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Water Management (General) Regulation 2018
Current version for 7 February 2020 to date (accessed 31 March 2020 at 10:40)
Schedule 2 Stream order of a watercourse
(Clause 3(1), definition of “minor stream”)
The method of determining the stream order of a watercourse shown on a topographic map is the Strahler system.
The Strahler system
The Strahler system is as follows—
(a) Any watercourse that has no other watercourses flowing into it is classed as a first order stream.
(b) If 2 streams join, the resulting stream is—(i) the same order as the highest order of the 2 streams, or(ii) if the 2 streams are of the same order, the order greater than that of the 2 streams.
For example, in the diagram below—
(a) If 2 first order streams join, the stream becomes a second order stream (2).
(b) If a second order stream is joined by a first order stream, it remains a second order stream.
(c) If 2 second order streams join they form a third order stream (3).
(d) If a third order stream is joined by a first or second order stream, it remains a third order stream.
(e) If 2 third order streams join they form a fourth order stream.