An anabranch is a section of a river or stream that diverts from the main watercourse channel (or mainstem) and rejoins the mainstem downstream.

In the simplest case, an island or rock in the river creates a main course and an anabranch course; a more significant anabranch would diverge for a distance of several kilometres before rejoining.

River deltas branch into large numbers of courses, though these are not normally regarded as anabranches, as the net result is usually multiple discharge points rather than a rejoined unified flow.

Word usage and related terms

The term anabranch, in its hydrological meaning, is rarely used outside of Australia.

Elsewhere, terms distributary, or more colloquially arm or channel may be used for subsidiary streams that branch from the main stream; however, unlike the more specific anabranch, these terms (especially distributary and arm) do not necessarily imply that the branch will rejoin the main course later downstream. In the United States, the term braided river describes watercourses which are divided by small islands, but the term does not cover extended separation.

Examples

• In the Fraser River delta of British Columbia, Canada, North Arm Fraser River, Middle Arm Fraser River, and South Arm Fraser River each fall into Georgia Strait separately. On the other hand, Annacis Island splits (South Arm) Fraser River into the (main) Annievile Channel and the (smaller) Annacis Channel, which rejoin below the island.
• On the Darling River in New South Wales, the river divides south of Menindee for a hundred kilometres before rejoining. The anabranch contains flowing water only in wetter years.
• The Bahr el Zeraf in southern Sudan splits from the Bahr al Jabal section of the White Nile and flows for 240 km (150 mi), before rejoining the White Nile proper upriver from Malakal.