Response of the Karst Phreatic Zone to Flood Events in a Major River (Bohemian Karst, Czech Republic) and Its Implication for Cave Genesis
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Abstract
Hydraulic and hydrochemical relationships between a medium gradient river and a karst aquifer were studied by water level and temperature logging combined with water geochemistry and d13C. The cave lakes are separated from the river by a floodplain up to 150 m wide formed by a gravel and sand layer up to 13 m thick covered with finegrained floodplain sediments. During minor discharge peaks (water level in the river , 1.5 m above the normal river stage), a water level oscillation in the cave lakes situated 40 to 190 m away from river is induced by the river level oscillation, but the river water does not enter any of the lakes. The groundwater chemistry in the cave lakes differs from that of the river water. Low bicarbonate content and high d13C values indicate that some of cave lakes’ waters have undergone CO2 degassing and calcite precipitation. During a major flood (recurrence interval . 100 years, level rising 7 m above the normal stage), the river water rapidly flooded the caves through openings in the river canyon (floodflow injection), while those connected to the river via alluvium only were flooded by an elevated groundwater stage, and the resulting water level rise was only about 50 percent of the river level increase. A simple hydraulic model was successfully used to simulate and explain the water table oscillations in the cave lakes. Flood-flow injection has recently been substantially reduced by low-permeability, fine-grained late Holocene fluvial sediments that cap coarse gravels in the river floodplain. Fast speleogenesis by flood injection would be expected in periods when the river canyon was bare or filled by gravel alone (glacial periods, transition to Holocene). Ice jams causing local increases in the river level are recognized as one of factors that can be important in speleogenesis.
