How have hydrologic controls on the Rio Grande effected the morphology of the river downstream of Brownsville? Chris Braun

Site Map Showing Extent of Reach

Controls of River Morphology Natural controls: velocity of flow geology sediment load gradient climate time (sources: Costa and O’Conner 1995, Eschner, et al. 1983, Leopold and Wolam 1957, Pickup 1976, Shumm 1977) Anthropogenic controls: water diversions dams land use in the flood plain channel rectification (sources: Church 1992, Collier, et al. 1996, Hirsch et al. 1990, Huang and Pogge 1978, Lagasse 1981, Milhous 1997, Petts 1979, Sing 1987)

Hydrograph showing the Effect of Elephant Butte Dam on Rio Grande Discharge (Modified from Everitt, 1993)

Methodology: Aerial Photos / Topographic Surveys 1911 Topographic Survey 1950’s aerial photos 1995/96 DOQQS N/A ASCS NAPP Scale: 1:10,000 scale: 1:20,000 scale: 1:40,000 W.W. Follett TNRIS USGS

Registered 1950s Aerials and 1911 Topographic Survey to 1995 DOQQs in ARC/INFO using the Transform command Common features, such as major intersections, were used as match points.

Measuring River Features Using Arc/Info Digitized thalweg (deepest part of the river where water tends to move the fastest) Digitized 100 transects to compute an average value of wetted channel width

Results and Conclusions The decrease in discharge caused by hydrologic controls on the Rio Grande has resulted in: narrowing of channel width (average width of 100 digitized transects in meters) 1911 – 87.9 1950 – 52.8 1995 – 34.4 channel migration / increased sinuosity (length of digitized thalweg in meters) 1911 – 46766 1950 – 49917 1995 – 51472