THE SIGNIFICANCE OF TRIBUTARY SEDIMENT INPUT IN CONTROLLING MAIN STEM RIVER MORPHOLOGY

The role of tributary bedload influx as a controlling factor in main stem river morphology is a complex and dynamic relation between sediment supply and local transport capacity. This study applies a combination of high-resolution remotely sensed imagery and GIS to quantify markers of morphological change surrounding the major tributary in the lower San Antonio River watershed.

Cibolo Creek comprises roughly 20% of the total contributing area for the watershed and is known to deliver gravel to the sand-dominated main stem. Active channel width, measured from LiDAR generated contour lines at 150 m intervals for 6 km above and below the confluence, average 19.2± 0.5m and 21.3± 0.4m, respectively. Differences between upstream and downstream widths are shown to be significant at P < 0.003. Subtle changes in channel sinuosity can be seen in multiple years of digitized aerial imagery, as downstream tortuosity values are 8 to 9% lower than upstream values. However, when looking at only the first half of the downstream section (~9.6km), the change in sinuosity increases to over 15%. These differences between upstream and downstream morphological characteristics suggest that tributary sediment influx has disrupted the continuum of downstream channel evolution. While the quantity of sediment from Cibolo Creek may alone be insufficient to alter morphodynamic stability, the caliber of input sediment has been shown to be large enough to disrupt the downstream fining process in the lower San Antonio River.

Active channel width and planform sinuosity measurements reflect a break in downstream continuity that appears to be the result of bedload fluxes out of Cibolo Creek. Considering the current rate of climate change, uncertainty of future weather patterns and the impending replacement or destruction of over 1,400 dams of high-hazard potential in the region, the effects of large tributaries on the morphodynamics of major waterways will inevitably become more pronounced.