Hydrologic Connectivity and Residence Time Affect the Sediment Trapping Efficiency and Dissolved Oxygen Concentrations of the Atchafalaya River Basin

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.

Estuarine Hypoxia—Identifying High Risk Catchments Now and Under Future Climate Scenarios

“…Kroes et al. (2022) also identified the regular, seasonal occurrence of widespread hypoxia on inundated floodplains of the Atchafalaya River Basin, with DO levels recorded during a flood event in 2004 falling to 2.0–2.6 mg/L (lower 95% confidence interval and mean concentrations respectively) 7 days after the initial flood peak and 1.4–2.2 mg/L 4 days later. Similarly, mean DO concentrations below 3 mg/L were recorded within 5 days of the flood peak in 2003, while outlying values reached 3 mg/L within 4 days of the peak.…”