The Significance of Tributary Sediment Input in Controlling Main Stem River Morphology
The role of bedload flux as a controlling factor in river morphology is well established, but the quantity and caliber of sediment capable of initiating morphological change is still poorly understood. This is especially true of sediment delivered from major tributaries, where sediment sources and erosional mechanisms can be vastly different from those of the main stem. The aim of this study is to refine the threshold for tributary geomorphic significance by quantifying bedload flux and examining the measurable effects on distinct aspects of main stem morphology. A combination of empirically driven transport predictions along with remotely sensed imagery and GIS was used to quantify markers of morphological change surrounding the major tributary of the lower San Antonio River watershed. Using the Recking (2013) reach-averaged transport formula, rates where determined for tributary and main stem channels and combined with a flow duration analysis to estimate annual loads. Aerial orthoimagery of the 36 km study reach was analyzed in ArcGIS to assess main stem morphological components of sinuosity, meander development, width, and sediment storage. Despite a much larger flux of sediment from the main stem, tributary input was found to influence key aspects of morphology related to sinuosity and sediment storage, in particular. These findings provide new insights for the establishment of quantitative thresholds and indicate that morphological response may be impacted by smaller sediment loads than previously suspected.