EXPLOITING REPEAT REAL TIME KINEMATIC GPS TOPOGRAPHIC SURVEYS AS A MEANS TO EVALUATE SPACE-TIME CHANGES IN SEDIMENT STORAGE IN THE HEADWATERS OF THE TERLINGUA SUBBASIN, TEXAS

Extensive, large-scale development on the upper Rio Grande and río Conchos throughout the 20^th century has significantly altered the natural sediment regime of the lower Rio Grande, most notably, in the Big Bend region of Texas, Chihuahua, and Coahuila. This relatively rapid and pronounced modification of the lower Rio Grande’s water/sediment budget has resulted in significant geomorphic adjustments of both the channel and its floodplain. The channel of the lower Rio Grande has narrowed by more than 50% since 1941; additionally, the floodplain has seen vertical aggradation of up to 3.0 m, between 1993 and 2008 (Dean and Schmidt, 2011).

The Terlingua subbasin (HUC 13040204) is an important contributor of both water and sediment to the lower Rio Grande in the Big Bend region. Its north-south elongated area of approximately 2851 km² is principally drained by Terlingua Creek. Monsoonal rains and short-lived, intense thunderstorms account for virtually all of the flow in Terlingua Creek, thus dictating its flashy hydrologic regime. While subsistence flows range from 0.03 to 0.04 m³/s, peak flows can reach 480 m³/s. Furthermore, the 1 in 5 year flow for Terlingua Creek is approximately half of the long-term 2 year flood on the lower Rio Grande. Based on historical mean daily discharge data collected at IBWC 08-3745.00 Terlingua Creek from 1932 to 2011, Terlingua Creek contributes approximately 47.8E3 m³ (38,800 AF) of water per year to the lower Rio Grande.

In order to assess reach-scale, spatial and temporal sedimentation within the Terlingua subbasin, a 300 m stretch of Terlingua Creek was analyzed through repeat RTK GPS topographic surveys that collectively comprise a four-year dataset (2011-2014). Using Geomorphic Change Detection 6.0 software in which total uncertainty is quantified and propagated throughout the Digital Elevation Model (DEM) on a cell-by-cell basis, the resultant dataset was utilized to create DEM of Difference (DoD) maps. Ultimately, the DoD maps provided a mechanism by which volumetric changes in sediment storage were quantified within and across survey years.