An oxbow lake is a U-shaped or crescent-shaped lake that forms when a river cuts through the narrow neck of one of its own meanders, abandons the loop, and the two ends of the old channel silt up, leaving a still, free-standing body of water on the floodplain. It is named after the U-shaped wooden collar of an ox harness. Textbook examples include Lake Chicot in Arkansas, the largest oxbow lake in North America at about 34 km (21 miles) long, and Kanwar Taal in Bihar, India, a 2,620-hectare oxbow of the Burhi Gandak river that became Bihar’s first Ramsar wetland in 2020.
Thomson’s 1857 Teacup and Einstein’s 1926 Paper: Why River Bends Grow
Every oxbow lake begins as a meander that keeps getting bigger, and the reason a bend grows rather than straightens puzzled physicists for decades. The answer lies in what is now called the tea leaf paradox. Stir a cup of tea and the leaves gather in the centre of the bottom, not at the rim where you might expect a spinning liquid to fling them. The engineer James Thomson gave the correct explanation in 1857: friction slows the water touching the bottom, so the outward push is weaker there than near the surface. The result is a secondary, corkscrew-shaped flow that runs outward along the top and inward along the floor.
In 1926 Albert Einstein published a short paper applying the same reasoning to rivers. In a bend, the water surface stands slightly higher on the outside of the curve, and the slow layer along the bed is swept across toward the inside. This helical flow does two jobs at once. It scours the outer bank, called the cut bank or concave bank, and carries the loosened sand and gravel to the inner bank, where it builds a crescent-shaped deposit called a point bar. Erosion on one side and deposition on the other mean the whole bend migrates sideways and becomes more strongly curved, a self-reinforcing process.
Geomorphologists measure how far a river has gone down this road with sinuosity, the ratio of channel length to straight down-valley distance. A channel with a sinuosity of 1.5 or more is classed as meandering. Rivers with high sinuosity develop larger loops, so their floodplains end up with longer oxbow lakes than those of gently curving rivers.
Breaching the Neck: Shreve’s Cut on Turnbull’s Bend, 1831
As two neighbouring outer banks eat toward each other, the strip of land between them, the meander neck, gets thinner. Eventually the river breaks through, either by steady lateral erosion of both banks or in a single flood, when high water spills across the neck and takes the shorter route. The abandoned loop is called a cutoff. Depending on the river, the whole sequence can take anywhere from a few years to several decades.
One of the best documented cutoffs on the Mississippi was helped along by people. Beginning in the 15th century the river had developed a westward loop, later called Turnbull’s Bend, near present-day Angola, Louisiana. A steamboat needed several hours to travel the 32 km (20 miles) around it, only to end up about 1.6 km (1 mile) from where it entered. In 1831 Captain Henry Miller Shreve, the river engineer after whom Shreveport is named, dug a canal through the neck. The river adopted the shortcut, known as Shreve’s Cut, and the old loop was left behind as a side channel called the Old River. That abandoned bend still matters: the Old River Control Structure, completed by the US Army Corps of Engineers in 1963, sits in it to stop the Mississippi from diverting into the Atchafalaya.
Plugging the Ends: How a Cutoff Becomes a Still Lake
A fresh cutoff is not yet a lake. For a while water still enters at one end and leaves at the other. What turns it into an oxbow lake is deposition at the two junctions with the new main channel. Because flow through the old loop is now slow, sediment settles at its entrances until both are sealed by plugs of silt and sand. From then on the lake has no current running through it.
That stillness sets the lake’s lifespan. Floods still deliver fine sediment, and plants colonise the shallow margins, so an oxbow lake gradually fills, turning first into a marsh or swamp and finally into a dry, curved scar on the floodplain. Geologists call these filled channels meander scars, and aerial photographs of large floodplains show generations of them, from open lakes to faint arcs of different-coloured vegetation in farm fields.
While they last, oxbow lakes are valuable wetlands. They trap sediment and farm runoff that would otherwise stay in the main river, and in the southern United States they are a main habitat for bald cypress and water tupelo trees. In the Amazon basin, oxbow lakes are favoured habitat for the giant river otter. The flip side is that the pollutants they trap accumulate in them, and they are vulnerable to heavy-metal contamination from industry upstream.
Lake Chicot, Arkansas, and Kanwar Taal, Bihar: The Big Classic Oxbows
The largest examples form on the widest, flattest floodplains. Lake Chicot, on the east side of Lake Village in Chicot County, Arkansas, is a former bend of the Mississippi. It is about 1.2 km (0.75 miles) wide and roughly 34 to 35 km (21 to 22 miles) long, which makes it both the largest oxbow lake in North America and the largest natural lake in Arkansas. Its name comes from the French chicot, “stumpy”, after the cypress stumps along its shores. The lake shows how humans become part of an oxbow’s story: when the dam at Cornerly Bayou failed in the 1927 flood, silt poured into the southern end, and in 1968 state and federal agencies drained and restored that section.
