GEOMORPHIC CONTROLS ON HYDRAULIC PROCESSES OF BLACKTAIL DEER CREEK, YELLOWSTONE NATIONAL PARK

Stream hydraulics and channel morphology are influenced by biologic and geologic factors. Trophic Cascade theory postulates that wolf removal in Yellowstone National Park initiated a chain of ecosystem changes: elk proliferated and heavily browsed vegetation, beavers were extirpated, and beaver dams deteriorated. Loss of beaver dams and riparian vegetation has been proposed to have caused channel widening and incision on Blacktail Deer Creek. Our study investigates fluvial geomorphic controls on channel form and stream hydraulics for two adjacent tributaries (with similar ecosystem controls but different drainage basin areas).

We simulate how 1.5-2, 10, 100-yr flood discharges inundate channels and floodplains using HEC-RAS. We integrate RTK-GPS cross section topographic field surveys with LiDAR DEMs to generate three-dimensional channel and valley floor geometries. To account for basin size differences, our models simulate Blacktail Deer Creek’s west and east forks individually. To consider how underlying geology impacts inundation, we further divide the east fork into a lower reach and three similar upper reaches.

At the same recurrence interval flood discharge, the east and west forks inundate different percentages of the channel and floodplain. Along the west fork, high magnitude discharges rarely overbank the channel. In contrast, the lowest east fork reach, crossing the same alluvial fan, fills the channel and spills onto the fan surface during low and high magnitude floods. Different stream behaviors may be due to distinctions in channel geometry. The west fork has a larger basin area and greater peak discharges. Greater discharges allow the stream to incise into fan sediments to form a wider and deeper channel. The east fork has less stream power and thus decreased potential for incision. Unlike the lower east fork reach, the upper three east fork sections overbank the channel during moderate (10-yr) flood discharges. Here, the river incises early Holocene valley-fill material instead of alluvial sediments. The differing responses to similar magnitude floods demonstrate that fluvial geomorphic history is an important control with persistent effects on the character of flood events.