Water Reservoirs as a Record of Watershed and Reservoir Processes: A Case Example Lake Abilene, TX

This study investigates a different approach for identifying water reservoir sediment source regions within watersheds. The method uses physical and chemical analysis of targeted vibracores augmented with multi-frequency acoustic profiling to examine the sediment in Lake Abilene, Texas. The acoustic survey was conducted using a sub-bottom profiling system with central frequencies at 200, 125, 50, 24 and 12 KHz. Vibracores were taken in 4 locations to verify the acoustic interpretations and perform sediment analysis. Fluctuations in the Cs 137 concentration were used as date markers to establish sedimentation rates. The C/N was used to discriminate between sources of organic material to indicate whether the sediment was deposited under base flow or flood conditions. Thin sections were examined to characterize the contact between sediment layers and the deposition pattern in each layer. XRD analysis was preformed to correlate the mineralogy of the layers in the sediment to sources in the watershed. Results show that the sediment cores are characterized by dark grey, cm-scale lamination with high C/N that consists of graded silt beds that fine upward in thin section. Approximately 35% of the sediment is derived from within the watershed, while the rest is eroded from the lakeshore.