Consequences of variations in flow magnitude and duration on water quality of the Apalachicola River floodplain

Large coastal plain river systems are often characterized by complex networks of secondary channels and sloughs that support wetland forests and estuarine productivity. These sloughs have exceptional ecological importance but have been impacted by hydrologic alterations such as upstream water withdrawals, dam operations, dredging, and channel modifications. These changes reduce hydrologic connectivity between the river and its floodplain, primarily through sediment accumulation and morphological degradation at slough inlets. This study examines the effects of reduced flow and disconnection on water quality in three sloughs connected to a major floodplain river. Water monitoring stations were installed upstream and downstream in each slough to collect water level and water quality data. Streamflow records were developed using regular in-situ discharge measurements. The data was collected from June 2021 to December 2023. The results show that as streamflow in the main river recedes, flow through each slough often recedes to intermittence. This affects water chemistry in the slough, lowering dissolved oxygen (DO) to 0-3 mg/L, effects on electrical conductivity (EC) and pH were less consistent. Principal component analysis identified flow as a key driver of seasonal variation in water quality parameters such as conductivity and temperature. The findings of this research indicate that mitigating stagnation and hypoxia in these sloughs can be achieved by extending the duration for which river water freely flows within the slough channels.