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In karst regions, the underground drainage system is usually well developed. As a result, major springs rise when geological conditions such as underlying impervious rock, valley slope intersection, or structural control in the rock force the water to the surface. Exsurgences refer to springs fed entirely by water which has never flowed as a stream on the surface; resurgences refer to springs fed entirely by water from sinking streams. The springs often are intermittent. In some springs, the water simply flows from a cave, while in others hydrostatic pressure forces the water to the surface.
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