FIRE AND ALLUVIAL CHRONOLOGY IN YELLOWSTONE-NATIONAL-PARK - CLIMATIC AND INTRINSIC CONTROLS ON HOLOCENE GEOMORPHIC PROCESSES

We employed a systemwide approach, a large and robust set of radiocarb on ages, and modern process analogs to interpret the Holocene history of forest fire-related sedimentation and overall alluvial activity in northeastern Yellowstone National Park. Debris-flow and flood events f ollowing the 1988 fires provided facies models for interpreting the st ratigraphic record of fire-related sedimentation within valley-side al luvial fans of Soda Butte Creek. Fire-related deposits make up approxi mately 30% of the late Holocene fan alluvium. Fifty C-14 ages on fire- related events cluster within the intervals of 3300-2900, 2600-2400, 2 200-1800, and 1400-800 yr B.P. and suggest earlier episodes of large f ires and fan aggradation around 7500, 5500, and 4600-4000 yr B.P. A ma jor pulse of fire-related debris-flow activity between 950 and 800 yr B.P. coincided with the height of the widely recognized Medieval Warm Period (ca. A.D. 1050-1200). Instrumental climate records over the las t similar to 100 yr in Yellowstone imply that the intensity and intera nnual variability of summer precipitation are greater during warmer pe riods, enhancing the potential for severe short-term drought, major fo rest fires, and storm-generated fan deposition. Along lower Soda Butte Creek, fill-cut terrace treads were created by lateral migration of c hannels and accumulation of overbank sediments ca. 8000 yr B.P. (terra ce level Tla), 7000-5600 (T1b), 3100-2600 (T2), 2000-1300(T3), and pos t-800 yr B.P. (T4). These periods coincide with overbank sedimentation on Slough Creek and the Lamar River but alternate with intervals of f ire-related fan deposition, implying a strong climatic control. Local paleoclimatic data suggest cooler, effectively wetter conditions durin g terrace tread formation. In warmer, drier intervals, reduced average runoff in axial streams results in meander-belt narrowing; concurrent channel incision may be caused by infrequent large floods. Greater re sistance to down-cutting, however, allowed fewer terraces to be formed along Slough Creek and the Lamar River. Alluvial systems in northeast ern Yellowstone show a clear response to millennial-scale climatic cyc les, wherein alluvial fans aggrade and prograde over hood plains in dr ier periods. Axial streams widen their flood plains and trim back the fans during wetter periods. ''Small-scale'' climatic fluctuations of t he Holocene thus had substantial impact on postglacial landscapes in n ortheastern Yellowstone.