Geomorphic Transformation of In-stream and Riparian Habitat on the Rio Grande in the Big Bend Region

Authors: David J. Dean, John C. Schmidt

Year: 2010 (xlii)

Abstract

The endangered Rio Grande silvery minnow, Hybognathus amarus, resides in only 7% of its historic range. Population declines have generally been attributed to the loss of habitat due to reductions in stream flow by dams, diversions, and pumping, as well as channelization. As part of the silvery minnow recovery plan, the silvery minnow is being re-introduced to 2 parts of its historic range, one being the Big Bend reach along the U. S. - Mexico border. The Rio Grande in the Big Bend region, however, has changed dramatically during the past century, shifting from a wide river with transient channel margins, backwaters, and side-channels to a simple channel with steep, definable, vegetated banks and few channel bars. The channel narrowed by more than 50% since the early 1900s. Rapid narrowing continues today. Between 1991 and 2008, the active channel width of the Rio Grande in Big Bend National Park narrowed by 35-50%. Narrowing occurred by vertical accretion of fine-grained deposits inset within natural levees. Vertical accretion occurred on top of alternate bars of sand and gravel. In two floodplain trenches, 2.75 and 3.5 m of vertical accretion were measured, all of which occurred in this 17 year period. In some localities, nearly 90% of bare, active channel bars were converted to vegetated floodplains during the same period. Channel narrowing coincided with a rapid invasion of non-native riparian vegetation, Tamarix chinensis and Arundo donax) which led to increased bank roughness, and created a positive feedback of decreased flow velocities, an upward shift of the stage-discharge relations, overbank flooding at lower discharges, and continued vertical accretion. This resulted in a loss of channel capacity, and thus, although peak flows were reduced by 48% and the percent exceedance of both the two-year flood and mean annual flow declined, overbank flooding continued. The magnitude of these historic geomorphic and habitat changes, and the magnitude of flow depletion on the northern branch of the Rio Grande and on the Rio Conchos, cannot be ignored in the development of recovery plans for the endangered endemic fish species.