Recent large-magnitude floods and their impact on valley-floor environments of northeastern Yellowstone

The Lamar River watershed of northeastern Yellowstone contains some of the most diverse and important habitat in the national park. Broad glacial vall ey floors feature grassland winter range for ungulates, riparian vegetation that provides food and cover for a variety of species, and alluvial channe ls that are requisite habitat for native fish. Rapid Neogene uplift and Qua ternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine-riparian ecosyst ems. Uplift and glacial erosion have generated high local relief and extens ive cliffs of friable volcaniclastic bedrock. As a result, steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt. Recent major floods on trunk streams deposited extensiv e overbank gravels that replaced loamy soils on flood plains and allowed co nifers to colonize valley-floor meadows. Tree-ring dating identifies major floods in 1918, ca. 1873, and possibly ca. 1790. In 1996 and 1997, discharg e during snowmelt runoff on Soda Butte Creek approached the 100-year flood estimated by regional techniques, with substantial local bank erosion and c hannel widening. Indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996, with similar duration and m uch greater flood plain impact. Nonetheless, 1918 peak discharge reconstruc tions fall well within the range of maximum recorded discharges in relation to basin area in the upper Yellowstone region. The similar to 1873 and 191 8 floods produced lasting impacts on the channel form and flood plain of So da Butte Creek. Channels may still be locally enlarged from flood erosion, and net downcutting has occurred in some reaches, leaving the pre-1790 floo d plain abandoned as a terrace. Gravelly overbank deposits raise flood-plai n surfaces above levels of frequent inundation and are well drained, theref ore flood-plain soils are drier. Noncohesive gravels also reduce bank stabi lity and may have persistent effects on channel form. Overall, floods are p art of a suite of catastrophic geomorphic processes that exert a very stron g influence on landscape patterns and valley-floor ecosystems in northeaste rn Yellowstone. (C) 2001 Elsevier Science B.V. All rights reserved.