BEAVER-INDUCED CHANGES TO REACH-SCALE FLUVIAL GEOMORPHOLOGY IN THE ADIRONDACK MOUNTAINS, NEW YORK, USA

Beaver occupy and modify fluvial landscapes via dam-building and the associated slowing of water and sediment in ponds. Not all sites are suitable for beaver in terms of food availability, baseline hydraulic geometry, and watershed hydrology, but, if beaver can establish themselves within a floodplain, they can engineer that fluvial system to further suit their needs. This study investigates the impact of beaver on the reach-scale fluvial geomorphology of small catchments in the central Adirondack Mountains of New York.

Our primary catchment of interest is Panther Brook (D.A., 3.5 km²) which is located within Huntington Wildlife Forest near Newcomb, NY. First, we established a topographic baseline by surveying the stream’s longitudinal profile from its base level (Catlin Lake) to within 300 m of the watershed’s eastern drainage divide. At each survey station we measured bankfull width and depth. At each 50 m survey station along the longitudinal profile, we conducted three 100-pebble counts to characterize in-channel grain size distributions.

Two distinct morphologies emerge for Panther Brook: 1) low-gradient, beaver-impacted “meadow” reaches, and 2) steeper “boulder” reaches flowing over eroded till between meadows. The beaver-impacted reaches are primarily narrow channels incised into beaver meadow sediment. These have gradients ranging from 0.0001 to 0.0485 with a mean of 0.0173. The meadow stretches generally have low relative bed roughness, d_90­­/D, ranging from 0.00135 to 0.302, largely controlled by the fine sediment being remobilized from the beaver meadow stream banks. Bankfull shear stress for the meadow reaches range from 4 to 285 Pa. In contrast, the boulder reaches have gradients from 0.003 to 0.115, relative roughness values ranging from 0.0450 to 0.785, and bankfull shear stresses of 33-723 Pa.

Intermittent beaver activity on Panther Brook has forced reaches at the transition between “meadow” and “boulder” morphologies from supply-limited to transport-limited regimes. These beaver-forced shifts in reach-scale transport capacity dramatically impact sediment storage in Panther Brook. Our results suggest that without beavers’ engineering efforts, sediment would be rapidly routed from source to sink even across moderately low-gradient reaches that are now buried in beaver meadow sediment.