Before European settlement of Iowa, oxbows (off-channel habitats typically created by stream meander cut-offs) were ubiquitous across the landscape. These habitats provided a multitude of ecosystem services including habitat for fishes, aquatic plants, and aquatic macroinvertebrates, as well as nutrient sequestration and floodwater retention.

As the landscape of Iowa was converted to agriculture, streams across the region were straightened and many fields tiled to expedite the flow of water off the landscape. The straightening of streams eliminated much of the off-channel habitat that was historically present throughout the region. Consequently, the depletion of this habitat has led to declines in species such as the Topeka shiner which relies on off-channel habitat, such as oxbows, for reproduction and refugia.

To reduce the size, severity, and duration of the Gulf of Mexico hypoxic zone, there has been an effort to reduce nutrient runoff (nitrate and phosphorus) from agricultural fields within the Mississippi River basin, especially in Iowa and Illinois.

In Iowa, restoration of oxbows has increased over the last two decades to restore critical habitat for Topeka shiner and more recently to curtail nutrient runoff from agricultural fields. The restoration of these oxbow habitats has been the result of work done by the U.S. Fish and Wildlife Service, the Iowa Department of Natural Resources, The Nature Conservancy, the Iowa Geological Survey, and the Iowa Soybean Association.

Two projects will provide insight into the Topeka shiner/oxbow system relationship and inform management decisions regarding how and where oxbows should be restored.

The “Index of Oxbow Restoration Quality for Topeka Shiners” project addresses post-restoration assessment of oxbows. Currently, the only method to assess the quality of an oxbow restoration for Topeka shiner is to determine whether the species is present within the oxbow. Given the rarity of Topeka shiner, relying on their detection as an indicator of a successful restoration is likely inefficient and inaccurate.

To solve this problem, the project will create a tool that conservation practitioners can use to assess the quality of an oxbow restoration for Topeka shiner, without having to rely on the detection of the species.

Using fish community data from the previously mentioned projects as a model creation dataset, and data collected from 12 recent oxbow restorations as a test dataset, an Oxbow Quality Index (OQI) will be created which will allow for more efficient analysis of the quality of oxbow restorations for Topeka shiner. The findings of this project will inform those who restore oxbows on the quality of the restoration and will aid in shaping future oxbow restorations.

A second project, “Tile Effects on Restored Oxbows” will examine the potential ecological tradeoffs of using oxbows to intercept tile drainage. Oxbows that intercept tile drainage have demonstrated relatively high efficiency of total nitrogen retention at a lower cost compared to bioreactors. Recently, multi-purpose oxbows have been approved by the Science Advisory Team of the Iowa Nutrient Research Center to retain nutrients and provide habitat, which will likely lead to their proliferation across the landscape. It is necessary to determine whether this promising approach for reducing nutrient loss has unintended consequences for oxbow communities, for Topeka shiners, and other species of greatest conservation need.

This project will quantify biotic community structure and associated habitat conditions in 12 recently restored oxbows, six with and six without tile drainage, to test effects of tile drainage on conservation benefits of oxbow restoration. Relative abundances and community composition of fish, macroinvertebrates, and plants will be examined. The findings of this project will inform future decisions regarding the restoration of multi-purpose oxbows.

These projects are occurring concurrently and will include field seasons during the summers of 2019 and 2020. During these field seasons, 12 recently restored oxbow sites (10 within the Boone River basin in north-central Iowa, two within the North Raccoon River basin in west-central Iowa) will be surveyed for fishes, plants, macroinvertebrates, and physical habitat characteristics. Methodology for sampling will be based upon methods established by previous projects conducted by our research group.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new, fish and wildlife research projects that we believe our readers will appreciate reading about. Story by Dylan Osterhaus, Research Assistant-Fisheries Biology at the USGS Iowa Cooperative Fish and Wildlife Research Unit at Iowa State University.