“Reach” can have slightly different meanings, depending on how it is used.
A reach is a section of a stream or river along which similar hydrologic conditions exist, such as discharge, depth, area, and slope. It can also be the length of a stream or river (with varying conditions) between two streamgages, or a length of river for which the characteristics are well described by readings at a single streamgage.
In practical use, a reach is any length of a stream or river. The term is often used by hydrologists when they’re referring to a small section of a stream or river rather than its entire length.