Extraordinary sediment delivery and rapid geomorphic response following the 2008–2009 eruption of Chait en Volcano, Chile

Abstract

The 10 day explosive phase of the 2008-2009 eruption of Chaiten volcano, Chile, draped adjacent watersheds with a few cm to >1 m of tephra. Subsequent lava-dome collapses generated pyroclastic flows that delivered additional sediment. During the waning phase of explosive activity, modest rainfall triggered an extraordinary sediment flush which swiftly aggraded multiple channels by many meters. Ten kilometer from the volcano, Chaiten River channel aggraded 7 m and the river avulsed through a coastal town. That aggradation and delta growth below the abandoned and avulsed channels allow estimates of postdisturbance traction-load transport rate. On the basis of preeruption bathymetry and remotely sensed measurements of delta-surface growth, we derived a time series of delta volume. The initial flush from 11 to 14 May 2008 deposited 0.5-1.5 x 10(6) m(3) of sediment at the mouth of Chaiten River. By 26 May, after channel avulsion, a second delta amassed about 2 x 10(6) m(3) of sediment; by late 2011 it amassed about 11 x 10(6) m(3). Accumulated sediment consists of low-density vesicular pumice and lithic rhyolite sand. Rates of channel aggradation and delta growth, channel width, and an assumed deposit bulk density of 1100-1500 kg m(-3) indicate mean traction-load transport rate just before and shortly after avulsion (similar to 14-15 May) was very high, possibly as great as several tens of kg s(-1) m(-1). From October 2008 to December 2011, mean traction-load transport rate declined from about 7 to 0.4 kg(-1) m(-1). Despite extraordinary sediment delivery, disturbed channels recovered rapidly (a few years)