Oxbows for Conservation and Nutrient Reduction in Iowa Floodplains

Post submitted by Clay L. Pierce, U.S. Geological Survey and Keith E. Schilling, Iowa Geological Survey

Formed when looping stream meanders are cut off through bank erosion or as the result of artificial stream straightening, oxbows are a common geomorphic feature in floodplains of Iowa’s agricultural landscape. Early accounts of habitat use by many fish species in prairie streams in Iowa and other Midwestern states describe habitats such as slow pools, submerged and emergent vegetation, side channels, and backwaters – habitats that are rare or nonexistent in those streams today. Oxbows are frequently among the few remaining slow or standing water habitats associated with many streams in these regions. Previous studies have established oxbows’ conservation value for Topeka shiners, federally listed as an endangered species, as well as their habitat value for numerous other native fish species. Studies conducted over the last two decades in Iowa consistently report greater prevalence and abundances of Topeka shiners in oxbows than in their associated streams. Ten other fish species of greatest conservation need have also been found in Iowa oxbows.

Widespread nutrient loss from agricultural areas in the U.S. Midwest is impacting aquatic ecosystems at both local and regional scales, including the Gulf of Mexico. Enhancing ecosystem services in floodplains offers opportunities to achieve nutrient reduction benefits in agricultural watersheds while minimizing loss of crop production areas. Preliminary studies in Iowa suggest that oxbows intercepting surface and tile water can reduce the amount of nitrate-nitrogen reaching streams by roughly one half.

Ongoing conservation studies, which will be presented in the oxbow track of the upcoming 2018 Iowa Water Conference, describe efforts to identify oxbow remnants for restoration, landscape and habitat characteristics of oxbows associated with presence of Topeka shiners, restoration programs in Iowa to increase the number and quality of oxbows for Topeka shiners, and efforts to evaluate the success of oxbow restorations. Also to be presented in the oxbow track of the 2018 Iowa Water Conference are three studies addressing water quality benefits of oxbows describing quantification of nutrient reduction benefits at individual oxbow sites and exploring the value and benefits of oxbow restorations within a watershed water quality planning context. A comparison of the value of oxbow restorations within the context of statewide nutrient reduction strategies in Iowa will also be discussed.

Clay Pierce is the Assistant Leader of the U.S. Geological Survey’s Iowa Cooperative Fish and Wildlife Research Unit at Iowa State University in Ames.

Keith Schilling is an Associate State Geologist and Research Engineer for the Iowa Geological Survey at the University of Iowa in Iowa City.