BEAVER, CLIMATE, AND SCALE EFFECTS ON LATE HOLOCENE STREAM BEHAVIOR IN NORTHERN YELLOWSTONE NATIONAL PARK

Beaver dams have altered valley floor morphology and sediment character along many streams in northern Yellowstone National Park. Beaver-related features along small streams (basin areas <10 km²) such as Oxbow, Geode, and Yancey Creeks include up to 2.5 m of fine-grained organic-rich pond sediments, berms (abandoned dams) on flood plains and terraces, and stream meanders forced by relict dams. Radiocarbon ages indicate long-term deposition rates of 0.5-6.0 mm yr^-1 for beaver-pond deposits, and modest net aggradation. On larger streams such as the Gardner and Gallatin Rivers (basin areas >50 km²), evidence for beaver effects is limited to thick, fine-grained overbank deposits with unusually high organic content and beaver-chewed sticks in muddy, low-energy fluvial deposits with little evidence of ponding. In late Holocene terrace deposits on the Gardner River, these sediments are 0.5-1.5 m thick, and sedimentation rates (0.2-3.4 mm yr^-1) are less than on smaller streams. Terrace deposits are ¹⁴C dated from 3000-2700, 1700-950, and 300-50 cal yr BP, with intervening episodes of incision. Despite beaver activity, net incision of 1.5 m occurred over the late Holocene. The timing of terrace formation is similar to Soda Butte Creek in NE Yellowstone, where during late Holocene warm periods punctuated by severe drought, e,g, the Medieval Climatic Anomaly ~1050-650 cal yr BP, discharge variability was likely greater, and large floods inferred to stem from rapid snowmelt and rain-on-snow events caused channels to incise. In cooler, effectively wetter periods, e.g. the Little Ice Age 650-100 cal yr BP, more consistent snowmelt runoff and a lack of extreme floods promoted lateral channel migration, forming wide floodplain surfaces later preserved as terraces. The disparity between net Holocene incision on rivers and minor net aggradation along smaller streams suggests that total stream power is an important control on fluvial behavior in northern Yellowstone. Valley reaches with low total stream power have undergone modest aggradation or show net vertical stability, in part stemming from beaver damming. Conversely, on larger rivers with greater stream power, changes in channel behavior and net incision are predominantly a function of variations in discharge and sediment supply related to Holocene climatic variations.