Meandering Rivers and Oxbow Lakes: How They Form and Change
A meandering river forms looping bends because its fastest flow erodes the outside of each curve, and an oxbow lake forms when a flood cuts through a loop neck.
A meandering river is one that winds through wide, looping bends across its valley, and those bends slowly migrate over time. An oxbow lake forms when a river cuts through the narrow neck of one loop, usually during a flood. The water takes a shorter path, and the old loop is left behind as a crescent-shaped lake. Both features come from the same process: erosion on the outside of each bend and deposition on the inside.
Why does a river form bends?
Water in a bend does not flow in a simple line. As it turns, it moves in a spiral called secondary flow. Surface water is pushed toward the outer bank, and near the bed water moves back toward the inner bank. The fastest current hits the outer bank, which erodes it into a steep cliff called a cut bank.
The spiral carries sediment, the sand and silt the river has picked up, toward the inner bank. There the water slows and drops its load, building a gently sloping ridge called a point bar. Erosion on the outside and deposition on the inside push the bend farther outward, which makes the loop tighter and deeper.
The word itself comes from the Maeander River in western Anatolia, now called the Büyük Menderes in Turkey, whose twisting course was well known in antiquity.
How do bends migrate?
Each bend grows and shifts. The cut bank retreats while the point bar grows on the inside, so the bend moves sideways across the floodplain and downstream along the valley. Geomorphologists often note that the wavelength of meanders, the distance from one bend to the next, is roughly 10 to 14 times the width of the channel.
As the loop grows, the land between its two arms narrows. This strip is called the neck. In a large flood, the neck can become very thin, and the river is then close to breaking through it.
How does an oxbow lake form?
During a flood, the river may break through the neck along a short new channel, a process called a chute cutoff. The main flow now runs through the shortcut, and the old loop is abandoned. Sediment dropped at both ends of the old loop plugs its connection to the river, sealing it off. What remains is a curved lake shaped like an oxbow, the U-shaped wooden yoke fitted to a pair of oxen, which gave the feature its name.
The new, shorter channel is steeper than the old loop, so the river now flows faster through the cutoff. The old loop, now isolated, receives little fresh water from the river.
Where do meandering rivers form?
Meanders develop most readily under a particular set of conditions:
- A gentle slope, so the water moves at a steady and moderate speed
- A single channel that carries fine sediment, such as silt and clay
- Banks held together by fine soil and vegetation, which resist sudden collapse
- A relatively steady flow rather than frequent flash floods
The lower Mississippi River, which crosses a broad, flat floodplain, is a classic example, with many oxbow lakes along its course. Rivers with steep slopes, coarse gravel, and sudden floods tend to braid instead, splitting into many shifting channels. Rivers that carry their sediment to the coast can eventually build river deltas, where the same kind of deposition happens on a larger scale.
What happens to an oxbow lake over time?
Most oxbow lakes do not last. Water seeps out and evaporates, while silt and decaying plant material settle on the bottom. Plants colonize the margins, the lake shrinks, and it gradually becomes a marsh, then a meadow or woodland. Each stage is an example of ecological succession. While it lasts, the lake often supports fish, frogs, and wetland birds.
The filled loop can remain visible from the air as a curved scar on the land. Over much longer periods, the sediment that fills it hardens into sedimentary rock, which is part of the rock cycle.
Tags
earth science erosion geology hydrology
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