Use of Remote Imagery to Analyse Changes in Morphology and Longitudinal Large Wood Distribution in the Blanco River After the 2008 Chaiten Volcanic Eruption, Southern Chile

摘要

The 2008 Chaiten volcanic eruption generated significant changes in the channel morphology and large wood (LW) abundance along the fluvial corridor of the Blanco River, southern Chile. Comparisons of remote sensing images from the pre-eruption (year 2005) and post-eruption (years 2009 and 2012) conditions showed that in a 10.2km long study segment the Blanco River widened 3.5 times from 2005 to 2009, and that the maximum enlargement was nine times the original width. Changes in channel width were lower between the years 2012 and 2009. The sinuosity and braiding indexes also changed between 2005 and 2009. After the eruption the channel sinuosity was higher and specific river reaches developed a braided pattern, but by 2012 the channel was recovering pre-eruption characteristics. Huge quantities of LW were introduced to the study segment; individual LW per km of channel length were 1.6 and 74.3 in 2005 and 2009, respectively, and more than 30 log jams km(-1) were observed in the year 2009. Between 2009 and 2012 the quantity of LW was very similar. Statistically significant relationships were found between the number of log jams and channel sinuosity and between the number of pieces of large wood with sinuosity and channel width. Wood was highly dynamic between 2009 and 2012: 78% of individual pieces and 48% of log jams identified in the 2009 image had moved by 2012. Finally the supervised classification of imagery associated with ArcMap tools was tested to identify large wood.