Understanding fluvial topography : morphodynamic processes that build river levees and cut terraces

Floodplains capture extreme events on a river when water overtops associated riverbanks. As a river migrates, floodplains adapt to feedback in topography, sediment availability, and flow conditions. Describing how floodplains evolve will improve estimates of sediment, organic carbon, and nutrient storage and help predict changes to a stockholder’s river-adjacent land. This dissertation focuses on pathways between a river and its floodplains as terraces form and as natural levees evolve. Terraces form when floodplains are elevated above the active river due to changes in base-level or water to sediment discharge ratios. Natural levees comprise one of several constructional topography styles separating a river channel and its floodplain. As such, levees regulate the exchanges of solids, solutes, and flow between the river and its overbank surface. First-order questions exist about the amount of sediment transported to the floodplain and flow characteristics that natural levees preserve. The improved resolution provided by airborne lidar has enabled systematic studies of terraces formation and natural levees on the coastal Trinity River, Texas, highlighted below. This dissertation analyzes how floodplains are abandoned as terraces and how modern natural levees are formed on floodplains. In Chapter 2, proxies developed to describe both elevation variations and paleo-channel characteristics preserved on late Pleistocene terraces that confine the modern valley. Results show the abandonment mechanism varies for different terrace sets. In the remaining chapters, I combine high-resolution topography measurements, changes observed with repeated measurements, and field observations to quantify natural levee topography and describe how natural levees evolve. I show a first-order morphodynamic correlation between levee crest elevation and river hydraulics and between levee width and topographic floodplain lows. I further show that the percentage of banks with natural levees increases in the most ocean-ward reach. Levee channels are vital in transporting sediment to the levee-floodplain boundary as bedload transport building out into Gilbert-type deltas in standing water. Lastly, I use a framework that integrates across scales from the grain size to the entire >150 km river reach to show how water flow and sediment transport contributes to levee-scale depositional patterns