Let’s say that you live in northeast Nevada and ranch a stretch of sagebrush watered by a small cottonwood-lined creek. Or that you make your living from your senior water rights and an orderly orchard of fruit trees in western Colorado. Or that you grow alfalfa just north of the Gallatin Range in Montana, your water spilling from an old irrigation ditch. What matters is that your livelihood depends on the supply of water reaching your land.
Then, you find out that beavers have moved in. You don’t know how many, but in the willow-choked, protected gulch just upstream from your land, there is now a beaver lodge under construction and a silty pool of water bottled up by mud and cut wood.
Regardless of your personal animosity or appreciation of beavers in the abstract, there is probably now one concrete question at the front of your mind: will this impact the water reaching your land?
Unfortunately, that’s not a simple question to answer.
Photo Credit: James Garcia
A beaver dam neither creates nor destroys water. Rather, a dam changes how it flows through a watershed. Instead of running quickly down a stream, the water is slowed down or stilled, flooding nearby low-lying areas, creating a pond, and typically raising the local sub-surface water table (Westbrook et al., 2006). If water is building up in one section of a stream, the in-stream flow downstream of that section will be decreased during that same time period. More water pooling in one location means less in another. On the other hand, the slower release of water from a beaver-impacted area can also cause a relative increase in streamflow during the late summer when less water is typically available for wildlife, fish, cattle, or farms (Nash et al., 2021).
According to a 2021 review paper by Dr. Caroline Nash and colleagues, this increase in late-season streamflow is a frequent positive finding in studies that have investigated the impacts of beaver dams. This potential to restore watersheds and provide increased late summer water for livestock, fish, and wildlife has contributed in part to a recent flood of excitement around beavers as a tool to help the Intermountain West adapt to the aridification of climate change.
Nonetheless, digging deeper into the literature, Nash et al. finds that there are few studies that have used detailed discharge measurements from both before and after dam construction to understand the relationship between beavers and streamflow over the course of the year. These studies resist categorization. While one from Utah found that beaver dams increased streamflow during the growing season (Majerova et al., 2015), a different study from England found an overall decrease in streamflow (Puttock et al., 2017), while another from Utah reported that it depends on the age of the beaver pond (Clark, 2020). In short, while there are general trends, the streamflow implications of a beaver dam depend largely on situation-specific idiosyncrasies, such as “geomorphic setting, local climate and geology, and the number and type of dams” (Nash et al., 2021).
This might leave you–as a beaver-concerned landowner–with an unsatisfying answer to your pragmatic question. While general trends are better than nothing, what exactly these rodents mean in your specific situation in your specific slice of the West is a critical question for you.
To answer that, let’s hope that you began preparing for this fateful day several years ago by taking frequent stream discharge measurements from both above and below the location of the (then nonexistent) dam. With this baseline data in hand, all you need to do now is continue these measurements now that the beaver dam is there (along with preventing any other changes that could impact streamflow). This will essentially amount to a natural experiment that shows how streamflow changed as a result of the impact of the beavers’ ecological engineering.
If that sounds unrealistic, most biologists and restoration practitioners would agree. As mentioned earlier, there are very few studies with pre-beaver monitoring data. This is largely due to the high cost of monitoring, but also relevant is the fact that even in situations where beavers are being released into a watershed, there is really no way to control exactly where (or even if) those beavers will build dams (Nash et al., 2021).
Still without a satisfying answer, your next best option to resolve your question might be to find an alternative baseline. Rather than comparing your pre- and post-beaver in the same stream, you’ll instead want to compare your stream to another one that is as similar as possible in every way except the presence of beavers. Given that it is impossible to control for every relevant variable, this will not offer as clear of a picture of the impact of beavers. However, it may finally give you some sense of how the presence of beavers will impact your land and livelihood.
This is not an abstract question. As beavers return to more and more of their historic habitat and the American West wrestles with the many impacts of a decreasing water supply, the complex relationship between beavers and streamflow has real consequences for the people and ecosystems of the region. Do beavers pose a threat to thirsty cows, crops, and cities that don’t want to see their water rights impinged upon by a rodent? Or do they provide hope for sustaining late-season streamflow, an essential component of climate resilience in the Intermountain West? The answer might well vary from one stream to the next.
This question is one of several that I am helping to answer for one specific location in the Western U.S.: The Nature Conservancy’s Tensleep Preserve in Wyoming. If you’re curious about that work, you can read the project description here.
Sources
- Clark TR. 2020. Impacts of Beaver Dams on Mountain Stream Discharge and Water Temperature. Utah State University.
- Majerova M, Neilson BT, Schmadel NM, Wheaton JM, Snow CJ. 2015. Impacts of beaver dams on hydrologic and temperature regimes in a mountain stream. Hydrology and Earth System Sciences 19: 3541–3556.
- Nash, CS, Grant, GE, Charnley, S, Dunham, JB, Gosnell, H, Hausner, MB, Pilliod, DS, Taylor, JD. 2021. Great Expectations: Deconstructing the Process Pathways Underlying Beaver-Related Restoration. BioScience 71: 249–267.
- Puttock A, Graham HA, Cunliffe AM, Elliott M, Brazier RE. 2017. Eurasian beaver activity increases water storage, attenuates flow and mitigates diffuse pollution from intensively managed grasslands. Science of the Total Environment 576: 430–443.
- Westbrook CJ, Cooper DJ, Baker BW. 2006. Beaver dams and overbank floods influence groundwater–surface water interactions of a Rocky Mountain riparian area. Water Resources Research 42: W06404