AN ASSESSMENT OF LONG TERM AND SEASONAL CHANGES IN THE HYDROLOGY OF THE RIO GRANDE AND RELATED SPRINGS IN THE BIG BEND TO AMISTAD REACH

The purpose of this study is to observe and assess hydrologic conditions of the Rio Grande in the Big Bend region of Texas by utilizing stream flow and groundwater data. Another objective of the study is to assess the possible influences of climate change and groundwater pumping on the flow of important springs located within the reach of the Rio Grande from the confluence with the Rio Conchos to Amistad Reservoir. Surface water data can be combined with a stream budget analysis in order to estimate stream and groundwater flow relationships. These can be joined with subsurface geologic data to determine if the stream is gaining or losing spring flow over time. The area of interest for this study is the Rio Grande in the Big Bend Region of west Texas, beginning at the confluence of the Rio Conchos and continuing downstream through Big Bend National Park and the Rio Grande Wild and Scenic River to Amistad Reservoir (BBTAR). The Rio Conchos is the first perennial tributary after more than 460 kilometers, dramatically affecting the hydrology of the Rio Grande with increased stream flow. However during extreme low flow conditions, groundwater contributions from the Edwards-Trinity aquifer can contribute over 50% of total flow in the BBTAR reach. Average baseflow estimates from 1962 to 2011 show groundwater contributions ranging from 162 cfs (117,352 AFY) to 350 cfs (253,540 AFY). Average groundwater contributions separated by season show 228 cfs (54,503 AFY) during the spring, 230 cfs (54,981 AFY) during the monsoon season and 296 cfs (68,615 AFY) during winter. Precipitation data in the study region is insufficient to study the effects of climate change on baseflow. Groundwater wells in the study area also contain limited data to show water table fluctuations. This study identifies long term and seasonal changes that possibly affect spring flow within the BBTAR reach.