Lithologic Heterogeneity and Variable Valley Width in the Buffalo River Watershed, AR

Abstract:

Lithologic heterogeneity across catchments is thought to be a key control on the style of incision, whether dominated by vertical or lateral processes, but little field evidence is available to support the claims. Map and field observations from the Buffalo National River (BNR) indicate that valley width is related to variation in lithology along the length of the river. The BNR is a gravel-mantled, ingrown meandering, bedrock river that incises through a sequence of Pennsylvanian, Mississippian, and Ordovician carbonate and clastic sedimentary rocks. Using GIS, valley width was measured at 250 m intervals along the entire length of the river through two sequences of limestone and sandstone reaches that represent the dominant lithologies of the catchment. Preliminary results show measurable and statistically significant differences in valley width between limestone and sandstone reaches. Where limestone strata is exposed at river level the mean width is ~ 360 m and is highly variable with a standard deviation of 227 m. The mean width in sandstone strata is ~240 m with a standard deviation of 88 m. At each transition from limestone to sandstone there is a greater than 20% decrease in valley width downstream, with no visible change in channel width at map scales. It is possible the atypical pattern in valley width in the BNR is due to variation in lithologic resistance. These results support the assumption that lithologic heterogeneity exerts control on stream valley evolution.