The Changing Geomorphic and Ecologic Template of the Lower Rio Grande within Big Bend National Park, TX and Chihuahua and Coahuila, MX

The Rio Grande within Big Bend National Park (BBNP), TX and Chihuahua and Coahuila, MX has experienced a reduction in active channel width of over 35% over the last 18 years. Channel narrowing coincided with over 2.4 meters of vertical floodplain aggradation and 1 to 2 meters of channel bed aggradation. These rapid geomorphic changes occurred following two channel widening events in 1990. Ensuing drought conditions and tightened water management upstream on the upper Rio Grande in the U.S. and Mexico and the Rio Conchos in Mexico resulted in a significant reduction in stream flow as the sediment supply from tributaries remained relatively unaltered. Other geomorphic observations include the development of inset floodplains, the disconnection of the floodplain from the channel and overall channel simplification. Additionally, a lack of sufficient flooding during the recent drought years resulted in the increased colonization of the riparian corridor by both non-native (Arundo donax, Tamarix chinensis, Tamarix ramosissima) and native vegetation (Salix exigua, Prosopis glandulosa) which further promote floodplain deposition and the disconnection of the floodplain from the channel. These rapid geomorphic and ecologic changes appear to be significantly impacting both the quality and quantity of riparian and aquatic habitat along the Rio Grande as numerous fish and amphibian species have been extirpated (Rio Grande silvery minnow – Hybognathus amarus, Woodhouse’s toad – Bufonidae woodhousii) from or are threatened (Big Bend Slider – Trachemys gaigeae) within the Rio Grande in BBNP