CONTROLS ON CHANNEL FORM ALONG BEDROCK-INFLUENCED ALLUVIAL STREAMS IN SOUTH-CENTRAL INDIANA

The controls imposed by bedrock geology on cross-sectional channel morphology and local channel gradients were examined along bedrock-influenced alluvial streams in south-central Indiana. The streams were separated into two classes on the basis of the underlying strata: those underlain by siliciclastic (SC) rocks, and those developed primarily in carbonate (CC) lithologies. The controls imposed by bedrock geology on channel form (width, depth, width-depth ratio, and slope) were assessed by: (1) examining the relationships between bedrock geology and channel sedimentology, and (2) identifying statistical relationships between variable descriptors of channel sedimentology and form. Differences in bedrock geology exhibited by SC and CC basins affect channel sedimentology, and subsequently the parameters controlling both local channel gradients and width-depth ratios. For example, streams within SC basins exhibit short (< 150 m long) bedrock reaches that are separated by extensive segments where thick (> 2 m) alluvial deposits bury underlying strata. Width-depth ratios vary along SC streams as a function of bedload size. Additionally, downstream decreases in slope can be explained, in part, by grain-size variations and changes in basin area (i.e., flow magnitude). These relationships suggest that SC channel reaches are adjusted, by eroding or depositing unconsolidated materials that form the channel perimeter, to transport incoming sediment as constrained by the present hydrologic regime. CC streams are characterized by thin alluvial deposits and extensive bedrock reaches requiring adjustments in channel form to involve erosion of the underlying strata. As a result, channel form may be primarily adjusted to the erosional resistance of the underlying bedrock as indicated by weak relationships between bedload size and channel gradients, and poor statistical relations between bedload material size and width-depth ratios. Thus models developed to describe controls on channel form variables along bedrock-influenced streams may be enhanced by defining and mapping variations in channel sedimentology and developing a model for each type of sediment reach.