BEAVERS, CLIMATE, AND FLUVIAL GEOMORPHIC CHANGE OVER THE HOLOCENE IN THE NORTHERN RANGE OF YELLOWSTONE NATIONAL PARK

Through relict beaver-related deposits and landforms, small streams in the Northern Range of Yellowstone National Park provide the opportunity to document the long-term effectiveness of beavers as geomorphic agents and assess climatic and fluvial geomorphic controls on beaver occupation. Preliminary radiocarbon ages (n = 21) on beaver pond sediment suggest that beaver occupation over the late Holocene has been episodic on centennial-millennial timescales. Distinct periods of beaver activity fall within the intervals of 4350-3350, 1250-950, and 550-0 cal yr BP. Although preservation and exposure concentrate these ages within the last 1500 yr, there is a peak in activity within the middle Little Ice Age. A notable gap in dated beaver pond sediment exists 950-750 cal yr BP, coincident with widespread multidecadal droughts ca. 920 and 800 cal yr BP within the Medieval Climatic Anomaly as well as a major episode of debris flows following severe fires in Yellowstone 950-700 cal yr BP. The coincidence of changes in beaver activity with these episodes suggests that the ability of beavers to maintain impoundments is influenced by climate variability via changes in discharge and riparian vegetation on these small streams, some of which are ephemeral in drought conditions. Past beaver dams and associated deposits are located on stream reaches with favorable geomorphic conditions for preservation such as lower gradient and down-valley constrictions. Persistent beaver-related changes to streams and valleys include aggradation by the accumulation of thick, fine-grained deposits, subsequent incision, and terrace formation, but both aggradation and incision since deglaciation have been limited to a few meters. Many stream reaches show little net effect that can be attributed to beavers. Overbank aggradation not clearly related to beaver dams occurred on Geode Creek during the interval of 9450-7450 cal yr BP, followed by flood plain stabilization indicated by a buried soil, and then continued fine-grained aggradation until present. A 1.5 m terrace along Elk Creek is underlain by stacked, alternating channel gravels and fine-grained overbank sediments with an age of ~10,000 cal yr BP in the center of the section. Overall, beaver dams are just one of a suite of factors influencing channel and floodplain morphology on these relatively pristine small streams.