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Paper No. 87-7
Presentation Time: 10:05 AM
ACCELERATED GEOMORPHIC RESPONSE TO MASSIVE SEDIMENT LOADING BY THE 2008–2009 ERUPTION OF CHAITÉN VOLCANO, CHILE, FOLLOWED BY RAPID RECOVERY
The explosive phase (May 1–11, 2008) of the 2008–2009 eruption of Chaitén volcano in southern Chile distributed fine-grained pumiceous volcanic ash over a broad sector dominantly southeast of the vent, with proximal basins locally receiving >100 cm. The Chaitén River drainage basin (77 km2), situated south to southeast of the vent, exhibited rapid geomorphic changes and extraordinary rates of sediment transport in response to this initial sediment input. Modest rainfall (estimated 20 mm total, <3 mm/h) on May 11–12 triggered 4.5 m of channel aggradation of the Chaitén River bed 10 km downstream of the volcano by late afternoon on May 12, primarily by lahar deposition, although slightly greater rainfall on May 4–6 elicited no such response. The channel aggraded an additional 2.5 m through fluvial deposition by late on May 13 in response to an additional 60 mm of rain. With aggradation, pre-eruption bed material of mostly cobbles and boulders changed to coarse sand and fine pumice gravel. Over the next 10 days a major rainstorm pushed total sediment discharge from the Chaitén drainage basin to 4–8 x 108 m3 y-1 (equivalent annualized rate) or about 1 x 107 m3 km-2 y-1. This is an extraordinary unit-area sediment yield for a drainage basin >50 km2, if only for a short period. In late 2008 and early 2009 following the initial flush of tephra from the basin, two valley-filling pyroclastic flows supplied large inputs of additional but coarser sediment to the Chaitén River. Erosion and mobilization of these new deposits kept the downstream river bed elevated about 3 m above its pre-eruption level and kept a large delta growing at the river mouth through early 2010. Yet these responses to a major volcanic disturbance were short-lived. The Chaitén basin returned to near pre-eruption channel-bed elevations, dominantly coarse bed material, a single-thread channel form, and low sediment yields within four years.