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- Title
Hydrologic Connectivity and Residence Time Affect the Sediment Trapping Efficiency and Dissolved Oxygen Concentrations of the Atchafalaya River Basin.
- Authors
Kroes, Daniel E.; Day, Richard H.; Kaller, Michael D.; Demas, Charles R.; Kelso, William E.; Pasco, Tiffany; Harlan, Raynie; Roberts, Steven
- Abstract
Little is known about water movement, volume, or residence time (RT), and how those characteristics affect sediment trapping efficiency (TE) and dissolved oxygen concentrations (DO) in the United States' largest remaining bottomland hardwood swamp, the Atchafalaya River Basin. To better understand these dynamics, this study used bathymetry, lidar, and stage records to determine volumes in the Basin's hydrologically distinct water management units (WMUs). Discharge measurements determined flow distribution and RT. Residence time was compared with DO to identify conditions that coincided with DO increases or decreases. Suspended sediment concentrations (SSC) were used to determine TE relative to calculated and measured discharge and RT. Discharge through units (85–2,200 m3/s) and RT (0.37–231 d) depended on connectivity and river stage. At high stages, with water temperatures >20°C, DO in the largest WMU declined by −0.21 mg/l/day. DO trends indicated less well‐connected areas of the WMU contributed hypoxic waters as the flood wave lengthened and stages fell. In the two WMUs examined for TE, TE (−266% to 99% and up to 38 Gg/day) correlated with hydrologic connectivity, SSC, RT, water volume, and, in one WMU, discharge losses. Long RT and high TE indicated a high potential to process nutrients. These relationships varied among WMUs. Large volumes of sediment‐laden water moving over the floodplain combined with long RT, high TE, and hypoxia indicate that this ecosystem has continental‐scale importance in reducing nutrient loads to the northern Gulf of Mexico. Reports from other systems suggest similar processes may be operating on other large river floodplains globally. Plain Language Summary: The Atchafalaya Basin is the largest floodplain wetland in the lower United States and the last floodplain interaction of the Mississippi/Atchafalaya River (AR) system before entering the estuaries of the Gulf of Mexico. This study examined the distribution and duration of water on the floodplain of the AR Basin over a range of discharges from normal low water to a normal flood. Large volumes of water moved across the floodplain, carrying and depositing large masses of sediment and nutrients at high flows but exported sediment during low flows. Small inputs of water relative to floodplain volume indicated a high potential to reduce nutrient delivery to the Gulf of Mexico. Oxygen data indicated pockets of poorly connected water were present across the floodplain and contributed to low water oxygen. High water levels on the floodplain could not maintain oxygen levels during the late spring through summer when water temperatures were high. Because of the large amount of flow, sediment, and associated nutrients, the AR Basin is likely important at the continental scale for the reduction of nutrients delivered to the Gulf of Mexico. Reports from other large systems suggest that similar processes are likely occurring on large coastal floodplains around the world. Key Points: Residence times calculated from previous studies of the Atchafalaya River are accurate but do not reflect the conditions on the floodplain of the Atchafalaya River BasinFloodplain interactions in the Atchafalaya River Basin are not well represented by main stem river channel samplingThe Atchafalaya River Basin is likely of continental‐scale importance for nutrient trapping
- Subjects
GULF of Mexico; MISSISSIPPI; WATERSHEDS; FLOODPLAINS; SUSPENDED sediments; WETLANDS; RIVER channels; SEDIMENTS; SUMMER; SWAMPS
- Publication
Water Resources Research, 2022, Vol 58, Issue 11, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2021WR030731