INSIGHTS ON COARSE SEDIMENT ROUTING IN TRIBUTARIES OF THE BUFFALO NATIONAL RIVER, AR

Though bed load is a principal transport mechanism in gravel mantled bedrock streams, there are few in situ studies of gravel movement at the reach to watershed scale. Thus, gravel caliber and provenance variability in major tributaries of the Buffalo National River (BNR) of northwest Arkansas have been analyzed to identify how lithology and basin morphometrics influence tributary gravel routing in the watershed. Pebble counts and provenance sampling took place along gravel bars on four tributaries at least one stream bend up from the confluence with the BNR. Additionally, main stem gravel bars just above and below each tributary confluence were studied and later compared to identify potential tributary-derived coarse sediment inputs in the main stem. Comparisons between gravel populations at each tributary confluence included caliber data comparisons (mean, d50, d84), provenance percentage comparisons, and non-parametric statistical analyses (Wilcoxon Rank Sum Tests). The combination of these comparisons identified gravel population relationships between the tributaries and main stem that support the presence of significant tributary-derived sediment inputs from three of the four study tributaries. Basin morphometric data were then calculated and compared between tributaries to identify potential in situ controls on tributary sediment routing. As the one study tributary without a strong sediment input signal, Beech Creek has the smallest area, lowest stream order, and lowest drainage density of the four study tributaries. Each of these observations can likely be attributed to Beech Creek’s high relative position in the watershed, which causes it to be at a disadvantage for coarse sediment entrainment relative to other larger and more developed BNR tributaries.

This study found that tributary-derived sediment inputs have directly influenced gravel populations in the main stem and appear to be controlled by tributary basin area, stream order, and drainage density. These findings better constrain watershed-scale gravel routing patterns and controls in the BNR, which is critical for understanding channel and basin morphology in this ever-changing fluvial system.