Impact of record flooding of a subtropical river on estuary/ocean exchange

Measurements of current velocity profiles during and after cresting of the Suwannee River in Northern Florida, USA, were used to investigate the effects of increased river discharge on subtidal flows near the estuarine transition with the Gulf of Mexico. Three moored velocity profilers were deployed across a lower estuary cross-section. The cross-section bathymetry consisted of a channel (∼5.5 m deep) near the western bank of the estuary that shoaled monotonically eastward. Two-layer gravitational exchange developed only in the deepest part of the cross-section during the river cresting and persisted for ∼20 days. After this ∼20-day period, the net flow decreased and was seaward throughout the water column. Net flows outside the channel were seaward throughout the observation period and were modulated by the river pulse. By comparing the estuarine response in the 5.5-m channel to theoretical responses driven by a dynamic balance between pressure gradient and stress divergence, a condition required for two-layered flow was proposed. Gravitational exchange flow should be expected when the ratio of density-driven flow to river-induced flow is greater than 0.23 to 0.28. Smaller values of this ratio should produce unidirectional, seaward flows after a river pulse. Two-layered flows restricted to the channel can be explained also with this ratio because of the sensitivity of density-driven flows to local depth and eddy viscosity. These findings need to be tested against observations in other systems affected by extreme freshwater pulses.