Multi-purpose Oxbows Make the Grade for Nutrient Reduction

Illustration shows four stages of oxbow formation
Illustration shows stages of oxbow formation. Image courtesy of the Iowa Soybean Association. 

New research shows that “multi-purpose oxbows” can effectively reduce nitrate-nitrogen, earning them a spot in the Iowa Nutrient Research Strategy’s menu of conservation options.

Oxbows are floodplain wetlands that form when a stream or river cuts a straighter path through a loop of its natural meander as it migrates within its floodplain or when a stream is channelized (see images). They often flood, providing habitat for species that need shallow water habitats.

Oxbows tend to disappear as they gradually fill with sediment and organic material from surrounding land. To restore them requires removing fill material that has accumulated over time, reviving their unique habitat for species such as the endangered Topeka shiner, the orange-spotted sunfish, amphibians like leopard and chorus frogs, and waterfowl.

Multi-purpose functions

“We’ve learned that oxbows provide more than habitat, important as that is,” said Keith Schilling, State Geologist and Director of the Iowa Geological Survey, lead researcher on several studies documenting restored oxbows’ potential to reduce nitrate by 35 to 54 percent, levels comparable to other edge-of-field conservation practices recommended for nitrate reduction.   

Schilling’s research partners include The Nature Conservancy, the Iowa Soybean Association, U.S. Geological Survey’s Iowa Cooperative Fish and Wildlife Research Unit at Iowa State and the U.S. Fish and Wildlife Service. Since 2011, the groups have restored more than 100 oxbows in Iowa, primarily in the North Raccoon River and Boone River watersheds, with a focus on habitat for endangered species. Now they are working together to better understand oxbows’ benefits for water quality, with support from USDA’s Natural Resources Conservation Service and the Iowa Nutrient Research Center.

“To gain this added benefit for what we’re calling ‘multi-purpose’ oxbows, the research shows it’s important to capture more nitrate,” said Schilling.

His recent studies showed that where an oxbow only received water from overbank flooding and groundwater seepage that happened naturally, nitrate loading into the oxbow was relatively low, as was the resulting nitrate reduction. However, when agricultural drainage tiles were routed into the oxbows, their power to reduce nitrate increased significantly.

“Routing drainage tiles into wetlands isn’t possible everywhere,” said Schilling, “but there are many places in Iowa where we have restorable oxbows with drainage tiles in close proximity.” 

Impacts on fish

There have been many oxbow restorations already in Iowa, so the idea isn’t brand new, according to Clay Pierce, assistant leader for the US Geological Survey’s Cooperative Fish and Wildlife Unit and adjunct professor of natural resource ecology and management at Iowa State. He has been helping document the status of Topeka shiners in Iowa, which has been slowly improving as oxbows are being restored. Now Pierce is working with two ISU graduate students to assess how piping farm drainage tiles into oxbows might impact the quality and type of their fish habitat.

“My hypothesis is that it won’t make much difference for the species we’re most concerned with,” said Pierce. “They are used to conditions we usually think of as inhospitable. But there will be added nutrients, and the tile drainage will also provide a more stable water supply. If water levels in these normally intermittent wetlands increase to the point that they become stable habitat for game fish, it is conceivable they won’t be the best place for tasty small minnows like the Topeka shiner.”

Another option for landowners  

Adding “multi-purpose oxbows” to the Iowa Nutrient Reduction Strategy's menu of conservation options took time. The INRS science team reviewed the emerging research by Schilling and others for two years before concluding that multi-purpose oxbows -- those designed to both provide habitat and reduce nitrate loads delivered by tile drainage – are an effective practice for agricultural water quality improvement. Blind inlets and saturated riparian buffers are the only other practices that have been added to the strategy since it was adopted in 2012. 

Karen Wilke, Iowa freshwater specialist and Boone River project director for The Nature Conservancy, is excited about this new tool for landowners. She has been facilitating oxbow restoration effort in Iowa since the beginning, working as “boots on the ground” to help coordinate contacts with landowners, line up permits when needed, and oversee contracts and restorations.

“One thing we have learned,” she said, “is that once an oxbow is restored, farmers often really enjoy having this feature on their land.”

Corey McKinney, an Iowa Soybean Association natural resource specialist involved in the oxbow restorations, has had many positive responses from farmers.

“ISA’s role in this partnership has been to support adding more conservation practices into working landscapes in ways that don’t hurt production," he said. "Oxbows fall into this perfectly, as they are in wet, usually rough areas that are a hassle to farm and aren’t very productive. The oxbow restorations also can have other benefits, like providing some flood storage, especially when there are more of them in a watershed.”

“If we can just set these spots aside at little or no cost to farmers, it is an ideal way to ‘stack’ conservation practices for multiple benefits,” said McKinney.

Costs to restore an oxbow vary widely and compare to installing a bioreactor or other edge-of-field nutrient reduction practices. Funding for restorations so far has come from the Sand County Foundation, Syngenta, and the Fishers and Farmers Partnership for the Boone Watershed, with a focus on habitat improvement. Cost-share through the USDA’s Environmental Quality Incentives Program can often be used to help cover expenses, though Wilke says the practice criteria need to be updated for multi-purpose oxbow restorations. Wilke and McKinney are working with conservation program managers to make these changes. 

“Making oxbow restoration an approved practice in the Iowa Nutrient Reduction Strategy is going to open doors and bring new options for landowners through federal and state water quality programs,” said McKinney. “It also increases awareness about oxbows among conservation program staff who can help promote them for the many benefits they provide.”

The Iowa Nutrient Research Center was established by the Iowa Board of Regents in response to legislation passed by the Iowa Legislature in 2013. The center pursues science-based approaches to areas that include evaluating the performance of current and emerging nutrient management practices and providing recommendations on implementing the practices and developing new practices. Iowa State University leads the partnership that includes the University of Iowa and the University of Northern Iowa. Since its inception, the center has supported more than 92 projects.

Restored oxbow in Iowa. Photo by Dylan Osterhaus.