Assessment of Bank Erosion Associated with Riparian Vegetation Removal along the Colorado River in Western Colorado
Gigi Richard1 and Shannon Hatch2
1Fort Lewis College, Durango, CO, USA; garichard@fortlewis.edu
2Rivers Edge West, Grand Junction, CO, USA; shatch@riversedgewest.org
The purpose of this study was to assess changes in channel mobility following tamarisk and Russian olive (TRO) removal along a 51-km reach of the Colorado River in western Colorado via GIS analysis of repeat aerial photos, remote sensing from drone-collected imagery, and field surveying of channel cross sections at vegetation removal sites. The three-pronged investigation included: field surveys of 21 cross sections at three TRO removal sites between 2013 and 2018; measurement of bank erosion along the 51-km study reach using repeat aerial photos from 2002-2016; and, two experimental drone flights in 2017 over the three study sites to assess vegetative regrowth and channel change following vegetation removal. The GIS analysis of aerial imagery revealed that increased bank erosion was significantly correlated with TRO removal during the 2007-2012 time period, during which the annual peak flows exceeding the mean annual flood four times. No clear trend emerged from the GIS analysis to indicate that erosion increased or decreased with time following TRO removal. The Colorado River Island site was the most mobile of the field study sites, with bank erosion between the 2015 and 2018 field surveys of up to 4.8 meters. Erosion at this site was corroborated by aerial photo measurements from 2002-2016, field cross-section surveys, and two drone flights in 2017, and occurred pre- and post-vegetation removal. Significant erosion occurred on both the north and south banks of the river at this site, whereas TRO removal was only performed on the north bank. Results from the three investigation methods revealed that channel change and bank erosion are correlated with high flow events and occur along this reach regardless of vegetation removal efforts. Future work could include an assessment of the method of TRO removal and revegetation efforts, which could contribute channel stability or mobility following vegetation removal.