Streams interface the ground water in all kinds of landscapes (see Box B). The interaction comes up in 3 basic ways: sources gain water from inflow of ground water by using the streambed (gaining stream) they lose water to ground water by outflow due to the streambed (losing stream, Figure), or they can both, gaining in some reaches and losing in other reaches. For ground water to discharge into a stream channel, the altitude of the water table in the vicinity of the stream must be higher than the altitude of the stream-water surface.