GEOMORPHOLOGY OF SAND BARS IN BOQUILLAS CANYON, BIG BEND NATIONAL PARK

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. As part of a larger effort to study the biology, hydrology and geomorphology of the Rio Grande in the Big Bend region, we have initiated a survey project to monitor the changing geomorphologic condition of the river in the canyon. Recent studies have shown that the Rio Grande is an environmentally stressed river. In addition to water quality issues, it suffers from upstream diversions, the encroachment of invasive species such as Tamarix sp. (salt cedar) and Arundo donax s.p. (giant river cane), and an overabundance of sediment. The net results are aggrading inset floodplains, a narrow channel, decreased geomorphic complexity and dereased aquatic habitat. Periodic floods created by runoff associated with tropical systems occur every decade or two and reestablish a more natural meandering river system. One such flood event occurred in the fall of 2008. Our surveys began in 2004 and have continued periodically until 2011. The data have been collected using a total station survey instrument using a permanent local datum for each bar. Our strategies for data collection have evolved over time. The early data sets include several cross sections for selected bars in the canyon. Later data collection efforts include full topographic coverage of the bars for the development of three dimensional models for geomorphic change detection analysis. These models make for better assessment of overall change but are time consuming to construct, and as a result, we are not able to collect data on all bars in the canyon. Comparative analysis of pre and post 2008 flood morphology is possible and reveals estimates of elevation changes. For example, at the entrance bar, the flood resulted in a wider river channel and the conversion of a point bar to a remnant island and small secondary high flow channel on the inside of a river bend. There was an overall lowering of almost all surface topography on the point bar with a maximum elevation loss of 5 meters in the area where the high flow channel developed. Extrapolation of the cross section data indicate an overall loss of 52,000 m³ over the area of the Texas side of the bar (36,000 m²).