Profile maintenance in bedrock streams incising soluble strata

Abstract

This study employs quantitative hypothesis tests to explore variations in channel substrate, geomorphic setting, and incision processes against changes in velocity (ū), shear stress (τ). and unit stream power (ω) in streams incising soluble and insoluble bedrock: Buckeye Creek (14 km2 drainage area) and Greenbrier River (3800 km2), West Virginia. Both streams are incising at ≤ 40 m Ma-1 and transport coarse, insoluble sediment. The streams are incising by quarrying, abrasion, and corrosion. Comparisons in thirteen stream reaches reveal that τ and ω are 3 to 30 times lower atop soluble versus insoluble bedrock. For constant relative solubility, τ and ω are higher in reaches that interact strongly with hillslopes.

Values of ū are highest where evidence of abrasion is prominent on insoluble channel elements. Evidence of quarrying is associated with higher values of τ and ω than evidence of corrosion. By inference, the efficiency of corrosion-driven incision on soluble bedrock translates to minimization of mechanical energy expenditure, which is used to calculate τ and ω. Presumably, incision of insoluble strata requires more mechanical energy expenditure for a similar incision rate.

Rapid longitudinal changes in τ and ω are associated with similarly rapid longitudinal sorting and coarse-sediment storage in an alluvial reach in Buckeye Creek. Depth has adjusted in Greenbrier River such that changes in τ are of lesser magnitude. As a result, coarse sediment transport may be continuous across soluble and insoluble bedrock. Overall, variations of ū, τ, and ω appear to reflect relative substrate solubility, geomorphic setting, and incision processes.

Sculpted forms enhance abrasion and corrosion in both streams by localizing energy expenditure on channel margins. Analyses of the equations for a forced vortex show that the morphology of large sculptures is controlled by channel hydraulics, whereas a mathematical model of erosion by small sculptures reveals that their morphology is dependent on overall channel erosion rates and not simply channel hydraulics. Therefore, corrosion and abrasion in sculpted forms are not controlled absolutely by channel hydraulics, which places a limit on the use of hydraulic variables for modeling channel incision by these phenomena.