How do beaver dams influence the downstream delivery of water?
Beaver-based restoration and human-beaver coexistence have been gaining traction across the Western United States due to many benefits of beaver re-establishment including creating complex instream and floodplain habitats that support native wildlife, improving water quality, and increasing wildfire resilience. At the same time, major drought and climate extremes are straining local water resources leaving many water users concerned about future water availability. If you live downstream of a beaver dam complex or beaver-based restoration project, you may have questions about whether this will impact streamflow and/or water rights on your property. It’s a complex topic but here we summarize the current scientific understanding and bring you up to speed on what the most likely outcomes are.
During small floods beaver dams slow, spread, and store water, delaying and decreasing peak flow.
During high flow events beaver dam complexes facilitate groundwater recharge. Later in the year the stored water seeps back into the stream, increasing baseflow.
More riparian vegetation and surface water can increase evapotranspiration, causing a slight reduction in annual streamflow. Increased
groundwater storage may be able to improve drought resilience.
The magnitude and timing of any changes are highly dependent on localized factors.
Background
Beaver dams (or human-made beaver mimicry structures) raise the water surface elevation. Higher surface water elevation relative to the initial pre-beaver water surface elevation combined with a larger area covered by surface water increases groundwater-surface water interactions. (Larsen et al. 2021) This increases local groundwater storage but groundwater doesn’t just sit still. Water movement is based on gravity, flowing through the gaps in the soil from high to low elevation. Water that infiltrates near a beaver complex flows underground and either resurfaces downstream or gets evapo transpired by plants. But what effects does this have on streamflow? It depends on what timescale you’re looking at! Water moves both above and below ground, illustrated in light and dark blue, respectively.
Beaver dams facilitate infiltration to alluvial aquifers (a). Groundwater flows underground due to gravity (b). Water can later flow back into the stream (c). Illustration by Robyn Holmes.
Annual Scale
Impacts on streamflow vary depending on the time of year. During periods of high flow such as spring runoff, beaver-influenced reaches facilitate the transfer of more water from the stream to groundwater. (Larsen et al. 2021; Pearce et al. 2021; Bobst et al. 2022) Later in the season when streamflow recedes, there is more water stored in the floodplain that can now flow back into the stream channel.
There are many anecdotal accounts of streams that had historically dried up during late summer becoming perennial (year-round flow) after beavers moved in. One study interviewed 21 ranchers based out of the high desert of Elko County, NV who had beaver move into their property following adjusted grazing management. Of those interviewed, 16 reported observing & increased water availability in streams and beaver ponds, and longer duration of stream flows, during the hot season and 14 reported observing higher water tables and increased groundwater storage.
Multi-year scale
Looking at streamflow over a longer time period, there tends to be a slight decrease in streamflow, likely due to increased evapotranspiration–a term that combines evaporation from surface water and water transpired through plants. According to a 2021 review paper (Larsen et al. 2021), eight quantitative studies had been conducted with seven of them showing decreased annual discharge and one showing no change. The limited number of studies may be partially due to overall water budgets being difficult to conduct. Streamflow in and streamflow out are relatively easy to measure but flow underground and evapotranspiration are more challenging. Measuring streamflow in the years before and after a beaver-based restoration project is difficult to compare due to differences in precipitation.
Given the difficulty in obtaining accurate subsurface and evapotranspiration measurements plus the broad array of site-specific characteristics that influence site hydrology, there isn’t an agreed-upon estimated magnitude of change in annual streamflow in response to beaver-based restoration. By causing more surface area and vegetation, beaver dams may increase evaporation, represented here as white dots. Illustration by Robyn Holmes.
While reduced annual flow may sound like all-around bad news, don’t forget to consider the fact that human-made dams also increase evaporative losses on an annual basis but it’s a sacrifice many folks are willing to make in order to receive water at a later time in the year. Additionally, with beaver dams the water ‘lost’ to increased evapotranspiration is going toward more vegetation and natural aquatic habitat which can support more wildlife and create areas of
refugia and resilience in the face of fire and drought. (Jordan and Fairfax 2022; Fairfax et al 2024). That said, the drawbacks of downstream flow impacts for neighbors are a real and reasonable concern, especially in areas where water is already scarce and it will have to be a local consideration whether the benefits outweigh the costs. Ongoing research will help us better understand hydrologic responses under different conditions. (Larsen 2021; Peace et al 2021; Bobst et al. 2022)
While the previous sections explained the general theory of how beaver impacts streamflow, there are other localized complicating factors that influence the specifics of the magnitude of effects. (Larsen et al, 2021) Here are a few examples:
Valley bottom characteristics
Valley bottom width and slope influence the geomorphic characteristics of a stream (e.g., width, depth, number of channels, channel bed material) which influence beaver dam capacity and groundwater-surface water interactions. Aquifer geometry defines the storage volume which is a factor that governs how much water can be temporarily stored.
