A distributary, or a distributary channel, is a stream that branches off and flows away from a main stream channel. They are a common feature of river deltas. The phenomenon is known as river bifurcation. The opposite of a distributary is a tributary. Distributaries usually occur as a stream nears a lake or the ocean, but they can occur inland as well, such as in an endorheic basin, or when a tributary stream bifurcates as it nears its confluence with a larger stream. In some cases, a minor distributary can "steal" so much water from the main channel that it can become the main route.
Common terms to name individual river distributaries in English-speaking countries are arm and channel. They may refer to a distributary that won't rejoin the channel it has branched off (e.g., the North, Middle, and South Arms of the Fraser River, or the West Channel of the Mackenzie River), or one that will (e.g. Annacis Channel and Annieville Channel of the Fraser River, separated by Annacis Island).
In Australia, the term anabranch is used to refer to a distributary that diverts from the main course of the river and rejoins it later. In North America an anabranch is called a braided stream.
Examples of Distributaries
In Louisiana, the Atchafalaya River is an important distributary of the Mississippi River. Because the Atchafalaya takes a steeper route to the Gulf of Mexico than the main channel, it has captured more and more of the Mississippi's flow over several decades, including capturing the Red River, which was formerly a tributary of the Mississippi. The Old River Control Structure, a dam which regulates the outflow from the Mississippi into the Atchafalaya, was completed in 1963 to prevent the Atchafalaya from capturing the main flow of the Mississippi and stranding the ports of Baton Rouge and New Orleans.
An example of inland distributaries is the Teton River, a tributary of Henrys Fork in Idaho, which splits into two distributary channels, the North Fork and South Fork, which join Henrys Fork miles apart.
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