Braided river

A braided river is a complex network of multiple shallow channels. These channels constantly diverge and rejoin around small, temporary islands. These islands are known as braid bars, channel bars, or accreting islands. In British English, they are sometimes called aits or eyots. This pattern resembles the interwoven strands of a hair braid.

Unlike a meandering river, which follows one single sinuous path, a braided river is spread out. It is also distinct from an anastomosing river. Those consist of semi-permanent channels separated by a floodplain rather than small bars. Braided rivers are vital parts of many geological environments worldwide.

The formation of these rivers involves complex physical processes. Scientists believe braiding occurs when a river carries an abundant supply of sediment. A key part of this is bedload transport. This is the movement of sediment particles by rolling or bouncing along the river bottom. As the water moves, it creates net erosion at channel divergences. At the same time, it causes net deposition at convergences.

This cycle of moving material builds the bars that split the water. Numerical models suggest that significant bedload transport is essential for this to happen. Without it, the river might form a different system entirely.

Several specific factors determine if a river will braid or meander. One major factor is the slope of the riverbed. Experiments in flumes have helped scientists find a specific threshold for this. For a stream with poorly sorted coarse sand, the threshold slope is 0.016 feet per foot. Any slope above this threshold creates a braided stream. Any slope below this threshold creates a meandering or straight channel. The amount of water, or discharge, also plays a role. A higher discharge can actually lower the critical slope needed for braiding.

River banks also play a crucial role in channel development. Braided rivers often have weak or highly erodible banks. These weak banks allow the river to form wide, shallow channels. This prevents the helical flow of water that is necessary for meandering. In contrast, rivers with cohesive, resistant banks form narrow and deep meandering channels. Some rivers are also described as "flashy." This means they experience rapid and frequent variations in the amount of water they carry. These frequent floods can reduce bank vegetation and further destabilize the banks.

Braided rivers appear in many different environments. They are most common in wide valleys near mountainous regions. They also appear on alluvial fans and river deltas. You can find them in gravelly mountain streams and sand bed rivers. Many exist in areas with young, rapidly eroding mountains. For example, the White River in Washington carries sediment from the Emmons Glacier. This glacier is part of Mount Rainier, a young volcano.

Extensive systems are found in Alaska, Canada, New Zealand, and the Himalayas.

There are many notable examples of braided rivers across the globe. The Brahmaputra River in Northeastern India is a classic, enormous example. It is so large it can be seen from the Space Shuttle.

In the United States, the Platte River in Nebraska is a famous example. It is known for having many linguoid, or tongue-like, bar deposits. Other unique types include the Scott River in Alaska, which is a glacial outwash river. The Donjek River in the Yukon Basin shows repeated cycles of sediment deposition. Each cycle features finer sediments toward the top.

Understanding these rivers helps us connect many scientific ideas. Braided rivers are linked to geomorphology, which is the study of landforms. They are also connected to sedimentology, the study of sediments. Even human activity can change these patterns. The Piave River in Italy is currently transitioning from a braided to a meandering profile. This change is happening due to human interventions. By studying these shifting channels, scientists learn how water, sediment, and land interact to shape our world.