Riparian and Geomorphic Disturbance from a High-Magnitude Flood on the Blanco River in the Texas Hill Country
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Rivers of the Central Texas Hill Country are subject to significant bio-geomorphologic changes along the riparian corridor during low-frequency, high-magnitude catastrophic flood events. One significant change can involve floodplain stripping, which is the removal of alluvium and vegetation along the riparian corridor. Discharge magnitude, valley morphology, vegetation, and land cover can influence floodplain stripping processes. The Blanco River in Central Texas is an ideal system to study flood disturbances, which can include the process of floodplain stripping. The upper reaches of the river drain the high relief Hill Country landscape of the Edwards Plateau where the main stem channel is laterally confined by limestone bedrock, steep canyons, and thin soils. The lower reaches of the river transition across the Balcones Escarpment to a lower relief valley with deeper soils and a wide floodplain as it nears its confluence with the San Marcos River in the Coastal Plain. In May 2015, the Blanco River experienced catastrophic flooding with estimates of 33 cm of precipitation falling in its headwaters over only a few hours. Imagery from pre- and post- flooding was used to identify and categorize the various patterns of disturbance along the river that range from complete floodplain stripping to minimal disturbance. A custom method was developed to sample disturbance along the river. Relationships between total disturbance area, disturbance area per floodplain, and disturbance area per sample were examined. Results show the most severe disturbance occurred within the floodway near the channel and decreased with distance. Patterns of biogeomorphic disturbance mentioned in previous literature such as across meander scour and parallel chute scour were identified as well as patterns of severe disturbance at tributary confluences.