Post-fire sediment attenuation in beaver ponds, Rocky Mountains, CO and WY, USA

Abstract

We evaluated the post-fire sediment dynamics in beaver ponds to examine these ponds' contributions to sediment storage following disturbance. Beaver dams and beaver mimicry structures impound water and sediment, a function that is of growing interest in wildfire-prone landscapes. Wildfires typically lead to high sediment loading into rivers in the years following fire, constituting a disturbance to aquatic ecosystems and a challenge to water resource managers. Previous work establishes that beaver dams trap substantial volumes of sediment, but sedimentation appears spatially and temporally heterogeneous and it remains unclear the extent to which short-term pulses of sediment are attenuated by these structures. We examine the conditions under which beaver dams and beaver mimicry structures store post-fire sediment by quantifying the sediment volume of 40 ponds, about half of which were burned in large wildfires in the Colorado and Wyoming Rocky Mountains in 2020. The median relative volume of burned ponds is 85%, which is greater than the median for unburned ponds (58%), meaning that burned ponds store higher relative volumes of sediment when pond size is accounted for. Furthermore, sediment accumulated at a median rate of 3.0 cm/year over the entire history of the pond. Post-fire sedimentation rates, with a median of 20.4 cm/year, were an order of magnitude higher than pre-fire rates with a median of 1.8 cm/year. In addition, vegetation and geomorphic characteristics correlated with sediment storage in ponds. Sediment surveys confirmed that ponds with greater surface areas contain higher volumes of sediment. Additionally, older ponds and ponds abandoned by beavers stored higher volumes of sediment compared to recently constructed ponds, ponds actively maintained by beaver, and beaver mimicry structures. These findings demonstrate that beaver ponds and mimicry structures may function as sediment sinks capable of attenuating post-fire sediment. The biogeomorphic context, defined across multiple scales from the pond to the catchment, provides additional explanation for the wide range of sediment storage observed and remains an important consideration for beaver-based restoration, catchment sediment management, and resilience evaluation.