How Oxbow Lakes Form

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An oxbow lake is a uniquely shaped lake resulting from the path of a meandering river. Oxbow lakes are U-shaped or curved bends in a river that are cut off from the main river flow, forming a lake. 

An oxbow also refers to the horseshoe-shaped bends in rivers that can eventually become oxbow lakes. Every river contains twists and turns, or meanders, that cut through the surrounding landscape in a unique way. Oxbow bends and, later, oxbow lakes, are formed due to the movement of water. 

Streams meander on the flat landscape, Bering Land Bridge National Preserve, 2014.
Meandering streams alternate eroding sediments from the outside of a bend and depositing those sediments on the inside of the bend. This snaking pattern occurs as the stream meanders back and forth across the flat landscape, continually eroding sediments as it moves. Over time this erosive force cuts off the bends in meandering streams from the main body, leaving behind an oxbow lake. Streams meander on the flat landscape, Bering Land Bridge National Preserve, 2014. Photo: NPS, public domain.

As water flows down a river, it constantly erodes the sediment that makes up the riverbanks. Riverbanks made of soft sediments, like sand or clay, can erode quicker than harder rock surfaces, altering the riverbed at a more rapid rate.

The natural flow of water from higher elevations to lower in the form of a river or stream isn’t fixed; different water flow levels and sediment deposits alter the path of the river over time.

Oxbow lakes can also be formed by flooding, earthquakes, and other natural events that alter the flow of a river. 

(You can also watch this video on YouTube: Watch An Oxbow Lake Form: Ucayali River: 1985 – 2013)

River Hydrodynamics

There are a few things to know about the dynamics of a riverbend. The currents in a river can be varied, and debris like rocks or trees can alter the water flow.

When a river meanders, sediments along the bend with the smaller radius (the convex bank) are deposited by the water carrying debris from upstream. 

On the other side of the meander is the concave bank, or the cut bank. On this side, the river is eroding sediment away from the riverbank, and this lateral erosion can cause the water to undercut the riverbank. This curve has a larger radius than the convex bank does. 

Evolution of a meandering stream includes the following: (1) stream channel within meander belt; (2) development of a nearly closed meander loop; (3) high water flowing across the neck of loop, making a cutoff; (4) deposition of sediment sealing the loop and creating an oxbow lake.
Evolution of a meandering stream includes the following: (1) stream channel within meander belt; (2) development of a nearly closed meander loop; (3) high water flowing across the neck of loop, making a cutoff; (4) deposition of sediment sealing the loop and creating an oxbow lake. Diagram: Phil Reiker, NPS Geologic Resources Division, public domain.

The creation of bends in the river changes the pressure gradient of the water. Pressure is highest on the outside of a river’s bend (the concave bank), getting lower towards the inside corner (the convex bank). The water flows quickest on the concave side and slower on the convex bank. 

So, on one side of the river sediment is being deposited, while on the other side sediment is being eroded. Rather than flowing downstream at a uniform rate, water dynamics work to deposit this sediment in different ways. The primary flow of a river refers to the mass movement of water in the same direction- downstream. The secondary flow is the movement of water that pushes sediment laterally across the river, from the concave bank to the convex bank. 

Secondary Flow and Sediment Deposits

The same secondary flow that causes debris to build up on the convex bank works to create a point bar. A point bar grows into the river, making the meander more pronounced. As the meander expands, two adjacent concave banks grow closer to one another.

Eventually, continued erosion and high water events like floods or seasonal high water flows can join two concave banks together. The main river channel has changed its course, leaving the riverbend cut off from the main flow of the river.

Continued sediment deposit can result in this bend truly becoming cut off from the rest of the river. Once that process has occurred, the formation is known as an oxbow lake. Rivers with high sinuosity, or lots of bends, often create longer oxbow lakes than rivers that naturally flow in straighter lines. 

Aerial view of Fort Larned, coming from the east with the dry oxbow in the foreground, NPS.
Pawnee Fork Oxbow: The depression surrounding this area was once part of the Pawnee River. Before Fort Larned was built, the river naturally changed course. The dry oxbow was used for wood piles and hay stacks. Photo: Aerial view of Fort Larned, coming from the east with the dry oxbow in the foreground, NPS.

Bodies of water that have been artificially straightened to reduce flooding impacts to cities can leave behind oxbow lakes. Canals, agricultural use, and the reduction of winter snowpack in some locations can also alter natural river flows. While flooding can be detrimental to human infrastructure, the sediments deposited by rivers and the movement of their meanders create fertile, abundant landscapes that help sequester carbon, provide essential habitat to migrating animals, and much more.

