CHANNEL MIGRATION OF THE LOWER GUADALUPE RIVER: SEGUIN, TX TO THE SAN ANTONIO RIVER CONFLUENCE

Across Earth, rivers are found meandering across the landscape. To accommodate dynamic flow regimes and sediment loads, rivers are in constant adjustment within the channel and adjacent valley. Extreme floods as well as base flows alter the course of the river at different rates. Geomorphic characteristics control the extent of adjustment of a river over time. Numerous geomorphic and watershed characteristic variables influence the course of the river at different spatial and temporal scales. Each river is unique across Earth, and each reach of river is different from another, thus, the mechanisms driving, and the rates of change in one reach of river may not be the same for another.

This study addresses channel adjustment and mechanisms of change on a 380 km reach of the Guadalupe River, from Seguin, TX to the San Antonio River confluence. The following questions are addressed: (1) at what rate does channel migration occur along the lower Guadalupe River, and (2) what are the driving mechanisms controlling channel migration? To determine this, a GIS (Geographic Information System) analysis was performed from 1960 to 2014. An analysis at three spatial scales was performed including the 380 km reach, seven sub-reaches, and 70 individual meander bends.

Statistical analysis was performed for the entire reach and the sub-reaches and show that the channel is relatively stable within itself. For the 380 km reach, lateral rates of migration range from 0.11 to 8.15 m yr^-1 with a mean of 0.36 m yr^-1. To understand the drivers of lateral migration, sub-reach analysis concluded that presence of vegetation and high silt-clay soil composition relate to lower rates of migration. The highest and lowest average rates of migration occur in adjacent zones, the Balcones Fault zone (BFZ) and Upper Coastal Plain (UCP) at the top of the reach, and the Upper and Middle Delta (UD, MD) at the bottom. Several dams outside of and within the study reach influence the BFZ and UCP, whereas an active zone of subsidence contribute to migration in the UD and MD. Individual variables alone do not control rates of migration, but work together as mechanisms of migration in this river system.