Abstract
The Sao Francisco (SF) river (AB = 634.000 km2 , L= 2863 km) and its coastal zone (Lat. 10o 36´ S Long. 36o 23´W, NE-Brazil) have been subject to the impact by a dam cascade constructed between the seventies and the nineties in the middle-lower sector of the basin. This study addresses today’s conditions of the river load and behavior of suspended matter in the coastal zone, after river flow was ultimately regulated by the damps in 1995. The study was conducted from 11/2000 to 03/2002 and comprehended monthly measurements of river discharge (Qr) and suspended particulate matter (SPM) loads at the Propria gauging station downstream of the damps (80 km from the coast), and 11 surface water sampling transects of SPM along the estuarine mixing zone (EMZ) and the turbidity plume on the continental shelf. Information on the pre-dam conditions was obtained from national hydrological data banks. River discharge (Qr) during the annual cycle was estimated at 50,3 km3 /yr (1595 m3 /s), being similar to the annual average of 55,5 ± 7,4 km3 /yr (1760 ± 235 m3 /s) of the post-dam period (1995-2001) and 44% lower than the annual average of 94,9 ± 26,9 km3 /yr (3010 ± 850 m3 /s) of the pre-dam period (1938-1973). The SPM load amounted to 2,28 x 105 t/yr and the yield 0,3 t/km2 /yr, corresponding to a reduction of 94% to pre-dam conditions. The river evolved from a turbid to a transparent system due to material retainance by the dam reservoirs and the basin´s SPM yield is now the lowest in comparison to other Brazilian coastal rivers impacted by damps. The lower mesohaline portion (S > 5 to < 15) of the EMZ is now generally set at the river´s mouth and over the pro-delta shoals, being more turbid (SPM ~ 10 to 40 mg/L) than the river itself (SM ~ 5 mg/L). The impoverishment of riverine SPM in conjunction with the gain of SM (non-conservative behavior) over the pro-delta shoals indicates, that the coastal turbid plume of today is being fed and maintained by resuspended matter from shallow relict muddy sediments, deposited in earlier times, as well as, from adjacent coastal erosion.
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