Other Names for Oxbow Lakes

An oxbow by any other name might be a billabong if you’re Australian or a resaca if you’re in south Texas. Billabongs are vital water sources for many animals and people in the arid regions of Australia, and individual billabongs often have names because they might be the only source of water for many miles. 

In Texas, the natural seasonal flooding of the Rio Grande leaves behind oxbow lakes on a regular basis. These lakes are often replenished during flood events as the Rio Grande alters its flow every year. 

Famous Oxbow Lakes in the USA

Oxbow Bend in the fall.  The reflection of Mount Moran can be seen in the water.  Photo: NPS, Grand Teton National Park, public domain.
Oxbow Bend in the fall. The reflection of Mount Moran can be seen in the water. Photo: NPS, Grand Teton National Park, public domain.

While many locations have their own famous oxbow bend or lake, perhaps the most well known is Oxbow Bend in Grand Teton National Park, Wyoming. This is an oxbow bend that is home to a variety of wildlife and allows for expansive views of the Grand Tetons, popular with photographers during sunrise and sunset throughout the year. 

A true oxbow lake, Lake Chicot in Arkansas is the largest oxbow lake in North America and was formed over 600 years ago from the meandering of the Mississippi River. This lake is also the state of Arkansas’s largest naturally formed lake. 

This photo, taken by an astronaut onboard the International Space Station (ISS), has been annotated to show the current state boundary (yellow line) between Arkansas and Mississippi. (Note that north is to the left in this image.) The image also shows how much the Mississippi River has meandered.  Lake Chicot, the largest oxbow lake in North America can be seen. Image: NASA, public domain.
This photo, taken by an astronaut onboard the International Space Station (ISS), has been annotated to show the current state boundary (yellow line) between Arkansas and Mississippi. (Note that north is to the left in this image.) The image also shows how much the Mississippi River has meandered. Lake Chicot, the largest oxbow lake in North America can be seen. Image: NASA, public domain.

There are two states that have had their boundaries formerly dictated by the flow of their biggest rivers: the Missouri River and the Mississippi River.

Carter Lake in Iowa was formed after a flood altered the flow of the Missouri River in 1877. A city built on its banks, the city of Carter Lake, is now the only city in Iowa located on the west side of the Missouri River. 

On the Mississippi/Louisiana border, a riverbend formerly known as Eagle Bend is now Eagle Lake. Eagle Lake was formed after an earthquake altered the main flow of the Mississippi River. Now, a piece of Louisiana rests on the eastern side of the Mississippi River because of this change. The Mississippi is home to more than a few oxbow lakes, as the river channel was often artificially straightened to make navigation easier. 

Reelfoot Lake in Tennessee was formed by an earthquake that altered the trajectory of the Mississippi River. Reelfoot Lake is considered to be a shallow oxbow lake and is a sought after destination for birdwatchers, as it is a nesting place for eagles. 

Wisconsin’s Half Moon Lake is an oxbow lake popular among fishermen.

Double Oxbow

Landsat 8 satellite image of the meandering Colorado River. Image: NASA
Landsat 8 satellite image of the meandering Colorado River. Image: NASA

Canyonlands National Park in Colorado is home to a double oxbow, called The Loop. The Loop is located about 8 kilometers (5 miles) upstream from the confluence of the Green and Colorado Rivers.  Unlike some of the examples above where the rivers flow through soft soil, the oxbow in Colorado is an “entrenched meander”, flowing through sedimentary rock canyons with walls that are 150 meters (500 feet) high.

Eventually, the Colorado River will erode the canyon walls to create a new channel and an oxbow lake.

Green River Oxbow, Canyonlands National Park. Photo: NPS/Neal Herbert
Green River Oxbow, Canyonlands National Park. Photo: NPS/Neal Herbert

References

The Snippet Seamstress. The middle course of a river – formation of meanders. Accessed February 21, 2021. Retrieved from http://snippetseamstress.blogspot.com/2009/01/middle-course-of-river-formation-of.html

Gardner, Emily. Exploring Geology Chapter 16 Rivers and Streams. Accessed February 21, 2021. Retrieved from https://slideplayer.com/slide/4753383/

Wikipedia. Oxbow Lake. Accessed February 21, 2021. Retrieved from https://en.wikipedia.org/wiki/Oxbow_lake

Kelly, Daniel. Friday Five: Oxbow Lakes Formed by Meandering Rivers. 25 April, 2014. Retrieved from https://www.lakescientist.com/oxbow-lakes-meandering-rivers/

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