Anabranch
Anabranch , noun
[Anastomosing + branch.]
A branch of a river that reenters, or anastomoses with, the main stream; also, less properly, a branch which loses itself in sandy soil. [Australia]
Such branches of a river as after separation reunite, I would term anastomosing branches; or, if a word might be coined, anabranches, and the islands they form branch islands.