Stream
Streams in geographic terms are awarded order designations.  A stream of the first order is a blue-line stream which does not have any other blue-line stream feeding into it.  A stream of the second order is one which is formed by the joining of two or more blue-line streams.  A third-order stream is one below the confluence of two or more second-order streams; a fourth-order stream is formed by the confluence of at least two third-order streams, and so forth.