THE IMPACT OF SNOW AVALANCHES ON WOOD DELIVERY AND GEOMORPHIC CHANGE IN MONTANE STREAMS

It is widely acknowledged that snow avalanches have a tangible role in the wood dynamics of mountain streams, but studies that explicitly quantify their impact with regards to such processes are comparatively minimal. In the late winter of 2019, the central and southern mountains of Colorado experienced an extreme avalanche cycle of historic magnitude that resulted in observable damage to alpine forests across the region; indeed, images of swaths of tortuously twisted conifer trunks made both statewide and national news. Utilizing this significant and temporally well-constrained period of events, we seek to further establish the role of snow avalanches as suppliers of wood to montane valley bottom streams and, consequently, drivers of fluvial geomorphic adjustment in mountain watersheds. Preliminary investigation in three catchments spanning gradients of avalanche magnitude and density – from large and frequent to small and rare – and stream management response – from in-stream wood removal to no interference – indicates that the total area of forest disturbed during the winter of 2019 ranges from 5 to 45 hectares per watershed, representing between 1-3% of the total forested area in each catchment. Damaged forest from individual avalanches ranged from 0.1 to 17 hectares. Areas of impacted forest are similar in magnitude to disturbance events (blowdowns) whose notable impacts on in-stream wood loads are better constrained. Such similarity in extent provides initial quantitative support for snow avalanches as an important supplier of in-stream wood in forested alpine catchments. Moreover, preliminary analysis of aerial imagery indicates that the majority of slides in each watershed (~75-90%) delivered observably substantial wood volumes to the stream corridor and roughly 65% fully overran the channel. Early results suggest that the relationship between avalanche density and fluvial geomorphic impact is non-linear, implying that large-magnitude events may have an outsized impact on stream morphology in such watersheds. Results of this work have important ramifications for the management of mountain watersheds; as climate change continues to influence snow and avalanche dynamics in complex ways, a thorough understanding of avalanches as a geomorphic process driver for headwater streams is essential.