LITHOLOGIC INFLUENCES ON A BEDROCK STREAM IN SOUTH-CENTRAL INDIANA

A 1500 ft long bedrock tributary channel of a trunk stream in Perry County, Indiana was mapped and measured to determine the effects of lithologic type and stratigraphic arrangement on stream geometry. The channel is incised into bedrock dominated by shale, with thin limestone (micrite), dolostone (dolomicrite) and sandstone (lithic arenite) beds. Channel width and minimum stream depth data collected at points along the channel showed limited relationships to lithological changes. The stream downcuts into the shale, frequently undercutting the thin beds of relatively resistant limestone and sandstone, and causing the resultant overhanging banks to collapse into the channel. The slumped banks appear as large boulders in the channel that decrease stream width in some cases and increase it in others. However, lithology showed a qualitative relationship to channel gradient. The interbedded resistant lithologies and shale units interact in some cases to create knickpoints along the channel. Channel reaches underlain by non-shale lithologies (e.g., sandstone, limestone) had the steepest gradients (0.056-0.128 ft/ft) whereas shale sections were less steep (0.018-0.043 ft/ft). The lithologically-controlled gradient changes apparently affected the planview geometry of the channel, as seven out of eight channel meanders occur within the shale reaches of the stream. The relationship between resistant and non-resistant lithologies is crucial toward understanding variations in stream geometry.