A Record of Extreme Sedimentation Events in Central Idaho: Fire, Climate, and Changing Sediment Yields Over the Holocene

In the central Idaho mountains, most of the annual runoff in small streams occurs with snowmelt, suggesting that most sediment transport would occur during high seasonal discharges with recurrence interval of perhaps a few years. A spate of recent major debris-flow events in small basins, however, implies that infrequent severe storms where mass movement threshold conditions are crossed are also be major contributors to sediment yields. This landscape is characterized by deeply weathered Idaho batholith granitic rocks, and slopes mantled by thick, poorly cohesive grussy sediment. An important control on debris-flow generation is fire, which dramatically lowers threshold conditions for surface runoff generation and also lowers the cohesive strength of slope sediment after roots decay. A series of storms culminating in intense rain on melting snow in January 1997 caused numerous colluvial failures, debris flows, and sediment-charged floods in South Fork Payette River tributaries, with the majority in either burned forested or unburned rangeland basins. This event produced sediment yields up to 44,000 Mg km^-2 in ∼0.5 km² basins, which equals several thousand years of background sediment yields (2.7-30 Mg km^-2 yr^-1) measured over a few decades of trapping and gauging in Idaho batholith watersheds, where major debris-flow events are not included (Clayton and Megahan, 1986). Recurrence intervals of ∼ 400 years for such extreme events would account for 10,000-yr average Idaho batholith sediment yields of ∼ 116 Mg km^-2 yr^-1 (Kirchner et al., Geology 29, 2001). Shaub (2001) found that of > 200 colluvial failures in the 1997 event in the South Fork Payette drainage, 75% occurred in burned areas. Over the last several decades, however, other large debris flows have been generated from unburned slopes by unusually intense summer convective storms and winter rain-on-snow events; we hypothesize that these events are more probable in warmer climates. Long-term effects on the main South Fork Payette channel are strongly dependent on sediment size transported as well as the total mass. Deposit characteristics from 10 alluvial fan sections in small basins show that of the total stratigraphic thickness, over 90% is probably or possibly related to fire. Of ∼ 54 depositional units within these fans, 5 fire-related debris-flow deposits account for ∼ 25% of the total stratigraphic thickness. Radiocarbon dating of these 5 large events show that all occurred between ∼ 750-1100 cal yr BP, corresponding to the "Medieval Warm Period", a time of increased fire-related sedimentation in Yellowstone and severe droughts in parts of the western USA. Most other large debris flows appear to occur during relatively warm episodes in Idaho. Event frequency reaches a maximum at other times corresponding to cooler climate (e.g., ∼ 300-500, 1100-1400, and 6600-7400 cal yr BP), but fan aggradation rates from these frequent small events are relatively low, and one well-preserved fan built over the earliest period provides a low average sediment yield comparable to modern background rates. These results suggest that infrequent large events account for the majority of sediment transport out of small mountain basins, and that sediment yields are strongly nonstationary over millennial timescales. Variations in climate and associated changes in fire regimes and storms exert a major control on the occurrence of extreme erosional events.