River Reach

A reach in geography is an expanse, or widening, of a stream or river channel. This commonly occurs after the river or stream is dammed. A reach is similar to an arm.

'Reach' definition in wikipedia

An arm in geography, is a narrow extension, inlet, or smaller reach, of water from a much larger body of water, like an ocean, sea, or lake. Although different geographically, a sound or bay may be called an arm.

'Arm' definition in wikipedia