Characteristics and Transport Rates of Mud Aggregates in the Lower San Antonio River




Jahangiri Gohar, Maryam

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It has long been assumed that all fine-grained sediment is transported in suspension and deposited in low energy environments. However, recent studies have shown that mud can also be transported as bedload in the form of aggregates. This study investigates the characteristics of mud aggregates and their contribution to the total bedload in the lower San Antonio River, Texas. Using bedload samples collected at sampling sites near Floresville, Kenedy, and Goliad, aggregates larger than 2 mm in size were assessed for clast size, constituent grain sizes, color, roundness, and density, and compared to potential source material from the channel. Rating curves developed for the transport rates of aggregates, the gravel fraction, and total bedload were used to computed annual loads with the magnitude-frequency approach. Mud aggregates range from 2 to 11 mm in size, consist mainly of mud, appear gray in color, are moderately rounded, and average 1.99 g/cm3 in grain density. The contribution of aggregates to the total bedload is small, with proportions ranging from 0.001% to 0.32% at the study sites. The effective discharge transports 11% of the annual aggregate load at Floresville and Goliad but 15% at Kenedy. Half of the aggregates are transported by low to intermediate flows. While the presence of aggregates enhances overall bedload magnitude, their limited contribution suggests negligible impact on channel morphology. This study provides challenges to the interpretation of paleoenvironments associated with mud dominated units. Additionally, it demonstrates that mud aggregates are transported as bedload and appears to be the first study to quantify transport rates in a modern environment.