2019 CSDMS meeting-070


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From Distributary to Tributary: Formation and Morphometry of Coastal Stream Networks Governed by Depositional Processes

John Swartz, University of Texas at Austin Austin Texas, United States. jmarshallswartz@utexas.edu
David Mohrig, University of Texas at Austin Austin Texas, United States. mohrig@jsg.utexas.edu
Paola Passalacqua, University of Texas at Austin Austin Texas, United States. paola@austin.utexas.edu

The formation and evolution of channel networks is a critical control on coastal landscapes and fluvial stratigraphy. Analysis of drainage networks often divides them into two regions: a dendritic upstream catchment with behavior governed by erosional processes resulting from the interaction of climate and tectonics, and a transition to a distributary reach governed by depositional processes close to the coast. The landscape built by these larger coastal distributaries is typically dominated by low-relief floodplains and numerous smaller stream networks. Despite the importance of these networks in governing the routing of fluids and sediments that build these landscapes, network geometries and characteristics remain poorly studied and understood. The northern Gulf of Mexico coastal plain is a depositional landscape characterized by the channels and deposits of large fluvial systems that have been prograding into the Gulf of Mexico since the Mesozoic, and hosts smaller stream networks locally known as the Coastal River Basins. Using a compilation of lidar bare earth elevation datasets we systematically identify and map these tributary stream networks across the coastal plain. We calculate for each basin a series of stream metrics that include local relief, slope, and length/contributing area. Additionally, our high-resolution (2m) elevation data allows for detailed analysis of the stream heads and drainage divides between each identified basin. We find that the basin divides for these networks are older distributary channel belts built by the larger fluvial systems. This indicates that the organization and geometry of these coastal networks is initially set and controlled by depositional processes, but the resulting basin morphology is nearly identical to those of drainage networks in predominantly erosional settings. We explore how drainage networks can form in depositional settings as the consequence of sedimentary processes such as river avulsion and ridge formation, with important implications for understanding drivers of drainage network formation, the speed and scale of drainage reorganization in coastal settings, flow routing during floods, and fundamental controls on the creation and preservation of fluvial stratigraphy.