QUANTIFYING AND DESCRIBING CHANNEL MORPHOLOGY OF THE COASTAL BACKWATER: THE LOWER TRINITY RIVER, TEXAS, USA

Gradually varied flow occurs as a river approaches the coast, resulting in a hydraulic transition from quasi-normal flow to backwater flow within the channel. The hydraulic transition occurs in concert with changes to the channel’s geometry. In this presentation we describe and show a systematic evaluation of the geomorphic consequences of a change in flow hydraulics from quasi-uniform flow to backwater flow in the Lower Trinity River in East TX, USA. This is done using data from repeat bathymetric and topographic surveys, aerial photo analyses, and bed-material sampling spanning several decades. The channel transitions occurs over 28 river kilometers. Through this transition there several pronounced changes in the channel geometry that include: a 28 percent decrease in point bar volume; a 10 percent decrease in the percentage of channel width that the point bars occupy; an increase in channel depth by 1.95 times and; a downstream narrowing of the channel by 30 percent. Ultimately, over this zone of transition the width to depth ratio decreases 66 percent. Changes in the morphology are reflected in a decrease in channel migration rates and a decrease in bend deformation. Through analysis of channel observations over the zone of transition we show that channel geometry and river bend kinematics are related to the connection between river flow and sediment transport.