The Impact of an Extreme Flood upon the Mixing Zone of the Todos os Santos Bay, Northeastern Brazil

River dams, by impacting the discharge pattern of a drainage basin, may cause problems to the lower river course and associated estuarine areas, especially in relation to channel morphology and material exchange with the coastal environment. With the aim of investigating the effect a natural flood upon the hydrodynamics and mixing condition of an estuary (Paraguaçu River) inside Baía de Todos os Santos, a simulated 8-day flood event was released by Pedra do Cavalo Dam, located 20 km up the river, on November 2001. Peak discharge reached 1534 m³s⁻¹ (15 times the predam average discharge) in the very first day of the simulation, and was reduced daily in a step-like fashion in the following 7 days. The flood had a major impact upon the water level of the closest monitoring station, 5 km downstream from the dam, but was not noticeable in the farthest station another 32 km downstream. Current measurements, performed at a fixed station 32 km downstream from the dam, registered velocities up to 1.8 m.s⁻¹ during the ebbing time, and an apparent inversion of the residual current vector between spring (ebb directed) and neap (flood direct) tides. The salt intrusion zone travelled 14 km up river during the spring-tidal cycle prior to the flood. With the onset of the event it was pushed 16 km downstream and was maintained out of the lower river course as the water column changed from well to partially mixed until the discharge dropped below 118 m³s⁻¹.