Oxbows -- cut off river bends that are often found as small, seasonally wet depressions along field edges -- can be multipurpose conservation tools. Oxbows can store floodwater, filter runoff or drainage from tile lines, and act as nurseries for breeding fish.
Most have silted in during the years, fading into the landscape. But, Aleshia Kenney, U.S. Fish and Wildlife Service biologist, in Moline, Illinois, notes that digging out the silt and reconnecting them to a nearby stream allows high water to reach the oxbow and puts both hydrology and biology to work enhancing local rivers.
"The main biological processes are uptake or assimilation of the nutrients -- nitrate and phosphate -- by plants, algae and bacteria, and transformation processes conducted by microbes within the oxbow," Kenney said.
"A study in ecological engineering showed that restored oxbows remove, on average, about 45% of the nitrates from the tile line water," she added. "During certain times of the year, the efficiency was almost 90%. The nitrate-retention efficiency of oxbows is very similar to other practices such as bioreactors, wetlands and saturated buffers, but restored oxbows also add in a large ecosystem benefit, as well."
Karen Wilke, of The Nature Conservancy, in Webster City, Iowa, studied under Kenney and now helps arrange restoration of oxbows on farmland in central-Iowa's Boone River Watershed.
"We've seen 30 different fish species and 54 different bird species [in restored oxbows], nine of which the Audubon Society said would not be here if we hadn't restored the oxbows," Wilke reported.
Wilke said The Nature Conservancy has identified 160 oxbows in the Boone River Watershed alone and has restored 21 of them so far in cooperation with the Iowa Soybean Association, the USDA Natural Resources Conservation Service, U.S. Fish and Wildlife Service, Iowa Department of Natural Resources, the Iowa Department of Agriculture and Land Stewardship, National Fish and Wildlife Foundation and local soil- and water-conservation districts, among others. The Nature Conservancy has been able to bring private donations and grants to the effort. This has been especially helpful because, although many oxbows may be financed in part through the Environmental Quality Improvement Program (EQIP), restoration funding for oxbows isn't yet included in Iowa's Nutrient Reduction Strategy.
Restoration itself is not complicated, Kenney noted. "When we restore the oxbows, we dig them down to the old stream channel bed, which is usually where the groundwater is," she said. "We have also started directing tile lines into restored oxbows. The tile water provides an extra water source to the restored oxbows during dry times, and the oxbows catch and process the nutrients before the water enters the stream."
That's a win-win for farmers and their communities, noted Sarah Lehmann, National Aquatic Resource Surveys team leader for the U.S. EPA Office of Water.
"Edge-of-field conservation practices are important components in the agricultural landscape to remove nitrogen and phosphorus that is lost through tile drainage," Lehmann said. "Our last National Aquatic Resource Surveys found that 65% of the river miles -- two out of three -- in the ecoregion that covers Iowa were rated "poor" for excessive total nitrogen, and 36% -- one-third -- were rated "poor" because of high levels of total phosphorus.
"When farmers incorporate these linkages to the oxbows at the edge of the field to their in-field nutrient-management practices, there are good benefits to both the farmer and the community," she added.
A NATURAL CHOICE
Back in 2007, Ray Frye dug up a pair of plugged tile lines in a field he had purchased the year before near Webster City, Iowa. The lines drained 25 to 30 acres of gently sloping land. Frye ran them into a depression a couple hundred yards long near the woods along White Fox Creek. The Nature Conservancy and the Iowa Soybean Association had identified the depression as an old oxbow and offered to cover the cost of excavating it back to its original depth and carving shallow channels that would allow the creek to connect with the oxbow during high-water events.
After it was restored in 2011, the White Fox Creek oxbow was one of the sites that Kenney studied for a paper on the ecological impacts of restored oxbows. Frye said, "We had 10 milligrams of nitrogen going in and zero coming out, and they counted 1,100 fish in it."
The oxbow -- and the rest of Frye's history of conservation practices on the land -- drew Selden Spencer to purchase the property in 2016. Spencer, a commissioner of the Story Soil and Water Conservation District, in Nevada, Iowa, said the oxbow opened his eyes to new approaches to protecting water quality.
"On the soil and water commission, we're always trying to find, 'Is there a way we can reduce nitrates?'" he said. "My answer has always been cover crops, cover crops, cover crops. But, if this is one other way we can reduce the nitrates, more power to you."
Spencer added the "unused" acreage that contains the oxbow has become his favorite feature of the farm.
"It's very valuable in more ways than growing a crop," he noted.
The Nature Conservancy, the Iowa Soybean Association and their partners are searching the landscape in central-Iowa's Boone River Watershed for signs of silted-in oxbows. So far, they have identified 160 in high-priority areas where restoring oxbows could yield big reductions in nutrient loading in local creeks.
Oxbow expert Aleshia Kenney, a biologist with the U.S. Fish and Wildlife Service, in Moline, Illinois, said that's just the tip of the iceberg.
"The landscape in the Midwest is littered with oxbow scars," she said. "Landowners can recognize them as depressions along streams that usually hold water at some point in time, usually spring or late fall. They are usually horseshoe or kidney bean shaped. If these areas fill during spring floods but then dry up during the summer, they may be in need of restoration."
Restoration includes excavation to restore the oxbow's original depth, resloping and reseeding banks, and introducing channels that connect the oxbow to its original river during floods. Floodwaters bring fish and other wildlife into the oxbow. As the flood recedes, and the oxbow is once again isolated, it can serve as a nursery for a wide range of fish species, as well as a haven for birds and other animals. Fall floods allow growing fish to get back into the river.
Taking a Shine to the Topeka Shiner
The Topeka shiner isn't much to look at -- it's a minnow just a couple of inches long -- but seeing the endangered species excites a lot of biologists in the Midwest. As Iowa's rivers became straighter, faster-flowing and more eroded, the Topeka shiner and many other native fish lost the slack water pools they need for breeding and early development.
Restoring oxbows can give young shiners a new lease on life and help demonstrate that farmers can voluntarily improve habitat and protect water quality -- great news for outdoor lovers and excellent public relations for the farm community. For their part, regulatory agencies are trying to encourage growers to invite the endangered species onto their properties.
"There are no regulations or problem creating this habitat," emphasizes Karen Wilke, of The Nature Conservancy, which has financed two dozen oxbow restorations in Iowa's Boone River Watershed. "The only thing they have to agree to is not to fill that oxbow back in."
Biologist Aleshia Kenney, of the U.S. Fish and Wildlife Service, said wildlife quickly finds and inhabits restored oxbows.
"Adding these off-channel areas back to the landscape provides the much-needed missing habitat that Topeka shiners, among many other species of fish, need to reproduce," she explained. "We have collected up to 23 different species of fish in these restored oxbows that all use that habitat for reproduction. By helping the Topeka shiner, we are also helping many other species of fish, birds, amphibians and reptiles who all depend on that type of habitat."
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