TOPOGRAPHIC CHANGE RELATED TO CHANGING FLOW CONDITIONS IN THE RIO GRANDE, BIG BEND NATIONAL PARK, TEXAS, USA

The Big Bend reach of the Rio Grande is the aquatic centerpiece of a binational conservation area encompassing 3.5 million acres within western Texas, USA and northern Mexico. Declining flows and water quality, channel narrowing and the influx of invasive vegetation and sediment have negatively affected the geomorphic condition and the aquatic habitat of the river. The Boquillas canyon reach of the Rio Grande includes approximately 17 miles of steep wall limestone canyon in the eastern part of Big Bend National Park. Topographic surveys of selected sand and gravel bars in the canyon have been conducted periodically since 2004 in order to monitor the geomorphic condition in the canyon. This study reports results for 4 topographic data sets collected along the river near the entrance to Boquillas canyon. Data were collected in 2004 in the form of 3 cross sections, in 2011 and 2013 in the form of traditional survey techniques using Total Station and RTK GPS, and in 2012 in the form of an aerial LIDAR survey. An area of interest was established as a bounding polygon that included full coverage for the 2011 to 2014 data, and a technique was developed to extrapolate the 2004 cross section data to estimate topographic change from 2004 for the same area. Results indicate a net volume loss between 2004 and 2011 of 40,517 m³. This loss was due to a large channel resetting flood event that occurred in 2008. The net volume change between 2011 and 2012 was 99 m³ gained. This was a period of drought and overall low flow resulting in very little topographic change. An overall net gain of 4,482 m³ was recorded for the 2012 to 2013 time period. The bulk of this gain was recorded as deposition in a small high flow channel. This observed sediment gain supports the hypothesis of gradual channel narrowing following periodic high flow channel resetting events (Dean and Schmidt, 2010).