CLIMATE CHANGE AND LOCAL GEOMORPHIC CONTROLS ON FLUVIAL PROCESSES ON SMALL STREAMS IN NORTHERN YELLOWSTONE

Small streams in northern Yellowstone have responded to Holocene climate change, but the nature and magnitude of response is strongly influenced by local geomorphic factors such as hillslope sediment sources, tributary junctions, beaver damming, and local base level controls that vary greatly between reaches. Many streams were vertically stable or slowly aggrading in the early to middle Holocene; this was followed by downcutting, of variable magnitude and timing, in the late Holocene. On Blacktail Deer and Lost Creeks, incision of a middle Holocene minor fill terrace (~2 m above bankfull) began prior to ~1450 cal yr BP. We infer that the late Holocene tendency for incision was caused by a general change to a wetter climate, contemporaneous with the onset of regional Neoglaciation and an increase in the magnitude of snowmelt-runoff floods. More localized cases of modern channel incision on Elk and Geode Creeks and the Gardner River are related to stream capture, channel avulsions, tributary fan effects, beaver dam abandonment, and floods from severely burned basins.

A generally wetter late Holocene climate also corresponds to evidence for increased beaver damming after ~4000 cal yr BP, likely aided by more consistent summer base flow. Geomorphic effects of beaver damming depend on stream system scale. On small streams, these include local aggradation of 1-2 m of fine sediment and berms from abandoned beaver dams that locally affect channel morphology. On the larger Gardner and Gallatin Rivers, beaver-pond sediments exist locally in terrace deposits, but dams are now present only on side channels and spring-fed tributaries, and little evidence exists for long-term beaver effects on main channel or valley floor morphology.

An overall wetter late Holocene climate was punctuated by warmer periods that included prolonged severe droughts; e.g., evidence for beaver-related sedimentation is sparse during Medieval time, 950-750 cal yr BP, also a time of severe forest fires in Yellowstone. At the same time, deposition of anomalously coarse flood gravels on Elk, Geode, and Tower Creeks (~ 820, 790, and 990 cal yr BP, respectively) indicates major floods, potentially from burned basins, rapid snowmelt, and/or extreme storms. In recent severe droughts, some perennial streams occupied by beaver in the early 1900s have become ephemeral.