Distributary

A distributary is a stream channel that branches off from a main river channel. This process is known as river bifurcation, which means the river splits into two or more paths. Distributaries are the exact opposite of tributaries. A tributary is a smaller stream that flows into a larger river to add water. In contrast, a distributary takes water away from the main channel and moves it elsewhere.

These branching channels are most common in river deltas. A delta forms when a river approaches a large body of water, such as an ocean or a lake. As the river slows down, it divides into complex networks of distributaries to spread its water out. This spreading often creates wide, marshy areas or new landforms.

However, distributaries do not always appear at the coast. They can also form inland on alluvial fans. An alluvial fan is a fan-shaped deposit of sediment made by a river. They can also occur when a tributary splits just before it meets a larger stream.

Sometimes, the way a distributary behaves can change the entire river system. A minor distributary might divert so much water that it eventually becomes the main route. In North America, these types of branching rivers are often called braided rivers. In Australia, people use the term anabranch for a distributary that leaves the main course and then rejoins it later. Some branches, however, never rejoin the original river. In English-speaking countries, these individual paths are often called arms or channels. For example, the Fraser River in Canada has the North, Middle, and South Arms.

The Atchafalaya River in Louisiana provides a famous example of how distributaries can shift power. It is a major distributary of the Mississippi River. Because the Atchafalaya has a steeper route to the Gulf of Mexico, it has captured more of the Mississippi's flow over several decades. To manage this, the Army Corps of Engineers completed the Old River Control Structure in 1963. This dam regulates the water to prevent the Atchafalaya from taking the entire Mississippi flow. Doing so protects the important ports of Baton Rouge and New Orleans.

Distributaries can also create unique connections between different parts of the world. The Casiquiare canal in South America is a remarkable inland distributary of the upper Orinoco. It flows southward into the Rio Negro. This creates a natural canal between the Orinoco and Amazon river systems. It is the largest river on Earth that links two major river systems. In Europe, the Rhine river uses distributaries like the IJssel, the Waal, and the Nederrijn to navigate the Rhine–Meuse–Scheldt delta.

In many parts of the world, these channels define the local geography. In China, the Huai River splits into three distinct streams. One stream even travels 168 kilometers to enter the Yellow Sea. In Africa, the Nile River has two main branches called the Rosetta and the Damietta. Ancient history tells us that the Nile once had seven distributaries. In Papua New Guinea, major rivers like the Fly River split into many branches. This creates wide, many-branched deltas near the Gulf of Papua.

Finally, some distributaries lead to very different destinations than others. Most rivers flow toward the sea, but some end in closed systems. The Okavango River in Africa ends in a large inland delta. Its distributaries do not flow into any other body of water. In Australia, many rivers from the Great Dividing Range form distributaries that flow into deserts. These often only flow during times of high water. They eventually end in shallow lakes or simply disappear into the sand. Whether they reach the ocean or end in a desert, distributaries are essential to how water shapes our planet.