Investigation on the basin-scale factors of bedrock shoals distribution in the Flint River, Georgia, USA

Abstract

Bedrock outcrops create shoaled sections in many rivers of the eastern United States. Bedrock shoals can consist of a variety of channel morphologies and lithologies and are important forms of river habitat by having higher species richness than other locations along the same river. Despite their ecological significance, the system-scale factors that determine their occurrence in rivers are not well understood. In this study, shoaled and non-shoaled sites along the length of the upper Flint River, Georgia, were analyzed to determine the factors responsible for the presence of shoals. Strike-flow ratio, rock integrity, confinement ratio, unit stream power, and geologic dip were selected as possible variables based on existing literature and observations on bedrock shoals and general bedrock morphology. The results of a logistic regression showed that confinement ratio and unit stream power were the most statistically significant predictors of a river reach’s shoal status (p=0.001 and 0.014, respectively), suggesting that variables comprising the driving forces in the Flint River currently dominate system-scale expression of channel morphology. A predictive equation developed from the binary logistic regression analysis correctly classified 80% of known shoaled sites as “shoaled locations” in the watershed and 60% of known non-shoaled sites as “non-shoaled locations” in the watershed. While these results are promising and suggest that system-scale heterogeneity of channel morphology may be controlled by spatial variability of confinement ratio and unit stream power, there is room for improvement, particularly in regard to correct classification of non-shoaled locations. In comparing the findings of this research to those previously made on bedrock shoal occurrence in the Cahaba River, Alabama (Bishop, 2013) – visually similar to the Flint River – questions arise concerning the prevailing view of how dynamic equilibrium operates in fluvial geomorphic systems. Although the Cahaba and Flint Rivers contain bedrock shoal morphologies for long expanses, the occurrence of bedrock shoals in the Flint River are controlled by driving forces, while bedrock shoals in the Cahaba River are controlled by resisting forces. This finding supports the idea that dynamic equilibrium, a state of balance between driving and resisting forces, may be temporally and spatially limited in geomorphic systems.