CHANNEL FORM AND PROCESS IN A TECTONICALLY-ACTIVE, GLACIATED RIVER VALLEY: UPPER YELLOWSTONE RIVER, PARK COUNTY, MONTANA, USA

The Upper Yellowstone River flows through a tectonically-active, glaciated valley in south-central Montana. Contemporary fluvial geomorphology of channel and floodplain deposits is strongly influenced by: active faulting, which lowered and tilted the valley; spatial distribution of poorly-sorted, cobble-boulder till (Bull Lake and Pinedale) and gravel outwash; and confining terraces formed by incision into the coarse till and outwash. Very large paleofloods occurred about 13.7 ± 0.5 ka (cosmogenic ¹⁰Be) and 5.6 cal ka (4900±300¹⁴C yrs.B.P) (Kenneth Pierce, pers. comm. 2002), and left their imprint in the form of mega-ripples south of Chico Hot Springs and flood modified gravels further downstream. Contemporary spatial distribution of Montgomery-Buffington channel types (e.g. plane-bed, pool-riffle, braided, anabranching-braided) is largely explained by these tectonic, glacial and post-glacial influences—as is occurrence of riparian vegetation, large woody debris, side channels, aquatic habitat, and human channel modifications (e.g. revetment) seeking to stabilize the channel.

Channel changes measured after recent large floods ( near 100-year, recurrence-interval floods of 1974, 1996, 1997, and 2011) occurred primarily through lateral erosion in pool-riffle channel segments and avulsion and lateral erosion in braided and anabranching channel segments. The Upper Yellowstone River deviates from the general model of channel response to large floods, in several ways: 1)terrace-bounded, plane-bed, channel segments, where stream power is confined to a single channel are essentially static and have not incised or eroded laterally, 2)pool-riffle, braided and anabranching segments have changed through avulsion and bank erosion which maintains the dominant plan-view channel configuration. Between large contemporary floods more frequent flood flows, with return periods close to the conventional "bankfull" discharge (e.g. 2 to 5-year floods), shape and maintain average characteristics of the main channel and individual anabranches. Consequently, the channel, floodplain, terrace complex consists of a mosaic of fluvial features influenced by geomorphic processes spanning the past 13,000 years.