Reach‐scale changes in channel geometry and dynamics due to the coastal backwater effect: the lower Trinity River, Texas

In this paper we use multiple field surveys spanning several decades to systematically evaluate the geomorphic consequences of a change in flow hydraulics from uniform flow to backwater flow for the lower Trinity River in east Texas, USA. Spatial changes in lateral migration rate, channel geometry, and point bar size correspond to two distinct geomorphic zones. Within the upstream uniform flow reach, the river channel is defined by fully developed point bars and a high rate of lateral channel migration. This zone transitions where the median channel bottom elevation drops below sea level. At this point flow is affected by the backwater influence of the Trinity Bay water surface elevation, as opposed to being bed slope control dominated. The change in hydraulics within the backwater zone is reflected in the channel morphology, which is characterized by smaller point bars, narrower and more symmetrical cross‐sectional channel geometry, lower channel migration rates, and little to no bend deformation or cutoffs. Studying the connection between channel geometry, river bend kinematics, sediment transport, and fluid mechanics in each zone provides a deeper understanding of the relationship between channel shape and river mechanics. © 2019 John Wiley & Sons, Ltd. Estimated point barvolume, surface area, and height in the study area compared with change in point bar volume as a result of a single large flooding event (Mason and Mohrig, 2018). The backwater zone is shown in grey. Bar volumes and bar surface areas have similar trends, showing an overall decrease with a slight peak just upstream of the backwater zone. In the backwater zone bar heights increase, while volume of material added to point barsdecreases.