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United States Department of Agriculture

Agricultural Research Service

Research Project: UNDERSTANDING AND PREDICTING THE IMPACT OF AGRICULTURE ON THE ENVIRONMENTAL INTEGRITY OF MANAGED WATERSHEDS

Location: Water Quality and Ecology Research

Title: Management of an Abandoned River Channel Wetland for Mitigation of Nonpoint Source Pollution

Authors
item Shields Jr, Fletcher
item Wilson, Casey
item Bryant, Charles
item Testa, Sam

Submitted to: Mississippi Water Resources Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: October 1, 2009
Publication Date: February 1, 2010
Citation: Shields Jr, F.D., Wilson, C.N., Bryant, C.T., and Testa III, S. 2010. Management of an Abandoned River Channel Wetland for Mitigation of Nonpoint Source Pollution. Proceedings, 2009 Mississippi Water Resources Research Conference, Mississippi Water Resources Research Institute, Mississippi State, Mississippi, pp. 103-113.

Interpretive Summary: Remnants of once-extensive natural wetlands occur across the agricultural landscape, and may be managed to yield improved wetland function in terms of trapping and retention of nonpoint source pollutants. Capabilities of such systems must be measured to refine design and management criteria. An existing wetland in a severed meander bend beside the Coldwater River in Tunica County, MS was modified by the construction of weirs equipped with water control structures. The resulting wetland cell was a segment of old river channel about 500 m long and 14 m wide. Inputs to the wetland cell included sporadic flows due to runoff events from about 100 ha of cultivated fields. Volumes of water entering and leaving the wetland cell were estimated for 18 months using measurements made at weirs and at a culvert. Estimates of loads entering and leaving the wetland cell were computed based on concentrations of grab samples collected at the wetland cell inflow and outflow locations. Mean values of turbidity, suspended sediment concentration, and concentrations of filterable and total phosphorus were 25% to 40% lower at the wetland cell discharge weir than in the slough. Mean concentrations of ammonia were 38% lower, but mean nitrate and nitrite concentrations were essentially unchanged by the wetland cell. Comparison of estimated input and output loads indicated that the wetland cell retained about 18% of input suspended sediment, 24% of phosphorus, and 29% of nitrogen input from cultivated fields. Wetland cell sediment and nutrient retention efficiency was greater for drier months, and declined during wetter periods with frequent runoff events.

Technical Abstract: Reduction of nonpoint source pollutants, principally sediment and nutrients moving from cultivated fields to surface waters, is a major challenge. Remnants of once-extensive natural wetlands occur across the agricultural landscape, and some workers have suggested that these areas might be managed to yield improved wetland function in terms of trapping and retention of nonpoint source pollutants. An existing wetland in a severed meander bend cut off in the 1940s from the Coldwater River in Tunica County, MS was modified by the construction of weirs equipped with water control structures. The wetland was a segment of old river channel about 500 m long and 14 m wide. Inputs to the wetland cell included sporadic flows due to runoff events from about 100 ha of cultivated fields and less frequent but larger flood events from the river. This type of flood event occurred only once during the study. Weir drainage structures were operated to retain water during March – November, and were opened to allow flow to and from the Coldwater River during December, January and February. Weir elevation during March – November corresponded to a mean water depth of ~ 0.15 m. Volumes of water entering and leaving the wetland cell were estimated for 18 months using measurements made at weirs and at a culvert. Estimates of loads entering and leaving the wetland cell were computed based on concentrations of grab samples collected at the wetland cell inflow and outflow locations. Water concentrations of sediment and nutrients were generally lower at the downstream end of the wetland cell than in the major inflow, an ephemeral slough. Mean values of turbidity, suspended sediment concentration, and concentrations of filterable and total phosphorus were 25% to 40% lower at the wetland cell discharge weir than in the slough. Mean concentrations of ammonia were 38% lower, but mean nitrate and nitrite concentrations were essentially unchanged by the wetland cell. Comparison of estimated input and output loads during periods when the wetland cell was not flooded by the river indicated that the wetland cell retained about 18% of input suspended sediment, 24% of phosphorus, and 29% of nitrogen input from cultivated fields. Wetland cell sediment and nutrient retention efficiency was greater for drier months, and declined during wetter periods with frequent runoff events.

Last Modified: 4/25/2014
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