A short length of river channel. Unstable river reaches that display rapid planform change are described as active reaches; see Martin and Pavlowsky (2011) Phys. Geog. 32, 5. However, Parker et al. (2012) River Res. & Applications 28, 10, 1708 state that there is a notable lack of consistency in its definition. Even so, they explain how to establish the boundaries of a river reach.