watershed terms Tools A B aquifer soil or rock below the land surface that is saturated with water confluence where two streams meet estuary where the river meets the sea flood plain a strip of land along a river ground water water that flows or seeps down and saturates rock head waters the water upstream from a point or stream riparian water zone owners of land adjacent from a stream can make use of the stream tribuatary smaller river or stream that flows into a larger river or stream water table the top of the water surface in the saturated part of an aquifer wetlands areas characterized by saturated soils10