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The history of sediment accumulation in an oxbow lake located on the Mississippi alluvial floodplain was reconstructed based on sedimentation rates determined using 14 C activities from bulk sediment fractions and from 210 Pb and 137 Cs measurements. Higher rates of sediment accumulation consistent with frequent flooding when first abandoned 3,800 to 5,000 years before the present were followed by slower sedimentation rates consistent with migration of the Mississippi river away from the oxbow and less frequent flooding. This low sedimentation rate persisted for several thousand years until the surrounding land was cleared for agricultural use in the late 19th century. A 50-fold increase in the rate of sediment accumulation has persisted from the time of land clearing to the present, doubling the total mass of accumulated sediment in a single century.

The Balcones Escarpment of south-central Texas is the eroded topographic expression of an en echelon fault zone where Cretaceous carbonates have been modifi ed by karst processes infl uenced by structural and stratigraphic controls. While the modern confi ned Edwards Aquifer fl ows through cavernous voids at the base of the escarpment, air-fi lled caves perched in the escarpment are relicts of paleoaquifer hydrology. The structural geology of the Balcones Escarpment and the phylogeography of its endemic cave spiders provide mutually informative frameworks from which to establish relative dates for the activation of discrete groundwater recharge areas. The mitochondrial genetic variability of troglobitic spiders is correlated with the structural elements of the Balcones fault zone in and around the San Antonio relay ramp. Older (basal) genetic lineages occur in structurally high, mature karst terrains, while the younger (derived) lineages occur in structurally low, emergent karst terrains. Based on mitochondrial deoxyribonucleic acid (mtDNA) data, Cicurina diversity is interpreted as the product of the progressive availability of vadose zone habitat as discrete recharge areas have developed. Genetics analysis was funded by the La Cantera Development Corporation and the U.S. Fish and Wildlife Service. Cave access was graciously granted by many private landowners, the USAA Corporation, Mabe Inc., the University of Texas at San Antonio, and the Texas Department of Transportation. We thank all of our caving partners for their time and effort. This paper benefi ted from reviews by S.J. Taylor and J.M. Sharp Jr., and graphics assistance by K.M. White. Where required, all specimens were collected by permit. (75)90084-0. Barr, T.C., 1968, Cave ecology and the evolution of troglobites: Evolutionary Biology, v. 2, p. 35-102. Brower, A.V.Z., 1994, Rapid morphological variation and convergence among races of the butterfl y Heliconius erato inferred from patterns of mito chondrial DNA evolution: