The landscape of 53,000-acre Walnut Creek Watershed flows from the farm fields to the east of Dallas Center into the growing residential neighborhoods and business districts of the Des Moines Metro. The watershed touches 9 communities and 2 counties, eventually draining into the Raccoon River. The confluence with the Raccoon is less than one mile upstream of the water intake for the Des Moines Water Works plant, a public water supply serving nearly half a million people in central Iowa. Walnut Creek is a priority watershed for local jurisdictions working to reduce flooding impacts and improve water quality.
In 2017, The Nature Conservancy partnered with Polk Soil & Water Conservation District to identify and fundraise oxbow wetland restorations in the Walnut Creek Watershed. Traditionally these oxbows are restored on the agricultural landscape, but as work began to identify sites along Walnut Creek several opportunities were found within natural areas in both Urbandale and Clive. After coordinating with local parks and community development staff, six oxbows were eventually identified and restored in late 2017.
What is an oxbow?
An oxbow is a meander of a stream that has been cut off from the present flow of water. Over time oxbows fill in with sediment and loose the habitat and water quality benefits they once provided.
Restoring oxbow wetlands provide multiple benefits including improving water quality, providing flood storage capacity, and improving wildlife habitat. Several oxbows in Iowa have been researched to quantify these benefits. Some of the findings include:
- Improve water quality- Restored oxbow have the natural ability to remove excess nutrients, such as nitrates, from the water. On oxbows that intercept farm tiles, nitrates were reduced by 45-90%.
- Flood Reduction- Each ¼ acre of oxbow restored can store an estimated 250,000 gallons of flood water. By allowing the stream to flow into the oxbow during high flows, flooding can be reduced downstream.
- Fish Habitat- Research found thousands of fish, representing 23 species living in restored oxbows. These oxbows provide important habitat for young fish to grow before entering the stream as adults.
- Bird Habitat- As part of a study in northern Iowa, 54 species of birds were identified at restored oxbows, including 9 species not found before restoration.
The restoration process for these oxbows is rather simple. During periods of dry weather, typically summer or fall, construction crews excavate out soil that has been deposited into the wetlands over time. Excavation continues until the historic riverbed depth is reached, typically finding a layer of gravel and reconnection to groundwater. This connection to groundwater is key to providing a regular source of oxygen, allowing fish to survive during low water. Once excavation is complete, banks are sloped back and revegeted.
This oxbow restoration was made possible by a collaborative effort between the City of Urbandale and the City of Clive, The Nature Conservancy, Iowa Department of Agriculture and Land Stewardship, Natural Resources Conservation Service, and Polk Soil & Water Conservation District.