Most people are familiar with tributaries to rivers -- smaller streams that empty into larger streams, adding the water and sediment collected from the smaller stream's drainage basin into the larger stream.

Sometimes, however, the process happens in reverse -- smaller streams branch off, carrying water *away* fom the main stream, and never rejoin it! Each of these smaller streams is a "distributary". Sometimes, one distributary is recognizable as a principal stream; other times, the river appears to break up into a maze of little channels.

Distributaries usually develop in river deltas -- The small islands formed by the sediment dropped at the mouth of a river divide the river up into smaller streams. In some deltas, distributaries come and go with each flood. In others, distributaries are scoured deeper by tides.