BEAVER DAMMING EFFECTS AND PERSISTENCE ON A MOUNTAIN STREAM, CENTENNIAL VALLEY, MONTANA

Some river restoration specialists advocate using beaver to raise water levels and increase floodplain-stream connectivity, vegetation production and overall heterogeneity. However, effects of beaver damming are likely dependent on specific stream characteristics and data on dam effects, especially their longevity, are limited. To increase understanding, 9 reaches were studied on Odell Creek at Red Rock Lakes National Wildlife Refuge, Centennial Valley, Montana. Odell Creek has a basin area of 46 km² and a snowmelt dominated hydrograph with peak flows between 80-200 cfs. It flows down a fan with a decreasing gradient (0.011–0.001) as a mostly single-thread, sinuous channel, except where beaver damming has caused overbank flooding and created multi-thread channels. The chosen study reaches represent longitudinal variability and beaver dams built and abandoned over the last 20 years. Present dams are concentrated in middle reaches and the delta; elsewhere stream power and high confining banks appear to limit beaver damming.

We investigated in-channel sediment characteristics and storage through pebble counts, fine sediment surveys and sediment mapping. The site of maximum accumulation from beaver damming covers ~660 m² where the main channel is filled with ~70 m³ of fine sediment (≤ 2mm). The main dam (52 m long), and associated smaller dams, diverted 50% of the flow from the main channel in May and June 2010 flows. This complex represents a significant discontinuity, but its persistence is unknown. 46 cross-section profiles, 9 water surface and bed surface profiles have been surveyed to analyze differences in channel geometry in relation to dams identified from air photos.

Observations suggest that persistence of individual dam effects on Odell Creek is limited to the decadal scale. However, if beaver remain in the system, sites along the creek will be in different stages of return toward a pre-dam state, maintaining heterogeneity and discontinuity along the stream corridor.