What Is an Oxbow?
Learn how TNC is restoring oxbow wetlands to help wildlife and people in Iowa.
An oxbow wetland is a meander of a stream, river or creek, that has become separated from the flow of water. Oxbow wetlands store excess water that might otherwise lead to flooding, filter water to improve water quality and provide habitat to a variety of wildlife. Over time, some oxbows fill in with sediment due to erosion of soil in surrounding areas.
Restoring and maintaining oxbow wetlands provides a natural solution to water resource management challenges. The Nature Conservancy in Iowa works with partners to restore oxbow wetlands in three priority watersheds: Cedar River Watershed, Boone River Watershed and Des Moines River Watershed.
In order to restore these important wetlands, The Nature Conservancy in Iowa works to identify locations where oxbows once existed and then partners with landowners to plan the restoration process. The area is cleared of obstructions and the u-shaped oxbow is excavated. Once the newly excavated wetland is reconnected with a nearby source of water, the oxbow fills and small fish can move in, often spawning and hatching in the oxbow. Learn more below about the benefits of restored oxbows.
Improving Water Quality
The ability of an oxbow wetland to process and filter out excess nutrients depends on how long the oxbow can store water before it flows into the nearby stream. Nitrates are a common nutrient necessary for agricultural production but are detrimental to water quality when introduced at high levels. Restored oxbows remove nitrate from flood waters. Studies have shown that, on average, oxbows filter 42 percent of excess nutrients from water that flows into them.
Oxbows that intercept agricultural tile drainage can filter nearly 100 percent of nitrates from the water, if given enough time. Tile drainage is a type of drainage system that removes excess water in soil from below its surface. In many Iowa streams, a significant percentage of water flow is from tile drainage. Urban oxbows also capture and treat a variety of urban contaminants when linked to stormwater discharge.
Increasing Flood Storage
Oxbows store up to one million gallons of water per acre of oxbow and can slow stormwater before it enters rivers and streams. When stream flows increase during and after precipitation events, oxbows can intercept some of the flood water and slowly release it back into the stream, helping to reduce the peak flows. As precipitation events become more extreme due to climate change, this role for oxbow wetlands will become more important. When coupled with stormwater wetlands or basins, oxbows can lead to significant reductions in flooding.
Providing Wildlife Habitat
Restored oxbow wetlands provide important habitat for young fish to mature before entering the stream as adults. They also offer vital refuge to smaller fish seeking protection from larger, predatory fish. Fish surveys completed in restored oxbows have found 31 species of fish including green sunfish, fathead minnow and the federally endangered Topeka shiner. Oxbow wetlands are also used by migratory birds and waterfowl as feeding grounds and nesting sites. Ducks and geese often begin using newly restored oxbows as soon as water is restored to them.