CLIMATE AND SCALE EFFECTS ON STREAM BEHAVIOR IN THE NORTHERN RANGE OF YELLOWSTONE NATIONAL PARK

Riparian corridors are key ecological zones in semiarid regions and are typically more biologically diverse than the overall landscape. These corridors are particularly important in the relatively dry Northern Range of Yellowstone National Park. The removal of wolves, an apex predator, in the early 20^th century may have transitioned Northern Range streams from a stable beaver-willow ecosystem to an unstable elk-grassland ecosystem by way of a trophic cascade. Some research suggests that wolf extirpation increased ungulate herbivory, decreased riparian vegetation, decreased beaver damming, and initiated unprecedented channel incision. To assess stream variability during the historical period (here defined as the time of written and photographic records), it is necessary to quantify baseline natural stream variability during the Holocene. Along small streams (basin areas <75 km²) such as Elk, Geode, Oxbow, Lost, and Blacktail Deer Creeks, valley floor sediments include up to 2.5 m of fine-grained organic-rich beaver 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. A 1.5 m terrace above the modern floodplain of Blacktail Deer Creek accumulated 7100-3300 cal yr BP and incision had initiated by 1450 cal yr BP, thus predating the historical period. On relatively large streams such as Fan Creek, the Gardner River, and the Gallatin River (basin areas >75 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, 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 formed 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 fluvial geomorphic record suggests that Northern Range channel incision is not solely related to historic trophic cascade-related events. Natural variability, in part driven by Holocene climate fluctuations, is a significant control on stream behavior.