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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.

Summary

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.

GW_illustration

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.

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Localized Factors

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.

Conclusion
Overall, downstream land managers and water users should know the most likely outcome of beavers moving in upstream are slight changes in flow and timing, with slightly less flow annually due to increased evapotranspiration, slightly decreased flow during small flood events due to attenuation, and slightly more baseflow during the dry season due to previously stored water returning to the channel. The magnitude of these changes depends on a variety of factors specific to your local stream such as soil, climate, groundwater elevation, valley topography, vegetation, and dam characteristics. The best available science is always emerging, but we can make the best decisions with the data available, and continue to monitor beaver-based restoration and beaver-human habitat conflicts from inception to resolution while continuing to collect data for aggregate analysis – to understand trends over space and time and to inform best practices to continually improve freshwater distribution outcomes for people and wildlife.

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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

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”

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):

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”

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”