"Sub-drainage basin" means the drainage area of the highest order salmonid bearings stream containing the subject property impact area. Stream order is the term used to define the position of a stream in the hierarchy of tributaries in the watershed. The smallest streams are the highest order (first order) tributaries. These are the upper watershed streams and have no tributaries of their own. When two first order streams meet, they form a second order stream, and when two second order streams meet they become a third order stream, and so on. (Added by Amended Ord. 06-061, Aug. 1, 2007, Eff date Oct. 1, 2007).