Beaver as drivers of hydrogeomorphic and ecological processes in a mountain valley

Abstract

In aquatic and semi-aquatic environments of North America, beaver (Castor canadensis) affect landscape structure and dynamics at a level rivaled only by humans. The objective of this dissertation is to identify the influence of beaver dams on hydrologic, geomorphic, and ecological processes in a mountain valley. The influence of two in-channel beaver dams and a 10-year flood event on surface inundation, groundwater levels, and flow patterns was examined in a 1.5-km section of the Colorado River valley in Rocky Mountain National Park, Colorado during the summers of 2002-2005. The two beaver dams and associated ponds controlled surface water and groundwater flow patterns over a larger portion of the valley and for a longer duration than a 10-year recurrence-interval flood. Most importantly, the primary hydrologic effects occurred downstream of the dams rather than being confined to the near-pond area. The dams attenuated the expected water table decline in the drier summer months for 9 and 12 ha of the 56-ha study area. My results suggest that beaver can create hydrologic regimes suitable for the formation and persistence of wetlands on large floodplains. The mechanism of beaver meadow formation was also examined by measuring sediment deposition, nutrient availability and plant species cover in a 4.3-ha area of the valley that was hydrologically influenced by one of these beaver dams. The in-channel dam triggered overbank flooding in the study area, killing vegetation in areas that were deeply flooded and deposited ~750 m3 of sediment on the floodplain and terrace west of the river. The study area formed a spatially heterogeneous beaver meadow after the dam failed and the area drained. Bare sediment was quickly colonized by Carex utriculata and C. aquatilis forming sedge-dominated communities on wet sites and early successional grasses such as Critesion jubatum and Agrostis scabra forming grasses-dominated communities on dry sites. Willow and aspen seedlings were found throughout the beaver meadow, suggesting that the sedge and grass plant communities may succeed to a shrub-carr community, facilitating future reoccupation of the site by beaver.