A watercourse is a natural or artificial channel through which water flows. A watercourse may be either on the surface, such as a river or stream, or subterranean, such as an underground stream or river. A watercourse need not have water present, as it is a channel for water, not the stream or river itself.
One example of a watercourse is a flood control dam's spillway. The spillway can be open with water flowing through it in times when water levels are high, but closed when water levels are at or below their optimum elevation.
Trenchlesspedia Explains Watercourse
A watercourse may be obvious, if a flow of water is present at all times, or it may be hidden: a small valley, ravine or gulch may be dry during times of no rain and might be a roaring river in times of heavy rain.
Underground streams and rivers called aquifers are more likely to maintain a steady flow because they are not directly dependent on rainfall. Rather, they depend on water that has worked its way through the soil over a period of time to recharge the aquifer.