What is an oxbow?

If you look at an aerial of Horseshoe Bend Wildlife Area in Milford, you'll see an area on the west that is a small body of water that juts off from the main meander of the Little Sioux River.

This area is called an oxbow.

Map of Horseshoe Bend with its oxbow

Basically, this was once a part of the Little Sioux River that was cut off when the current changed or straightened through the park. Now, it is its own body of water until a point when the Little Sioux River has enough water capacity to reconnect it.

Oxbows are extremely important to the environment, and many are being restored on rivers and streams throughout Iowa and the United States.

First, they are important for water quality.

Oxbows are basically a type of wetland, because they help to filter water. They do this in a variety of ways. As water is stored in the oxbow system, the nutrients in the water are processed out of the water system, so when that water does rejoin the river, it is much cleaner. Studies have shown more than 40 percent of nutrients are purged from the water, and up to 100 percent of nitrates can be cleaned from the system if allowed to sit long enough.

Oxbows are also an area that sediment can be removed from a river. Natural stream meanders create slower moving water, allowing sediment to fall to the bottom, out of suspension. That sediment is then deposited and not carried downstream, creating cleaner waters.

Second, oxbows are important for stormwater runoff and flood reduction.

When storms come and river levels rise, even if an oxbow is temporarily cutoff from the current, it will reconnect with rising water levels. This allows the oxbow area to contain some of that rising water --- oxbows can hold up to 1 million gallons per acre.

The flow into the oxbow can also decrease the current in the rising river, delaying flood peaks downriver.

Finally, oxbows are important for wildlife habitat.

Connected oxbows are perfect, calm areas for young fish to grow before entering the faster current of the connected river. In addition, it is also a place for smaller fish to avoid larger predators that are in the main body of water. The federally endangered Topeka shiner, a type of minnow, finds its home in oxbows.

Other creatures find oxbows as perfect food sources. Waterfowl stop in oxbows to feed on fish and vegetation, and raccoons use oxbows to find fish, amphibians, mussels and other food out of the fast current.

Oxbows are also often used as safe nesting sites for waterfowl.

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