A CONTEMPORARY APPRAISAL OF SOME KEY FISKIAN CONCEPTS WITH EMPHASIS ON HOLOCENE MEANDER-BELT FORMATION AND MORPHOLOGY

Looking back five decades and reflecting on the contributions of Harol d Fisk to Lower Mississippi Valley (LMV) geological knowledge, it is a pparent that he was highly successful in making engineers aware of the critical role of glacial advance and retreat, sea level changes, vall ey slope and river gradient, and time in affecting geomorphic processe s, sedimentary patterns, and river regimes. Perhaps his greatest contr ibution 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 o f his time with regard to glacial chronology and stratigraphy and such matters as global structure and tectonics. Major advances in these ar eas with new concepts and techniques have shown his LMV glacial respon se model to be oversimplified and invalid in some respects. More cycle s of valley filling and cutting are now known to be represented in the alluvial sequence and his concept of deep valley entrenchment as a di rect response to sea level changes is untenable in light of substantia l new data. The presence of a regional, rectilinear fracture pattern w ith a corresponding network of major fault zones has not been substant iated by subsurface data. The most widely known aspect of Fisk's work is his detailed interpretation and colorful depiction of the complex p attern of overlapping, cross cutting, and discontinuous Holocene meand er belts and their hundreds of abandoned channels. However, it is now recognized that Fisk's elaborate chronological reconstruction is large ly invalid both because of some incorrect basic assumptions and his la ck of radiocarbon and other numerical dating techniques for precise co ntrol. A newly emerging model of meander belt formation is more genera lized and relies heavily on archaeological evidence. It places emphasi s on episodes of multiple active river courses (partial flow channels) , frequent formation of distributary channels, and the significant inf luence of Mississippi River tributaries and minor basin drainage. Inte resting regional variations over time in meander belt configurations a nd 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.