Identifying and Quantifying Nutrient Reduction Benefits of Restored Oxbows
An oxbow is a remnant meander of a stream that has been cutoff from the active channel. Oxbows provide opportunities for water quality improvement and flood storage. Yet over time, deposited material in a degraded oxbow reduces the water depth and limits its potential for flood storage and nutrient cycling. No comprehensive analysis has been conducted that would directly link oxbow restoration activities with the Iowa Nutrient Reduction Strategy.
Several groups have partnered to restore oxbows in the Boone River watershed. This project will work with the Iowa Soybean Association and The Nature Conservancy to quantify the nutrient reduction benefit of oxbow restorations and identify the potential for future restorations in agricultural watersheds.
GIS-based tools will be used to characterize important oxbow metrics that are not reflected in aerial photography, such as depth, age, flood frequency and stream connectivity. This analysis will help identify additional oxbow restoration sites in the Boone River watershed. Water quality monitoring is being conducted at five of the restoration sites. This project will compile data and evidence from these sites, then analyze the data to evaluate and quantify the nutrient reduction benefits from restored oxbows.
FINAL REPORT: This project led to the development of a new geographic information system (GIS) toolkit that can be used to identify potential oxbow restoration sites in watersheds. The toolset identifies depressional features in river corridors, creates metrics describing slope, depth, area, and shape, then uses these metrics to identify and rank potential oxbow remnants. The tools were applied to the Boone River watershed, which has been the focus of oxbow restoration activities. The toolset successfully identified oxbows that already have been targeted for restoration, as well as numerous additional sites with restoration potential, demonstrating its utility as a screening device. Since project completion, colleagues at the Iowa State University Department of Natural Resources Ecology and Management continue development of the toolset. Another key finding of this project was that reconstructing oxbows to receive tile drainage water should be considered a viable practice in agricultural areas. Over a two-year period, water and nitrate concentrations and loads into a reconstructed oxbow were studied. Researchers found the nitrate retention efficiency was similar to other water treatment practices such as bioreactors, wetlands and saturated buffers.
During the last quarter, analysis of the reconstructed oxbow at White Fox Creek was completed, and a manuscript submitted and accepted for publication in Ecological Engineering. Over a two-year period, water and nitrate concentrations and loads into the oxbow were dominated by tile drainage inputs compared to groundwater seepage. Overall, the nitrate retention efficiency of the oxbow was similar to other practices such as bioreactors, wetlands and saturated buffers.
For the reconstructed White Fox Creek oxbow, hydrogeology and nitrate loading patterns have been analyzed and the average and seasonal nitrate retention efficiency quantified. Over a two-year period, water and nitrate concentrations and loads into the oxbow were dominated by tile drainage inputs compared to groundwater seepage. Nitrate concentrations were highest in tile drainage water, similar in upgradient groundwater and in the oxbow itself, and lowest in downgradient groundwater. Nitrate retention efficiency from May to September was estimated to range from 44% to 47% in 2014 and 2015, respectively. On a monthly basis, greater retention efficiencies were measured in late summer and early fall. Overall, the nitrate retention efficiency was found to be similar to other practices such as bioreactors, wetlands and saturated buffers.
Water quality data for the Frye oxbow site in the Boone River Watershed was analyzed. Daily water and nitrate budgets for the oxbow were developed for 2014 and 2015 using inflow loads from two tile inlets, groundwater seepage, precipitation and export from groundwater seepage. A spreadsheet model for the oxbow was developed to estimate seasonal N loss for these two years. Inflow N loads to the oxbow were concentrated in late June and early July 2014, due to large rainfall events. During this time period, the two tiles that discharged into the oxbow provided more than 95% of the incoming N load to the system, with contributions from groundwater nearly insignificant. The N load reduction increased from approximately 30% in late May and early June to nearly 60% in mid-June. The N load reduction decreased following the influx of N load after the precipitation events, and then increased steadily throughout the remainder of 2014. A similar pattern occurred in 2015. Overall, the N budget results show oxbows can effectively remove incoming N loads during the mid-to-late-summer period.
Water quantity and quality data from various oxbow monitoring sites collected by project partners in 2014-2015 was compiled. A visit to one site was done to survey top casing elevations of monitoring wells, conduct tests to estimate hydraulic conductivity of unconsolidated sediments and determine the area and volume of water contained in the oxbow. A water and nutrient budget for the oxbow that accounts for groundwater inflow, tile inflow, precipitation inputs and groundwater outflow for those the 2014 and 2015 seasons is being developed, and from that, estimates of nutrient reduction rates will be made.
This project will work with partners to quantify the nutrient reduction benefit of oxbows and will identify the potential for future oxbow restorations in agricultural watersheds. Water quality monitoring is underway at five restoration sites in the Boone River watershed. Monitoring wells have been installed at two of the sites to characterize groundwater seepage. At another site, the wells and tile drainage flows are being recorded continuously. Water quality samples are collected biweekly at the sites.. Data from the sites will be analyzed to evaluate and quantify the nitrate and phosphorus reductions associated with restored oxbows.