Bank‐dwelling beavers contribute to the wood regime in a dryland river

While floating down the lower White River (a major tributary to the Green River, eastern Utah) during the spring of 2020, we encountered broad swaths of recently cleared riparian forest (Figure 1a). As we navigated our rafts to the riverbank to get a closer look, we noticed fresh cut stumps (Figure 1b) revealing that beavers (Castor canadensis) were the culprit of this riparian harvest which included both native Fremont cottonwood (Populus fremontii) and nonnative Russian olive (Elaeagnus angustifolia) trees. As we continued downstream, we observed additional beaver clearing. At times, navigation became difficult because we were forced to maneuver our rafts around overhanging tree trunks and wood jams that consisted of beaver‐felled trees (Figure 1c). Seeing all this beaver activity, we kept on the lookout for dams but found none. This lack of dams suggests that this disturbance was the work of bank‐dwelling beavers rather than dam‐building beavers.

FIGURE 1.

(a) A harvested Fremont cottonwood forest along the White River, Utah, on April 24, 2023. (b) A fresh cut Fremont cottonwood beaver stump along the White River, Utah, on April 23, 2023. (c) A large wood jam of beaver‐felled trees looking laterally on the White River, Utah, on August 9, 2023. Photo credit: William W. Macfarlane.

These observations marked the beginning of 5 years of annual float trips and drone‐based surveys. Each year, we observed expanding beaver harvest zones, newly cleared patches, active bank dens, and well‐worn beaver trails. We documented many freshly felled trees, identified by intact leaves and fresh chew marks. Most of these felled trees were near the channel margin, some lying within or overhanging the channel, and some accumulating into new wood jams made up of additional beaver‐felled wood (Figure 1c). The persistence and extent of this beaver activity shows that bank‐dwelling beavers had substantially altered riparian vegetation, contributed to large‐wood availability on the floodplain, and contributed to in‐channel structure.

Beavers (Castoridae) are arguably the most well‐known ecosystem engineers with most research emphasizing the hydrologic and geomorphic effects of dam building (reviewed by Brazier et al., ²⁰²⁰; Fairfax & Westbrook, ²⁰²⁴; Grudzinski et al., ²⁰²²). This is not necessarily surprising as beaver dam‐ and canal‐building activities change lateral, longitudinal, vertical, and temporal connectivity of stream channels, floodplains, and adjacent uplands (e.g., Burchsted et al., ²⁰¹⁰; Majerova et al., ²⁰¹⁵). Whereas dam‐building beavers typically build dams and freestanding lodges in impounded ponds or lakes, bank‐dwelling beavers excavate burrows in the banks of rivers or lakes (reviewed by Bashinskiy, ²⁰²⁰). In medium to large rivers with deep‐water refugia, beavers commonly occupy bank dens (Hartman & Törnlöv, 2006; Macfarlane et al., ²⁰¹⁷; reviewed by Bashinskiy, ²⁰²⁰), and in these settings their influence on geomorphology and large wood dynamics remains comparatively under studied (Breck et al., 2003; Meentenmeyer et al., ¹⁹⁹⁸; Wohl, ^{2013, 2017}).

Inputs of large wood by beavers have primarily been attributed to channel‐spanning or floodplain dams (Wohl, 2013, 2017), yet bank‐dwelling beavers may still heavily influence the wood regime (sensu Wohl et al., ²⁰¹⁹) where dams are rare or absent. The wood regime in rivers encompasses wood recruitment, transport, and storage (Wohl et al., 2019). Large wood recruitment arises from physical and biological processes that deliver woody material from riparian zones and adjacent uplands into the active river channel (Benda & Sias, 2003). These processes include natural mortality and senescence (Keller & Swanson, 1979), windthrow (Lienkaemper & Swanson, 1987; Scamardo & Rutherfurd, ²⁰²⁵), bank erosion (Gurnell et al., 2002), mass wasting (e.g., landslide, avalanches; Benda & Dunne, ¹⁹⁹⁷; Kemper & Scamardo, ²⁰²³), flood entrainment of wood on the floodplain (Wohl & Goode, 2008), anthropogenic tree removal, and biotic agents such as insects, disease, and beavers (Livers & Wohl, 2016; Wohl, ²⁰¹⁷; Wohl et al., ²⁰¹²). On the lower White River in eastern Utah, our observations suggest that bank‐dwelling beavers strongly influenced floodplain storage of large wood and large wood recruitment by felling trees along channel margins where they either fall directly into the channel or are later activated during floods (Appendix S1: Figures S1–S3).

