FIRE-INDUCED SEDIMENTATION IN ROCKY MOUNTAIN CONIFER FORESTS

We compare fire-induced sedimentation processes in lodgepole pine-dominated forests on andesitic and rhyolitic volcanic rocks in Yellowstone with ponderosa pine forests on the Idaho batholith. In glaciated Yellowstone, colluvium is typically <1 m thick. Most debris-flow events after the 1988 fires stemmed from reduced infiltration and pervasive runoff in thunderstorms, with sediment contributed from both rilled slopes and incised channels. Sediment yields in small (0.3-2 km²) basins ranged from 4800-23,000 Mg/km², with greatest yields in Gibbon Canyon, where weathered Lava Creek Tuff produces abundant grus-like sediment. More than 3 yr after the fires, regrowth of >30% vegetative cover suppressed runoff and limited the number and severity of events. Slump and slide failures of surficial sediments and shale were relatively few and less clearly attributable to fire, but some yielded >100,000 Mg/km² when saturated masses transformed to debris flow. Most occurred >7 yr after fire, implicating loss of root strength, but melting of heavy snowpacks in 1996 and 1997 was a major cause of failure. Millennial-scale Holocene climatic cycles (Bond et al., Science 278, 1997) have strongly controlled major fires and sedimentation in Yellowstone.

In Idaho, grussy colluvium is susceptible to 2-5 m thick slide failures, but runoff-related debris-flow events also follow fires, as when an Aug. 1995 thunderstorm in the N. Fk. Boise River drainage hit slopes burned in 1994. Areas burned in 1989 along the S. Fk. Payette River had dense herbs and shrubs by 1996, but a rain-on-snow event culminated on Jan. 1, 1997 in numerous colluvial failures, with small-basin sediment yields up to 44,000 Mg/km². Comparison of sediment yields in 1997 events with those estimated from ¹⁴C-dated alluvial-fan sediments, and with short-term sediment gaging and Holocene-scale cosmogenic nuclide estimates of Kirchner et al. (EOS Abstracts 79, 1998), suggests that events like those in 1997 have recurrence intervals of at least several 100 yr, and that sediment yields vary strongly over millennial timescales. Further dating should elucidate how Holocene climate and fire regime changes have controlled sedimentation in Idaho batholith ponderosa environments.