The Role of Tributary Inputs on the Downstream Fining Process in the Lower San Antonio River

Bed sediment tends to decrease in size systematically with respect to distance downstream but can be affected by lateral inputs that reset the downstream fining trend. To understand whether the downstream fining process is disrupted in the lower San Antonio River, bed sediment was evaluated in five reaches containing major tributaries (Cibolo, Ecleto, Escondido, Cabeza, and Manahuila) to determine the impact of the tributary sediment on the main stem. Field sampling involved collecting sediment upstream and downstream of tributaries as well as sediment from the tributaries. Data were analyzed using 50th and 90th percentiles of sediment at each respective site. Bed material metrics show a significant textural difference related to the Cibolo Creek tributary. Three models (exponential, logarithmic, and power) were utilized to observe fining trends within the study area. Low R2 values indicate that the conventional models of downstream fining fail to describe patterns in bed material texture, although the models perform somewhat better for the study reach downstream of the Cibolo Creek tributary. Confluence mixing models do not predict observed values well, as only one site within Ecleto is compatible with the 50th percentile but Manahuila values are successfully predicted for both percentiles. Overall, Cibolo Creek appears to be the only significant tributary within the study reach due to its sediment characteristics that cause a disruption to the downstream fining process.