POOLS AND RIFFLES IN THE BEDROCK-ALLUVIAL SOUTH RIVER VIRGINIA ARE SYSTEMATICALLY DIFFERENT FROM POOLS AND RIFFLES OF ALLUVIAL RIVERS

Pool-riffle sequence spacing in mixed alluvial and bedrock rivers is poorly understood. Past studies have defined methods by which these sequences can be identified and mapped in alluvial rivers, yet the application of these techniques to hybrid (mixed alluvial-bedrock) environments consistently falls short. This could be due to the irregular spacing of bedrock outcrops which interrupts the well-documented ‘rhythmic’ spacing of typical pool-riffle morphologies seen in alluvial rivers. In order to address these inconsistencies, over 4 km of the South River in Virginia (a mixed alluvial-bedrock river) was mapped using RTK GPS, total station, and auto level surveys to create a detailed longitudinal bed profile which will help shed light on morphologic feature spacing in rivers such as this. This study ultimately aims to utilize a number of different techniques to break down the bed profile and objectively classify pools, riffles, and other features along the study reach. Preliminary results and analysis on the bed profile supports the hypothesis that bedrock outcrops do have a significant impact on the formation of pool-riffle sequences, and that pool-to-pool spacing in hybrid rivers does not follow the widely acknowledged 5-7 channel widths that is documented in alluvial environments. Comparison of alluvial bed profiles created in past studies to the one produced by this study highlight the stark differences in feature spacing, with the former yielding clearly defined geomorphic units and the latter failing to show much (if any) regular spacing. To further investigate, all features associated with the bedrock and valley margins were removed from the bed profile, and still no regular feature spacing could be found. This suggests the presence of bedrock in these mixed alluvial-bedrock systems is very influential.