On the Indo-Gangetic plain, Kanwar Taal (also called Kabartal) lies 22 km north-west of Begusarai town in Bihar. It is a residual oxbow left by the meandering of the Burhi Gandak, a tributary of the Ganges, and covers 2,620 hectares. It is described as Asia’s largest freshwater oxbow lake. During the monsoon it floods to a depth of about 1.5 metres, soaking up water in a state where around 70% of the land is prone to inundation. India designated it a Ramsar site in November 2020; 221 bird species have been recorded there, including three critically endangered vultures.
Smaller oxbows can be just as well known. Cuckmere Haven in Sussex, England, where the River Cuckmere winds across its floodplain to the English Channel next to the Seven Sisters cliffs, contains several oxbow lakes and is often cited in physical geography textbooks as a model meandering river.
Carter Lake, Iowa: An Oxbow Made in a Single Flood
Not every cutoff is slow. In March 1877 a flood shoved the Missouri River about 2 km (1.25 miles) to the south-east of its old course near Omaha. The abandoned loop, called Saratoga Bend, became an oxbow lake, Carter Lake. Because the river had jumped rather than drifted, the land inside the old bend stayed part of Iowa even though it now sits on the Nebraska side of the river. Nebraska challenged that, and the US Supreme Court settled it in Nebraska v. Iowa (1892): state lines follow gradual shifts in a river, but not a sudden avulsion that cuts off a bend. The town of Carter Lake, incorporated in 1930, is today the only city in Iowa west of the Missouri, with 3,791 residents at the 2020 census.
Meanders produce other odd river borders too. The Kentucky Bend, a peninsula inside a tight loop of the Mississippi, is an exclave of Fulton County, Kentucky, encircled by Tennessee and Missouri. It exists because the surveyors of the Kentucky–Tennessee line only estimated where their line would meet the river; later surveys showed it crossing the loop, and with the Mississippi as Kentucky’s western border, the tip of the peninsula ended up Kentuckian.
Billabongs, Resacas and West Lake: One Landform, Many Local Names
Because oxbows form on almost every large meandering river, many regions have their own name for them. In Australia they are billabongs. The word is most likely from the Wiradjuri language of southern New South Wales; the Macquarie Dictionary gives the original as bilabaŋ, with bila meaning “river”. The second half is disputed. One reading takes it as bong, “dead”, while Frederick Ludowyk of the Australian National Dictionary Centre argued in 2004 that it was a suffix meaning continuation. The word first appears in Australian English in 1836, when the explorer Thomas Mitchell recorded “Billibang” as the Aboriginal name of the Bell River. In South Texas, oxbows left by the Rio Grande are called resacas.
Some oxbows have become city landmarks. West Lake (Hồ Tây) in Hanoi, the largest lake in the Vietnamese capital, with about 17 km of shoreline and an area of 500 hectares, formed from a cut-off bend of the Red River. Halfmoon Lake in downtown Eau Claire, Wisconsin, is an old course of the Chippewa River, and in New Zealand the upper Taieri River near Paerau has left a whole field of oxbow lakes across its valley.
Tulla’s Rhine and the Oxford Canal: Oxbows Made by Engineers
Engineers can make oxbows on purpose. Straightening a river for navigation or flood control cuts through its meanders and leaves the old loops behind. The best-known example is the Upper Rhine in the 19th century, where the engineer Johann Gottfried Tulla led a programme to shorten and straighten the river; agreements such as the 1817 treaty for rectifying the Rhine between Neuberg and Dettenheim covered the work. Loops, side branches and islands were removed, and the river’s length between Basel and Worms fell from about 355 km to 275 km. Local flooding eased, but the faster flow raised flood risk farther downstream on the Middle and Lower Rhine, which is why partial floodplain restoration still goes on today.
Even canals can do it. The Oxford Canal in England originally followed the contours of the land in long loops. Between 1829 and 1834 its northern section was straightened, cutting its length from about 146 km to 125 km (91 to 77½ miles) and leaving oxbow-shaped stretches of old canal stranded beside the new line.
The common thread from Arkansas to Bihar is the same sequence: helical flow enlarges a bend, the neck is breached, the ends are sealed, and the lake slowly fills. To see the first step in more detail, read our explainer on why rivers meander. The setting it all happens in is covered in what a floodplain is, and the river with more oxbows than any other in North America is the subject of our Mississippi River guide.