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Rivers flow across wide, river valleys and snake across flat plains, creating curves called meanders. When a river carves itself a new channel, some of these meanders get cut off, thus creating oxbow lakes that remain unconnected but adjacent to their parent river.
How Does a River Make a Loop?
Interestingly, once a river begins to curve, the stream begins to move more rapidly on the outside of the curve and more slowly on the inside of the curve. This then causes the water to cut and erode the outside of the curve and deposit the sediment on the inside of the curve. As the erosion and deposition continue, the curve becomes larger and more circular.
The outer bank of the river where erosion takes place is known as the concave bank. The name for the bank of the river on the inside of the curve, where sediment deposition takes place, is called the convex bank.
Cutting off the Loop
Eventually, the loop of the meander reaches a diameter of approximately five times the width of the stream and the river begins to cut the loop off by eroding the neck of the loop. Eventually, the river breaks through at a cutoff and forms a new, more efficient path.
Sediment is then deposited on the loop side of the stream, cutting off the loop from the stream entirely. This results in a horseshoe-shaped lake that looks exactly like an abandoned river meander. Such lakes are called oxbow lakes because they look like the bow part of the yoke formerly used with teams of oxen.
An Oxbow Lake Is Formed
Oxbow lakes are still lakes, generally, no water flows in or out of oxbow lakes. They rely on local rainfall and, over time, can turn into swamps. Often, they ultimately evaporate in just a few years after having been cut off from the main river.
In Australia, oxbow lakes are called billabongs. Other names for oxbow lakes include horseshoe lake, a loop lake, or cutoff lake.
The Meandering Mississippi River
The Mississippi River is an excellent example of a meandering river that curves and winds as it flows across the Midwest United States toward the Gulf of Mexico.
Take a look at a Google Map of Eagle Lake on the Mississippi-Louisiana border. It was once part of the Mississippi River and was known as Eagle Bend. Eventually, Eagle Bend became Eagle Lake when the oxbow lake was formed.
Notice that the border between the two states used to follow the curve of the meander. Once the oxbow lake was formed, the meander in the state line was no longer needed; however, it remains as it was originally created, only now there is a piece of Louisiana on the east side of the Mississippi River.
The length of the Mississippi River is actually shorter now than in the early nineteenth century because the U.S. government created their own cutoffs and oxbow lakes in order to improve navigation along the river.
Carter Lake, Iowa
There's an interesting meander and oxbow lake situation for the city of Carter Lake, Iowa. This Google Map shows how the city of Carter Lake was cut off from the rest of Iowa when the channel of the Missouri River formed a new channel during a flood in March 1877, creating Carter Lake. Thus, the city of Carter Lake became the only city in Iowa west of the Missouri River.
The case of Carter Lake made its way to the U.S. Supreme Court in the case Nebraska v. Iowa, 143 U.S. 359. The court ruled in 1892 that while state boundaries along a river should generally follow the natural gradual changes of the river when a river makes an abrupt change, the original border remains.