The Influence of Bedrock Geology on Knickpoint Development and Channel-Bed Degradation along Downcutting Streams in South-Central Indiana

Abstract

Geologic controls on knickpoint development and the mechanics of channel-bed degradation were eluci-dated by comparison of downcutting streams in south-central Indiana developed in lithologically "homoge-neous" carbonate units and layered siliciclastic strata. Knickpoints formed in a single lithologic unit are stepped; each step consists of a knickpoint face and tread. The height of each step is equal to and controlled by bed thickness. Spatially, knickpoints in "homogeneous" terrain are limited to jointed strata that dip: (1) upstream, or (2) downstream at angles less than the channel gradient. Knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units. Total knickpoint relief is influenced by thickness of the weaker strata. Knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision. Knickpoint distribution is also influenced by joint frequency, lithologic layering, and the relations between strata dip and stream flow direction. Channel incision occurs by the erosion of bedrock strata along bedding plane discontinuities and typically proceeds in the direction of dip. Geologic controls imposed on degradation lead to distinctive types of channel bed morphology. It is generally agreed that downcutting streams are disequilibrium systems that exhibit rapidly evolving landforms. However, the downcutting channels appear to develop an equilibrium form (determined by bedrock geology) maintained until longer-term (hundreds of years) profile adjustments to base level lowering can occur.