Reconstructed Multi-Purpose Oxbows Improve Water Quality and Provide Ecosystem Services

Reconstructed Multi-Purpose Oxbows Improve Water Quality and Provide Ecosystem Services

by Sophie Pierce

BELOW: Oxbow lake in Kossuth County, Iowa, sampled by the IGS as part of an INRC-funded project focused on the water-quality benefits of reconstructed multi-purpose oxbows. OXBOWS ARE FLOODPLAIN features that form when river meanders are cut off, leaving them partially or fully disconnected from streamflow. These wetlands provide valuable habitat for aquatic life and waterfowl and can improve water quality by taking up sediment and processing nutrients from runoff. Over time, oxbows often fill in with sediment and organic matter, but they can be restored by dredging out the materials and re-vegetating the oxbow area. Recent Iowa Geological Survey (IGS) research indicates that restored oxbows can reduce nitrate-nitrogen (NO3-N) from groundwater seepage, overbank flooding, and tile drainage.

The IGS and partners at Iowa State University, the Iowa Soybean Association, and The Nature Conservancy are measuring the effects of reconstructed oxbows on water quality and ecosystem services in agricultural areas of north-central Iowa. The project, funded by the Iowa Nutrient Research Center, aims to characterize the hydrogeology of four recently reconstructed oxbows that capture a range of input water and nutrient sources, evaluate their N and orthophosphate (OP) reduction capacity, and quantify mass load reduction and retention.

Four sites were selected, two oxbows fed by tiles and two non-tiled, in the heavily farmed Des Moines Lobe landform region of north-central Iowa. The IGS conducted geophysical surveys of the oxbow areas using electromagnetic terrain conductivity (EM31) methods to characterize subsurface conditions. Researchers installed water table monitoring wells and piezometers at each site and instrumented them with pressure transducers to measure continuous water levels. The research team is also monitoring discharge from tiles into the oxbow, as well as flow in and out of the oxbow. Water level measurements, flow data, and subsurface geology information will assist in characterizing groundwater flow directions and rates.

Researchers are collecting water samples from groundwater wells, oxbow and tributary surface water, and tile flow input for analysis by the Iowa Soybean Association for NO3-N, chlorine, sulfate, and OP. Because tile-fed oxbows experience significant fluctuation in NO3-N levels, a Nitratax probe was installed at the two tiled sites to measure continuous nitrate concentration patterns. The team will use this information to develop a mass balance for the oxbows to quantify water and nutrient budgets. Results from this study will be used to develop design criteria to implement oxbow reconstruction to maximize nutrient reductions and other ecosystem services.