(1) The source and upper reaches of a stream; also the upper reaches of a reservoir (2) The water upstream from a structure or point on a stream (3) The small streams that come together to form a river Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries
The small rivulets that are the source of a stream or river Mapped as the last crenulation in a contour line below a ridge or crest on a topographic map