Substrate controlled interactions between hydraulics, sediment transport, and erosional forms in bedrock rivers
Date
2009
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Abstract
Bedrock rivers are important components of the landscape that are distinguished from alluvial rivers by high sediment transport capacity relative to supply, and a direct link between the underlying geology and forms and processes. This dissertation examines how independent substrate controls influence the interactions among bedrock channel morphology, hydraulics, sediment transport, and incision processes at inter-reach and intra-reach scales. The majority of this research was conducted on the Ocoee River, Tennessee, which flows through the Blue Ridge province of the southern Appalachians. Substrate differences correlate with variation in reach morphology (i.e., gradient, bedform orientation and amplitude), such that less erodible substrates are associated with steeper reach gradient and with transversely oriented ribs of greater amplitude. Increased hydraulic roughness in reaches having steeper bed slopes, greater rib amplitude, and less erodible substrate, points to the importance of positive and negative feedbacks in these systems: Greater substrate erosional resistance limits profile lowering, which likely creates steeper bed slopes and greater stream power, creating a self-enhancing feedback. This local increase in stream power is balanced by increased roughness resulting from the erosional processes that produce bedrock ribs, which represents a self-regulating feedback. The overall result reflects quantifiable adjustments between substrate resistance and hydraulic driving forces in bedrock channels. Transport distance for coarse sediment is not a significant function of grain size, as has been reported for alluvial channels. Instead, the highly complex bed topography in this system leads to widely varying coarse sediment transport dynamics. Reach-scale differences in channel morphology correlate with transport distance. Local topographic controls exert the strongest influence on coarse sediment transport dynamics. Complex interactions among gradient and bed roughness appear to govern reach-scale differences in the degree of alluvial cover. In reaches with more resistant rock and heterogeneous bed topography, pothole dimensions are larger and follow an aggregated spatial pattern. Intermediate bed elevations show the highest likelihood of pothole formation, suggesting that local hydraulics and tools versus cover relationships govern pothole formation and maintenance. At different spatial scales, substrate characteristics play a key role in controlling the forms and processes of the bedrock channels examined in this study.
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Subject
bedrock rivers
erosion
sediment transport
streams
hydrologic sciences
geomorphology
sedimentary geology