From Distributary to Tributary: Coastal Drainage Network Position and Morphometry Are Set by Depositional Processes

The location and behavior of river drainage networks is a primary control on landscape topography and fluvial stratigraphy. Drainage network characteristics are typically viewed as the product of erosional processes. In particular, drainage divide location and migration are often modeled by the relative erosional potential of competing streams. The Texas coastal plain is a strongly depositional landscape characterized by forms and deposits of large river systems that have been prograding into the Gulf of Mexico since the Mesozoic. The landscape surrounding these larger distributaries is dominated by low-relief floodplains and smaller stream networks commonly known as the Coastal River Basins. Using a compilation of lidar elevation datasets we systematically identify and map these coastal river networks across the coastal plain. We find that basin divides for these networks are older distributary alluvial channel belts formed by one of the larger coastal rivers. We calculate for each basin a series of stream metrics and statistics such as length/contributing area, slope, and local relief. Additionally, our high-resolution (<2m pixel size) elevation data allow for detailed analysis of the stream heads and drainage divides between basins. Although the organization and geometry of the examined stream networks are initially set and controlled by depositional processes, the resulting basin morphology statistics are nearly identical to those of drainage networks located in dominantly erosional settings. We explore how drainage networks can form in depositional settings as a consequence of sedimentary processes such as river avulsion and alluvial ridge formation, with important implications for understanding drivers of drainage network formation, the speed and scale of drainage reorganization in coastal settings, and fundamental controls on the creation and preservation of fluvial stratigraphy.