Simple Model of Lower Mississippi River Backwater Ecological Function

Floodplain waterbodies (“backwaters”) comprise key components of large alluvial river ecosystems when they are intermittently connected to the main channel. Due to the large floodplain area between the mainline levees, backwaters along the Lower Mississippi River (LMR) are a significant ecological resource and potentially have an impact on total downstream flux of nutrients and biomass. To assess these impacts, we developed a simple model of a hypothetical LMR reach for interactions between the river and floodplain backwaters under a range of hydrologic conditions. The spatial domain of the model is a single hypothetical backwater (BWh) and an associated main channel reach. Nutrient and chlorophyll-a concentrations are based on water quality data obtained from a 28-km reach of the LMR over the last 15 years. Water movement is simulated using relationships developed using upstream gage records, site water level data, and simulations conducted using the Adaptive Hydraulics Modeling System (AdH). Using the backwater model, impacts of the hypothetical backwater on downstream flux of nitrate, phosphate and chlorophyll-a may be simulated for a range of backwater geometries and over any temporal domain within the period of record for the Helena, Arkansas gage (1871-2022). Simulations of backwater performance using hydrologic records for extremely wet, extremely dry and normal years indicated that 1.0% to 13% of the annual main channel flow passed through the backwater. Production of algal biomass from the single hypothetical backwater ranged from 2.9 to 9.1 kg chlorophyll-a/ha/yr, which comprised about 0.30% to 0.58% of the total LMR flux. The hypothetical backwater retained 52 to 912 kg NO3-N/ha/yr, or about 0.02% to 0.27% of the total LMR flux. Retention of PO4-P ranged from about 16 to 44 kg/ha/yr or about 0.12% to 0.33% of the total PO4-P flux. Backwater processes were sensitive to the geometry of the backwater connection to the main channel. The amount of water diverted through the backwater and the chlorophyll-a contribution to the main channel was inversely related to the river stage required for main channel connection to the backwater, while NO3-N and PO4-P retention was related to connectivity in a complex, nonlinear fashion.