Weir

A weir is a small overflow type dam commonly used to raise the level of a small river or stream. Weirs have traditionally been used to create mill ponds. Water flows over the top of a weir, although some weirs have sluice gates which release water at a level below the top of the weir. The crest of an overflow spillway on a large dam is often called a weir.

en.wikipedia.org/wiki/Weir

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

Anabranch

a secondary channel of a river that leaves the main channel and rejoins it further downstream.

Australia provides a continental setting conducive to the formation of anabranching rivers. Very low relief, within channel vegetation and cohesive fine-grained sediment in an arid environment with declining flow discharges and increasing sedimentation downstream encourage the development of anabranches.

www.artistwd.com/joyzine/australia/strine/a-5.php

from "define: anabranch" on google