but leaving much of the sediment in place to form new streambanks.

We’ve learned from human-made dams that gradual, deliberate removal can release less than 15% of an impoundment’s sediment. Why is this important? If we seek to remove a series of five beaver dams, for example, it will result in some sediment moving downstream. If all five were entirely removed simultaneously, and assuming that each held 50 cubic yards of sediment, a maximum of 250 cubic yards of sediment could move downstream all at once, severely impacting downstream areas. With each dam’s gradual breaching, this might be decreased to 37.5 cubic yards (15% at each). However, if done in sequence from upstream to downstream, with appropriate time in between, that 15% can be distributed across each downstream pond and won’t be sent downstream by the next removal. This optimal scenario might result in only 17.5 cubic yards of sediment moving downstream; that is, 10% is stored in the next pond, and only the remaining 5% subsequently moves downstream after removal of the next dam. Essentially, each subsequent downstream beaver dam serves as a sediment trap, minimizing impacts downstream.

Removal of a beaver dam may not lead to quick or complete restoration of the site. In some cases, restoration may largely occur passively over time. However, water and sediment dynamics at the site may not be able to passively restore all conditions. In some cases, it may be necessary to give the site a year or two of adjustment and then reassess to determine if additional active efforts are required to restore the stream as desired.

There exists plenty of opportunity for conflict with issues of beaver management on trout streams. Wildlife managers can disagree with fisheries managers about the balance of objectives for a stream. River stakeholders can have very different perspectives of the detriment of beaver dams versus the benefits of removal. There can be disagreements about taking action on particular stream reaches. And there can be disagreements as to the restoration end goal and how to achieve it best.

Humans and beavers both manipulate and reshape streams to a much greater level than other animals. Just as humans disagree about whether it’s best to remove or retain a humanmade dam, so too do humans disagree about the pros and cons of beaver dams. Hopefully, this article helps to explain how beaver dams can have negative impacts on Michigan trout streams. Deciding how great an impact and how to best prioritize and address beaver dams needs careful consideration. There is no one size fits all approach to beaver management. If you remove a series of beaver dams, consider our simple guidance and take a careful and strategic approach.

Chairman’s Report... continued from page 5 safety to environmental issues. The goal is to develop guidelines for the funding of future dam safety programs, operating policy, and dam disposition or removal.

From a financial perspective, the fiscal year 2021 marked the third year where Michigan TU exceeded our membership donation goals. These donations, being unencumbered by grant restrictions, provide the core funding for our team to engage in the legislative process and support your chapter’s conservation initiatives. The team’s success in these areas has contributed to Michigan TU being recognized as a leading conservation organization in the state. I would like to thank our chapters and members who donated so generously during our chapter, spring, Aquifer Club, and yearend fundraising appeals, and those folks who donated in the memory of a Michigan TU supporter.

In closing, I would also like to thank my predecessor, Greg Walz, and all the other past council chairs, officers, and committee chairs as I endeavor to fill the shoes of those who came before. I believe that their work and their team members contributed greatly to what Michigan TU is today and laid the foundation for what is yet to come. I think the Michigan TU team has never been stronger, and I assure all of you that Bryan and Kristin are committed to and ready to support your chapter’s conservation initiatives.

I look forward to assisting our chapters when possible. Please feel free to contact me, and I will do my best to address your questions or direct you to someone who can. We are “One TU,” and we are here for you.