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Abstract:
Migrating regional ground water divides can create unstable zones of relatively stagnant flow in upland areas. Unlike traditional upland ground water divides, the process of flow reversal causes these zones to reject recharge. Artesian pressure surfaces limit the downward infiltration of precipitation and form the subenvelope above which ground water sapping can create a ‘peneplain’ (Stearns, 1967). Only regolith and rock above the pressure surface subenvelope is available for epigenic erosion. Inertia is eventually overcome and ground water circulation substantially increases as hydraulically-advantaged, ‘entrenched’ river systems capture the isolated packets of stagnant ground water. As artesian pressure is lost in the upper story, losing streams form. The losing streams may eventually be consumed by the steep slopes of an entrenching stream, thus completing the reversal of flow. Water level data suggest that the dewatering of stagnant divide areas can be hastened by distant earthquakes.

A variety of observations in Missouri, including recent studies using heat pulse flow meters, show that pressurized sandstone aquifers are widespread beneath upland divides and at surprisingly high elevations. The ground water in the sandstones is confined by relatively tight carbonates. Ground water leaves these confined aquifers by slowly percolating upward through the confining carbonate into shallow bedrock fractures. Storm events then flush shallow mineral-laden ground water into surface streams, which is why floodwaters tend to be dominated by ground water (Frederickson & Criss, 1999). In the major valleys, transverse speleogenesis reverses the hydraulic role of the carbonate beds (Klimchouk, 2003). Classic artesian hydrology generally ignores these mechanisms and cannot explain why most large Ozark caves are associated with sandstones. Unlike classic artesian systems, artesian aquifers in the Ozarks typically lack a marginal recharge zone. Artesian pressures are maintained by ongoing vertical movements. A subsidence rate of approximately 1 mm/yr in the Northern Mississippi Embayment (Calais, 2008) would cause the Ozark ground water divide to migrate to the north and west at approximately 0.7 meter per year, assuming a constant gradient. Flat interfluve summits form as the flow reversal process unfolds.

Once thought to be remnants of ancient peneplains formed near sea-level, isotopic evidence now indicates that modern Ozark summits are actually being sapped by relatively shallow but significant zones of chemical migration. The flat summit surfaces and the steep stream valleys form simultaneously as the landscape is lowered and drainages are rearranged. There is no need to postulate the prior existence of a low elevation peneplain. The uppermost artesian pressure surface acts as the base level, not sea level. Flat interfluve surfaces can form at any elevation, depending on hydrologic conditions. The summit surfaces appear flat because they are essentially created by a regional ground water surface that is widespread and relatively flat.