Braided river

A braided river, also referred to as a braided channel or braided stream, is defined by a web of multiple river channels that split and rejoin around small, often short-lived islands. These islands are known as braid bars, or in British English, aits or eyots. This type of river typically forms where the water carries a heavy load of sediment or flows over coarse material, and where the riverbed has a steeper gradient than that of a typical straight or meandering river. Braided rivers are also common in waterways that experience sudden and frequent changes in water volume—known as flashy rivers—and in rivers with banks that are easily eroded.

The channels in a braided river are generally shallow and unstable, with the braid bars frequently shifting or being submerged during high water. The entire layout of the river can change dramatically during floods. When vegetation takes root on the islands, making them more permanent, they are sometimes called aits or eyots. This structure contrasts with a meandering river, which follows a single winding path, and with an anastomosing river, which has multiple semi-permanent channels separated by floodplain rather than by channel bars.

The exact conditions that cause a river to braid rather than meander are not completely understood, but a plentiful supply of sediment is widely considered a key factor. Experiments using flumes show that a river will braid once a certain threshold of sediment load or slope is reached. For example, in one experiment with a small stream carrying coarse sand, a slope of 0.016 feet per foot was the tipping point: steeper slopes produced braiding, while gentler slopes led to meandering or straight channels. The balance between suspended sediment and bedload also matters. More suspended sediment can deposit fine, erosion-resistant material on the inside of curves, which can deepen the curve and sometimes shift a river from braided to meandering. Higher water discharge lowers the critical slope needed for braiding, while larger grain sizes raise it. Aggradation, or net sediment buildup, encourages braiding but is not always necessary—rivers like New Zealand’s Rakaia and Waitaki are braided despite not aggrading. Variable water flow also plays a role, mainly by reducing bank vegetation and weakening banks during floods. Numerical models show that bedload transport—sediment rolling or bouncing along the riverbed—is essential for braiding, especially when the flow is not tightly confined. Braiding does not occur in simulations of pure scour or with cohesive sediments lacking bedload. Meanders form only when banks are stable enough to limit lateral flow, while weak, erodible banks produce wide, shallow channels that prevent the helical flow needed for meandering, leading instead to braiding.

Braided rivers are found worldwide, most often in wide valleys near mountains or their foothills, in areas with coarse sediment and sparse bank vegetation, and on alluvial fans. Extensive systems exist in Alaska, Canada, New Zealand’s South Island, and the Himalayas—regions with young, rapidly eroding mountains. The Brahmaputra River in northeastern India is a classic example, while the Platte River in central and western Nebraska is a notable large braided stream in the contiguous United States, characterized by abundant tongue-shaped bars and dune deposits. The Scott River in southern Alaska serves as the type example for braided glacial outwash rivers.