BEYOND THE DAM: BEAVER GENERATED DISTURBANCE ENHANCES STREAM MORPHODYNAMICS AND RIPARIAN PLANT RECRUITMENT

Beaver activity and beaver dam analogs have become important considerations for river restoration practitioners, with most of the focus on the dams themselves. We have found, however, that beaver can have system-wide effects on stream morphodynamics and the recruitment of riparian willow. Along study streams (basin areas 20 – 125 km²) in southwestern Montana, USA, beaver-chewed willow stems (beaver cuttings) from dam construction, food caches and herbivory float downstream and commonly accumulate within 1 km of dam sites. At the 90 randomly selected sites surveyed, beaver cuttings accumulated on 81% of point bar sites and 51% of all surveyed sites. Willows can sprout from cut stems, adding roughness that enhances sediment accumulation on point bars and abandoned dam sites. Sprouting beaver cuttings were present at 25% of all sites, thus beaver cuttings commonly provide a secondary pathway for willow recruitment and influence sediment dynamics.

As beaver cuttings and sediment accumulate on point bars, the channel migrates laterally, burying the cuttings. Thirty-four radiocarbon (¹⁴C) ages found in fluvial terraces 1.2 – 3 m above the bankfull channel, show that beaver cuttings range in age from ~6030 – 380 cal yr BP, demonstrating that deposition of beaver cuttings has been a common process over millennia. The long-term preservation of beaver-chewed wood in point-bar sequences also attests to the importance of beavers in carbon storage in beaver occupied stream systems.

We have also observed that the mosaic of site types created by beaver—including intact dams, recently breached or abandoned dams, and long-abandoned dams, interspersed with reaches unsuitable for beaver colonization—promotes habitat heterogeneity while enhancing potential reproductive sites for riparian plants. The temporal and spatial extent of beaver effects should be considered when practitioners are making decisions about restoration and the impact of beavers on fluvial processes.