Soil characteristics
Soil characteristics impact how much water can be stored and how quickly water can move through the soil. Soil types that water can flow through quicker (e.g., sand and gravel), recharge faster and drain water back to the stream faster. Computer modeling by Bobst et al. (2022) suggests soil with moderate hydraulic conductivity (ie. silty sand) provides the largest increase in baseflow, and soils with low hydraulic conductivity (ie. silt) has the most effect on vegetation near the stream though this is yet to be explored and validated by real-world studies.
Valley bottom characteristics
Not all beaver neighborhoods are built the same. A complex of many dams may have more potential to influence flow than an isolated dam. That said, the effects of multiple dams are non- linear and also dependent on the valley and hydrologic setting. Additionally, similar to soils, dams that are more leaky will drain faster and have less impact on late-season flows. ( Larsen et al. 2021; Hood et al. 2024) Dam function may also change throughout their lifespan as sediment deposition changes the storage capacity and hydraulic conductivity (ability for water to flow through soil) or as beaver continue to make renovations (Clark, 2020).
Initial groundwater elevation
If the groundwater table is significantly lower than the surface water elevation, there may be a larger initial decrease in streamflow as the increased infiltration fills the alluvial aquifer. Once the aquifer is replenished, streamflow will be more stable year-round.
Evapotranspiration perspective
Increasing groundwater elevation allows more plants to access groundwater which increases both vegetation robustness and evapotranspiration. Areas where groundwater is already accessible with plants before beaver will experience less vegetation response and therefore less evapotranspiration increase. All this said, vegetation and evapotranspiration have complicated feedback loops and there may be other interactions (wind, shade, humidity) influencing evapotranspiration too.
Climate
Precipitation type, amount, and timing have the largest impact on year-to-year flow with temperature and humidity also impacting evapotranspiration. Snowmelt-dominated vs precipitation-dominated systems will experience different flow regimes during the year.
Further Resources
Bobst 2020 youtube video:
“Hydrologic Effects of Beaver-Mimicry Stream Restoration”
- Presentation by Montana Tech PhD candidate Andrew Bobst that gives an
overview of a study looking at groundwater elevation in response to BDAs and a project using computer modeling to learn about groundwater-surface water
interactions near beaver dams. - https://digitalcommons.mtech.edu/campus_lectures/95/h
Bobst et al. 2022
- Groundwater modeling of multiple hypothetic dam scenarios to better understand groundwater-surface water interactions and how that affects downstream flow.
- https://doi.org/10.1111/1752-1688.13044
Clark 2020
- Graduate thesis that looks at the same site as Majerova but for a longer time period and finds flow effects may change over time as dams receive fine
sediment deposition and/or stop being maintained. - https://doi.org/10.26076/891b-18b3
Charnley 2019
“If you build it, they will come: ranching, riparian revegetation, and beaver
colonization in Elko County, Nevada”
- Interviews ranchers in Elko County, NV that had beaver move into their property. Paper covers their views on beaver, changes to streams, and challenges.
- https://doi.org/10.2737/PNW-RP-614
Jordan and Fairfax 2022
“Beaver: The North American freshwater climate action plan”
- Review paper focused on how beaver is relevant to many climate change based stream ecosystem issues.
- https://doi.org/10.1002/wat2.1592
Larsen et al. 2021
“Dam builders and their works: Beaver influences on the structure and
function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems” (review paper):
- Review paper of hydrogeomorphic effects of beaver that gives a detailed
discussion of the process pathways and complexities that shape streamflow in beaver influenced areas. - https://doi.org/10.1016/j.earscirev.2021.103623
Moore and McEvoy 2022
“In Montana, you’re only a week away from a drought”: Ranchers’
perspectives on flood irrigation and beaver mimicry as drought mitigation strategies”
- Not sure if this is relevant enough, didn’t cite it thought it is drought/water
availability + beaver adjacent - https://doi.org/10.1016/j.rala.2022.03.004
Nash et al. 2021
“Great Expectations: Deconstructing the Process Pathways Underlying
Beaver-Related Restoration”
- Explains the process pathways behind beaver based restoration, complexities to consider when deciding if it’s a good fit, and forming reasonable expectations on project outcomes. Includes a section explaining the 5 studies that address downstream flow.
- https://doi.org/10.1093/biosci/biaa165
Peace et al 2021
“Impact of beaver dam analogues on hydrology in a semi-arid floodplain”
- Study looking at groundwater levels near BDAs
- https://doi.org/10.1002/hyp.14275