POST-FIRE WATERSHED RESILIENCE: BEAVER PONDS AS SITES OF SEDIMENT ATTENUATION

Excess sediment generated by wildfires threatens stream water quality, riparian habitat, and infrastructure. Beavers construct dams that pool water and capture sediment. Beaver ponds may bolster watershed resilience by providing sediment storage following wildfire. We tested the hypotheses that (1) post-fire sediment stored in beaver ponds is coarser and has a higher abundance of organic carbon relative to pre-fire sediment, and (2) post-fire sedimentation rates exceed pre-fire and unburned rates.

We surveyed 25 beaver ponds in the Colorado Rocky Mountains. Approximately half of the ponds are in areas burned in 2020 wildfires while the other half are in unburned areas. We further stratified the ponds by position in relation to the stream channel and whether ponds were actively maintained by beaver. We probed the water depth and sediment thickness along transects to estimate the volume of sediment stored and compute the residual pool volume. Additionally, we collected sediment cores from each pond and analyzed stratigraphy and sampled for grain size and total organic carbon. Charcoal distinguished pre- and post-fire sediments while the time elapsed since fire gave sedimentation rates.

Preliminary results indicate that burned ponds have greater sedimentation rates (5 – 67 cm/yr) than unburned ponds (8 – 10 cm/yr). Ponds spanning stream channels in burned areas accumulate sediment more rapidly (35 -67 cm/yr) than off-channel ponds (5 – 33 cm/yr). Cores from burned ponds contain layers of coarse sands and gravels separated by layers of charcoal. In contrast, unburned cores consist of fine sediments and abundant organic material with less distinct layering.

The high sedimentation rates in burned beaver ponds suggest that these ponds effectively capture sediment in watersheds during the first years after a fire. Additionally, post-fire sediments appear to be fluvially transported from elsewhere in the watershed rather than aerially deposited or biotically produced within the pond. Even ponds that are not currently occupied by beaver or are disconnected from the stream channel store high volumes of post-fire sediment. These findings indicate that beaver ponds effectively capture post-fire sediment, temporarily preventing transport to downstream receiving waters, thus enhancing watershed resilience.