BEAVER ACTIVITY IN RIPARIAN LANDSCAPES DURING HOLOCENE CLIMATE CHANGE IN THE GREATER YELLOWSTONE ECOSYSTEM

Riparian landscapes are an ecologically critical, yet limited, habitat type within the arid and semi-arid region of the western United States. The North American beaver (Castor canadensis) depends on riparian habitat, but also helps to sustain it. In the western US, beaver are being used to aid in restoration of streamside habitats. Yet, questions remain about past distribution and ecological effect of beaver. To address questions of long-term variability in beaver occupation, we are developing a detailed chronology of beaver-assisted sedimentation and overall fluvial activity for Odell Creek (basin areas 83 km²) in Centennial Valley (CV), Montana. Concentrations of beaver-chewed sticks preserved in stream terraces and paleochannels are a proxy for the presence of beaver and riparian vegetation. The relative ages of sites within the study area were determined by elevation data from airborne LiDAR and ground surveys. Numerical ages were obtained from 36 ¹⁴C ages (~30 more are pending) of beaver-stick wood and other organic material. The ages of the terraces span the last ~11,000 years and show a general trend of post-glacial incision. The oldest and highest terrace lacks beaver-stick deposits which exist in the younger, mid-Holocene, terraces. Most of the beaver-stick deposits create a distinct layer of woody material, sometimes covered by sand, overlying gravel channel deposits. The upper part of the stratigraphy at most sites is composed of silt. The stratigraphy is consistent with observations of beaver-chewed stick accumulations on point bars and frequent dam breaching (≤ 5 years) in the modern channel of Odell Creek. Initial beaver-stick deposit ages for CV streams show notable clustering in colder-wetter intervals ca. 5200-5000 and 3800-3600. Beaver-stick deposits from 900-550 cal yr BP, however, overlap the arid Medieval Climate Anomaly (MCA) (~1050-650 cal yr BP), a time of major fire-related debris-flow activity in Yellowstone, and minimal beaver-pond sedimentation in smaller (basin areas 0.1-50 km²) Greater Yellowstone Ecosystem streams. The presence of beaver-stick deposits during the MCA may indicate the importance of larger streams like Odell Creek for maintaining riparian zones and beaver refugia in drought-prone episodes, a critical concern with current and future warming.