FLUVIAL GEOMORPHIC LEGACY INFLUENCES MODERN STREAM DYNAMICS IN NORTHERN YELLOWSTONE NATIONAL PARK

Riparian corridors are key ecological zones in the semiarid Northern Range of Yellowstone National Park. During the late 20^th century, Northern Range valley floors may have transitioned from a stable beaver-willow ecosystem to an unstable elk-grassland ecosystem that greatly diminished riparian habitat. The transition may have been initiated by wolf extirpation during the early 20^th century. The removal wolves increased ungulate herbivory, decreased riparian vegetation, decreased beaver damming, and triggered unprecedented channel incision. To assess stream morphology and behavior during the historical period, however, it is necessary to understand channel dynamics over millennial timescales because fluvial geomorphic history may influence modern stream dynamics and valley floor morphology. Along small streams in the Northern Range (basin areas <75 km²), valley floor sediments include up to 3.0 m of fine-grained overbank sediments and organic-rich beaver-pond sediments. Radiocarbon ages indicate long-term deposition rates of 0.5-6.0 mm yr^-1 for beaver-pond deposits, and modest net aggradation during the late Holocene (0-4 ka). On Blacktail Deer Creek, late Holocene deposits are inset in fine-grained terrace and alluvial fan deposits that comprise most of the valley floor. These deposits contain minimal organic material and have well-developed A horizons. Radiocarbon dates and an ash layer associated with Glacier Peak (11.2 ka) indicate deposition during the early and middle Holocene. Aggradation may be related to a drier climate in the early Holocene compounded by high rates of paraglacial sedimentation in the recently deglaciated landscape. The larger Gardner River preserves beaver pond sediments that are 0.5-1.5 m thick, and sedimentation rates (0.2-3.4 mm yr^-1) are less than smaller streams. Pond sediments are preserved in late Holocene terraces with intervening episodes of incision that coincide with drought. Despite beaver activity, net incision of 1.5 m occurred over the late Holocene. The fluvial geomorphic record suggests that Northern Range stream aggradation and degradation events are not exclusively controlled by historic ecological factors. Natural variability and geomorphic history, in part driven by Holocene climate fluctuations, are important controls on stream morphology.