INVESTIGATIONS OF LATERAL CHANNEL MIGRATION ON BEAVER DOMINATED STREAMS IN SOUTHWESTERN MONTANA, USA

Beavers are identified as ecosystem engineers for their ability to alter fluvial and riparian systems. Prior research on the geomorphic impacts of beaver has primarily focused on the short-term effects of beaver dams on water and sediment storage adjacent to dam sites. Recent work, however, has begun to explore system-wide effects of beaver and their interactions with fluvial geomorphic processes. Our work builds off these investigations, examining whether beaver activity has effects on rates of lateral channel migration. We compared seventy-two, 200 m long, reaches of two forks of a beaver dominated, high-elevation, snowmelt driven system in southwest Montana. High resolution aerial images were used to identify dammed and undammed reaches from three years; 1995, 2009, 2018. Dams were identified in 2009 data; the 1995 image served as the pre-dam image while 2018 was the post-dam image. Only reaches where dams were present exclusively in 2009, and not in the other two images, or remained undammed through all three periods were used in the analysis. The pre-dam (1995-2009) and post-dam (2009-2018) periods were used to calculate annual migration distances of the channel centerline within beaver dammed and undammed stream reaches. Reach-averaged migration rates for the post dam period (2009-2018), were compared between reaches dammed in 2009 to those that remained undammed throughout all images. Pebble counts, channel surveys and discharge data provide additional information on driving and resisting forces affecting migration dynamics. The pre-dam period migration rates range from 0.07 – 2.91 (mean 0.43) m/yr, for both dammed and undammed reaches, while post-damming rates range from 0.53 – 0.04 (mean 0.14) m/yr. In both forks there are significant differences for migration rates between time periods at the 95% confidence level while no significant difference exists between dammed and undammed reaches within time periods. The significance of time, rather than damming, suggests that the beaver influence on channel migration is less important than other factors in controlling migration in this beaver dominated system. Whether the observed pattern holds across other beaver dominated systems, and is comparable to non-beaver systems, remains an open question with implications for management of headwater streams.