Evaluating the Water Quality Benefits of Reconstructed Multi-purpose Oxbows

Issue

Oxbows are natural floodplain features formed when a river cuts off a meander loop as it migrates within its floodplain. Natural oxbows are among the most biologically diverse aquatic systems in the world, but over time, sediment and organic material often accumulate to fill the oxbow. Removing this fill material to restore habitat is considered oxbow reconstruction. To date, restoration of oxbows in agricultural areas has primarily focused on creating habitat for the federally endangered Topeka Shiner and waterfowl. More recently, new research by Schilling et al. (2017, 2018) is showing that restored oxbows can be used for nutrient reduction, with N load reductions ranging from 35 up to 76 percent. Oxbows reconstructed to receive inputs from tile drainage received considerably more nitrate than oxbows fed simply by groundwater discharge or from an occasional flood pulse. Overall, research on newly reconstructed oxbows suggests that these systems can reduce nitrate concentrations and loads delivered via tile drainage, groundwater seepage and overbank flooding. Although results are promising, at this point, monitoring has only been conducted at a few sites.

Researchers propose the term “multi- purpose oxbow” as the descriptor for the practice to indicate that the oxbow restorations be designed to meet water quality and ecosystem habitat goals. Maximum nutrient reduction effectiveness of these restored oxbows requires intercepting tile drainage before it reaches natural waterways, and future oxbows restored under the INRC program will be fed by tiles. However, there are still important questions regarding how the introduction of tile drainage may affect the conservation benefits of restored oxbows.

Objective

Researchers and project collaborators will monitor four reconstructed oxbow sites (two tile-fed, two non-tile) and assess their functions as a new conservation practice for nitrate reduction while also serving habitat goals. The primary objectives are to:

• characterize the hydrogeology of four new reconstructed oxbows that capture the range of input water and nutrient sources, including groundwater seepage, overbank flooding and tile drainage.
• evaluate the nutrient reduction capacity (including both nitrate and orthophophorus) of the four oxbows and quantify their capacity in terms of mass load reduction and retention.
• quantify the relationships of biotic responses to hydrological and nutrient dynamics.
• develop design criteria to implement oxbow reconstruction to maximize nutrient reductions while simultaneously enhancing ecosystem services.

Approach

Funding provided to the Iowa Soybean Association and The Nature Conservancy has been used to reconstruct new oxbow sites in the Des Moines Lobe region of north-central Iowa. This project will focus on hydrology and water quality assessment at four reconstructions: two sites fed by groundwater discharge and flood pulses, and two sites fed primarily by tile drainage. At all oxbow sites, a geophysical survey of the oxbow areas will characterize subsurface conditions. We will install a minimum of four water table monitoring wells at each site to characterize groundwater flow directions and rates, water levels (including in the oxbow and in the nearby river) and water quality in the floodplain. Precipitation will be monitored at all sites. Discharge from tiles into the oxbow will be measured, as will discharge from the oxbow. Dimensions (area, depth, volume) of the oxbows (as-built) will be determined. A depth-volume relationship will be developed to calculate the volume of water in the oxbow for a specified stage. Flood inundation and connection with surface water will be quantified between the stream and oxbow. Combined with turbidity and total suspended solid (TSS) measurements of stream surface water, the potential for oxbow sedimentation from flooding will be assessed and an estimate of oxbow life expectancy developed. Twice-monthly grab samples of groundwater, tile water and surface water will be collected using standard methods at all sites and analyzed at the Iowa Soybean Association certified testing laboratory in Ankeny. A mass balance will be developed for the reconstructed oxbow to quantify input and outputs of water and nutrients as well as concentrations.

Results of this project will be transferred in several ways to different audiences, including communicating directly with private landowners since private land is where most potential oxbow reconstruction sites exist. The Iowa Soybean Association and The Nature Conservancy will use articles in their newsletters. Communication with private landowners will include an extension publication and newsletter articles, and state state and federal level land management agencies ill be informed via annual and final reports, presentations and scientific publications.