Beaver‐contributed wood inputs to the White River were associated with observable geomorphic consequences. Large trunks appeared to enhance local scour and downstream deposition consistent with flow‐obstruction interactions (Yager & Schmeeckle, 2013), creating discrete pools and bars that increased local heterogeneity. The large wood inputs also created bank‐attached and mid‐channel bars and associated geomorphic units including pools, riffles, and backwaters, important habitat for native fishes that use the White River. Limb‐rich trees acted as strainers, trapping smaller wood, accumulating into multi‐piece jams that further increased channel roughness and flow divergence. These pulse inputs and resulting jams differed markedly from the more diffuse, gradual wood recruitment associated with senescence or windthrow (Benda & Sias, 2003; Figure 1c).

Bank‐dwelling beavers can further promote wood recruitment into river channels by destabilizing banks. Burrow excavation creates subsurface voids and preferential seepage pathways (Meentenmeyer et al., 1998), weakening banks that later collapse during high flows or rapid changes in stage. We observed scalloped bank lines adjacent to burrows with toppled trees and brush mats lodged at channel margins and dig marks along the bank (Appendix S1: Figures S4 and S5). This “bankfall wood” appears to represent a substantial wood source to the White River and may be comparable to or even larger than other potential sources (e.g., windthrow, mortality; Dunkerley, ²⁰¹⁴; Peterson, ²⁰⁰⁰). The interaction between beaver activity and lateral erosion might amplify wood recruitment in riverscapes where natural rates of bank erosion are otherwise low or suppressed by vegetation encroachment or flow regulation (Scott et al., 2018, 2025).

Bank‐dwelling beavers appear to enhance floodplain wood storage through repeated clearing of vegetation in low‐elevation terraces adjacent to the active channel. While this material may not immediately enter the river, it becomes increasingly susceptible to mobilization during overbank floods (Wohl et al., 2012, 2019). In this way, bank‐dwelling beavers might help maintain a supply of recruitable wood on the floodplain, particularly in arid rivers where woody vegetation is typically limited. Notably, bank‐dwelling beaver activity in these environments may be one of the important processes capable of producing fresh, mobilizable woody material in the absence of other recruitment sources.

Beavers typically harvest younger trees between 5 and 25 cm (Breck et al., 2003; Mahoney & Stella, ²⁰²⁰), producing distinctive clusters of cut stumps (Figure 1a). Similar to dam‐building beavers, bank beavers likely fell trees for forage and materials for construction, felling trees in close proximity to water to reduce predation risk by minimizing the distance traveled on land (Brazier et al., 2020). Observations in a boreal system suggested that beavers may fell more biomass than needed for construction or consumption (Johnston & Naiman, 1990), with no explanation for the excess tree removal. We observed a similar pattern on the lower White River, whereby beaver felled patches of large wood that ranged from 400 to 1400 m² on floodplain surfaces near the active channel (Appendix S1: Figures S1–S3). Some trees that were felled went into or overhung the channel in areas that are hydrologically connected during overbank flows. Because of their repeated use of specific areas, bank‐dwelling beavers may create temporally consistent sources of wood, analogous to chronic recruitment zones described in forested mountain streams (e.g., Benda & Sias, ²⁰⁰³).

Through tree felling, bank destabilization, and floodplain wood provisioning, bank‐dwelling beavers significantly shape large wood dynamics. Our observations indicate that their contributions may be particularly important in rivers experiencing invasive woody vegetation expansion or hydrologic alteration (Stromberg, 1993). For example, in some areas, we observed beavers targeting young Russian olive, an aggressive invader that reduces geomorphic complexity and outcompetes native riparian vegetation (Scott et al., 2018, 2025). By felling invasive woody vegetation, bank beavers may slow thicket formation, maintaining open floodplain structure favorable to geomorphic dynamism and cottonwood regeneration (Appendix S1: Figure S6). Future research should assess if bank‐dwelling beaver activity delays the establishment of dense Russian olive thickets, helping to maintain a more open floodplain structure conducive to a dynamic channel, cottonwood regeneration, and larger, more persistent wood contributions over time.

Do beavers retain their role as ecosystem engineers in the absence of dam construction? Our observations indicate that bank‐dwelling beavers function as ecosystem engineers, influencing riparian vegetation, large‐wood recruitment, and in‐stream habitat complexity in dryland rivers such as the White River. While their effects may not be as visually dramatic as a dam, based on our field observations they exert consistent, measurable pressures on riparian vegetation and geomorphic processes that can yield persistent, ecologically meaningful changes in wood dynamics and in‐stream habitat. Recognizing the role of bank‐dwelling beavers expands our understanding of biotic drivers of riverscape complexity and provides new opportunities to integrate ecology with fluvial geomorphology in managing and restoring dynamic river corridors.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Supporting information

Appendix S1.

Video S1.

Video S1 Metadata.

Pennock, Casey A. , Macfarlane William W., Budy Phaedra, Scamardo Julianne, and White Daniel C.. 2026. “Bank‐Dwelling Beavers Contribute to the Wood Regime in a Dryland River.” Ecology 107(5): e70396. 10.1002/ecy.70396