THE WORLD DELTA DATABASE

THE SAO FRANSICO DELTA

Delta ID   # 36

CLIMATE TROPICAL SAVANNA (Aw)

DELTA AREA 40 000 KM2

DISCHARGE WATER 28 000 M3/S SEDIMENT 6X106 TONS/YR,

IMAGE ACQUIRED OCTOBER 23, 2001

The Sao Francisco River rises in the coastal mountains of Brazil and roughly one-half of the basin lies within the Brazilian PreCambrian Shield of South America. The southern-most part of the basin drains Cretaceous and Silurian sedimentary rocks. The drainage basin has an area of 602,300 sq km and an average elevation of 563 m. The maximum basin elevation is 1,150 m and the minimum is 310 m. Relief within the basin is quite low, averaging only 63 m. The main channel of the river flows some 2,227 km from its headwaters to the river mouth along the Atlantic Ocean. Drainage density of the tributary system is relatively high, averaging 0.19 km stream length per 500 sq km (Figure 80). The average annual rainfall very high, 998 mm, with a maximum rainfall of 1,671 mm and a minimum rainfall of 422 mm. Rainy months begin in November and lasts through March, with the average monthly rainfall being 140 mm. The dry months are from late April through October and average monthly rainfall rarely exceeds 20 mm; thus there are distinct wet and dry periods annually. Most of the basin is sparsely vegetated and consists primarily of tropical and subtropical grasslands and savannas, while along the eastern margin of the basin, desert conditions exist.

The alluvial valley is extremely narrow (36-i39) and along most of its length, the river flows between narrow steep-sided hills. The average annual river discharge is 2,808 cu m/sec, with a maximum of 4,878 cu m/sec and a minimum of 1,215 cu m/sec. Extreme chemical and biological weathering in the drainage basin results in shedding considerable quantities of fine-grained sediments, and the river transports an extremely high-suspended sediment load. The river debouches into the Atlantic Ocean via a single distributary and has formed a triangular delta with an area of 1,742 sq km. The subaerial delta is about twice as large as the subaqueous; strong longshore drift carries the sediment in a southeastward direction. Only a small part of the delta is active and the ratio of the abandoned to active delta is 2.30. The average tidal range is 1.86 m, but spring tides can attain a range of up to 2.5 m. The generally high relief in the delta, however, precludes tidal inundation and only a small percentage of the delta is subjected to tidal inundation. Offshore slope is 0.19 degrees. As a result, swells generated in the southern Atlantic Ocean are not appreciably attenuated, and the delta shoreline is pounded by high wave energy. The average annual wave power along the shoreline is 30.420 x 10⁷ ergs/sec/per m of coast and the average root mean square wave height is 1.49 m. Stated in other terms, more wave energy is expended in 10 hours along the coast in the Sao Francisco than in 365 days in the Mississippi River.

Figure 81 illustrates the average monthly wave power at several points along the coast. This high wave energy produces a relatively smooth delta shoreline with only a minor protrusion at the river mouth (36-i39).

The delta plain is composed almost entirely of closely spaced beach ridges with narrow swales and transgressive eolian dunes (36-g01). The delta shoreline is composed of large, broad sandy beaches. The sand is clean, well-sorted, and highly quartzose, approaching 95 % quartz. Eolian dunes have transgressed several kilometers across the beach ridges, and some dunes, especially to the north of the active river mouth, attain elevations in excess of 22 m.