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Abstract:
New data fail to support the prevailing theory that meandering bedrock valleys inherit their sinuosity from ancient alluvial rivers. In the Ozarks, observations indicate that bedrock meanders emerge during incision as a result of erosion by emergent groundwater and surface flow. Crustal tilting pressurizes deep aquifers that feed a huge base flow to large springs. Because of their large size and persistence in time, these artesian conduits have the potential to create new base levels of erosion. Transverse speleogenesis causes groundwater flow lines and surface streams to converge toward the springs, thereby further increasing the rate of landscape lowering and creating bedrock meanders. Groundwater outflow accelerates stream piracy, creates asymmetric drainage patterns and cuts channels across structural upwarps. By contrast, the antecedent meander theory favors long-term drainage stability that cannot explain the incredible diversity of the freshwater fauna found in the Central Highlands. Widely disjunct species of highland fish that thrive only in clear, high-gradient streams indicate that the Ouachitas, the Ozarks and the Eastern Highlands were once a continuous upland connected by a “land bridge” in southern Illinois. This connection allowed ancestral species to become widespread enough to be affected by a vicariant event, usually attributed to onset of glaciation. However, a 400-km eastward shift in Gulf of Mexico sedimentation indicates this vicariant event may have occurred in the middle Pleistocene, when it is proposed that the Mississippi River dissected the Central Highlands, separating the Interior Highlands from the Eastern Highlands.