Earth and Planetary Sciences ETDs

Publication Date

7-1993

Abstract

The Yellowstone fires of 1988 provided an opportunity to observe the geomorphic impact of widespread, intense forest fires on a mountain environment. Post-1988 fire-related sedimentation events also served as geomorphological and sedimentological analogs which were used to interpret a Holocene stratigraphic record of fire-related alluvial activity. Research focussed primarily on the steep-walled glacial trough valley of Soda Butte Creek, and parts of the Slough Creek and Lamar River drainages in northeastern Yellowstone.

All of the examined major post-1988 fire-related sedimentation events involved the generation of widespread surface runoff from brief, intense summer convective-storm precipitation on steep slopes in intensely burned basins. Sheetwash and rillflow entrained large quantities of fme soil-surface sediment. Where runoff became concentrated in low-order channels, deep incision and progressive sediment bulking occurred, with most smaller basins ( <2 km2) producing debris flows in initial phases of early postfire events. Deposition occurred mainly on alluvial fans along the sides of trough valleys. The depositional sequence in each event implied a progression from higher- to lower-sediment concentration flows. The relative importance of flow processes in an event is partly determined by basin characteristics such as area, relief, percentage of exposed bedrock, lithology, and available sediment. Later events were dominated by dilute flows, probably because of (1) flushing of channel storage in prior events, and/or (2) compaction of burned soil surfaces, thus reduced fine sediment availability even in previously inactive basins. In general, debris-flow dominated events caused fan aggradation, whereas more erosive streamflow-dominated events caused progradation of fans.

Post-1988 fire-related debris-flow deposits commonly displayed an abundance of mud-rich matrix bearing coarse charcoal. Similar charcoal-rich fire-related debris-flow facies were identified in Holocene alluvial fan stratigraphic sections and dated by 14C methods, calibrated to calendar years. Charcoal is typically scarce in fire-related hyperconcentrated-flow and streamflow sediments, but units of these facies were interpreted as probable fire-related sediments where they overlie well-preserved charred 0 horizons (soil surface layers), also 14C-datable.

An estimated 30% of the late Holocene fan alluvium in the Soda Butte drainage consists of fire-related debris-flow and probable fire-related sediments. Fire-related depositional events cluster strongly within the intervals of ca. 6500-6100 BC, 4500-4000 BC, 3500-2400 BC, 800 BC-350 AD, and 650-1200 AD. A major peak in fire-related debris-flow activity occurred at the height of the globally-recognized Medieval Warm Period ca. 1050-1200 AD, and notable correspondence exists between other warm and dry periods in high-resolution climate proxy records and fire-related sedimentation. Historical climate analogs imply that convective-storm activity during warmer periods is increased, but that high interannual variability of summer precipitation at such times may enhance the potential for both severe short-term drought and storm-generated fan deposition in northeastern Yellowstone.

Along lower Soda Butte Creek, fill-cut terrace surfaces are created by extensive lateral erosion of channels and concurrent accumulation of overbank sediments. Overbank aggradation occurred ca. 7300(?)-6700 BC (terrace level Tla), 5800-4500 BC (Tlb), 1400-800 BC (T2), 100 BC-650 AD (T3), and 1200-1700 AD (T4). Local paleoclimatic indicators imply effectively wetter conditions during these intervals. Because most present-day runoff, overbank flow, and suspended sediment yield occurs during the snowmelt period, floodplain widening and overbank sedimentation probably occurs during periods of generally cooler and wetter climate with higher winter precipitation. In transitions to drier intervals, reduced average runoff results in meanderbelt narrowing, and incision may be driven by extreme flood discharges during warm rain-on-snow events.

Alluvial systems in northeastern Yellowstone show a clear response to millennial-scale Holocene climatic cycles, where alluvial fan aggradation and progradation over valley floors during drier periods alternates with floodplain construction and trimming back of fans by axial streams during wetter periods. "Small-scale" climatic fluctuations of the Holocene thus had substantial impact on postglacial landscapes in northeastern Yellowstone.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Stephen G. Wells

Second Committee Member

Roger Y. Anderson

Third Committee Member

Kenneth L. Pierce

Fourth Committee Member

Gary A. Smith

Fifth Committee Member

Leslie D. McFadden

Project Sponsors

National Science Foundation; Geological Society of America and GSA Quaternary Geology and Geomorphology Division; Sigma Xi; UNM Graduate Student Association; UNM Department of Earth and Planetary Sciences (Kelley-Silver Fellowship)

Language

English

Keywords

geomorphology, wildfire, alluvial fan, debris flow, fluvial terrace, Medieval Climatic Anomaly

Document Type

Dissertation

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