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Looking back five decades and reflecting on the contributions of Harold Fisk to Lower Mississippi Valley (LMV) geological knowledge, it is apparent that he was highly successful in making engineers aware of the critical role of glacial advance and retreat, sea level changes, valley slope and river gradient, and time in affecting geomorphic processes, sedimentary patterns, and river regimes. Perhaps his greatest contribution to river engineering was in recognizing and elaborating on the effects of bed and bank materials on stream meandering. Fisk, however, could only work within the prevailing geological theory and method of his time with regard to glacial chronology and stratigraphy and such matters as global structure and tectonics. Major advances in these areas with new concepts and techniques have shown his LMV glacial response model to be oversimplified and invalid in some respects. More cycles of valley filling and cutting are now known to be represented in the alluvial sequence and his concept of deep valley entrenchment as a direct response to sea level changes is untenable in light of substantial new data. The presence of a regional, rectilinear fracture pattern with a corresponding network of major fault zones has not been substantiated by subsurface data. The most widely known aspect of Fisk's work is his detailed interpretation and colorful depiction of the complex pattern of overlapping, cross cutting, and discontinuous Holocene meander belts and their hundreds of abandoned channels. However, it is now recognized that Fisk's elaborate chronological reconstruction is largely invalid both because of some incorrect basic assumptions and his lack of radiocarbon and other numerical dating techniques for precise control. A newly emerging model of meander belt formation is more generalized and relies heavily on archaeological evidence. It places emphasis on episodes of multiple active river courses (partial flow channels), frequent formation of distributary channels, and the significant influence of Mississippi River tributaries and minor basin drainage. Interesting regional variations over time in meander belt configurations and morphology may reflect climate changes, but at present broad-scale variations in the lithology of the Quaternary alluvial sequence appear to be the dominant control. |
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