TRANSPORT OF GRAVEL-SIZED MUD AGGREGATES IN THE LOWER SAN ANTONIO RIVER, TEXAS

Sand and silt-sized aggregates are increasingly documented in the ancient rock record, rebuking the notion that fine-grained facies represent only low energy environments. Studies of modern depositional settings are more limited and focus on floodplains. In this study, the transport of gravel-sized aggregates is quantified in the lower San Antonio River, Texas, to better understand their contribution to the annual load in a modern channel. At three locations, Floresville, Kenedy, Goliad, transport rates of bedload sediment were determined through field sampling using a Helley-Smith bedload sampler. Rates cover discharges from near baseflow to bankfull capacity. Samples were dried and sieved following standard protocols. Aggregates with diameters of at least 2 mm were identified and weighed during the sieving process. Flow duration curves were developed from mean daily discharges measured at US Geological Survey stations. Annual loads of the aggregates as well as all bedload grains were computed using the magnitude-frequency concept. The maximum size of aggregates at Floresville and Kenedy equals 11 mm and is associated with discharges that occur only 0.4 and 5% of the year, respectively. At Goliad, the largest mobile aggregate (8 mm) emerged at a flow rate expected < 1% of the year. Aggregates are mobilized at all flow rates sampled except at Kenedy where no aggregates were captured at the lowest flow. Rates of aggregate transport span from 1.6e-6 to 1.2e-3 kg/s for discharges between 7.7 to 319 cms at Floresville, 1.1e-5 to 3.6e-4 kg/s for discharges between 8.3 and 224 cms at Kenedy, and 3.4e-6 to 4.9e-3 kg/s for flows between 9.5 and 372 cms at Goliad. On an annual basis, the aggregate load equals 9721 kg at Goliad, 2843 kg at Floresville, and 1655 kg at Kenedy. This load makes up from 0.08-0.74% of the total bedload in the study reach. Overall, the chance of mobile aggregates at Goliad is greater than that at Kenedy and Floresville, which suggests more sources of aggregates at Goliad.