CHAPTER 6 . The Argentina Estuaries: A Review
105
22000 m3 s-" governs the movements and mixture of river and marine waters being
also controlled by wind and tidal action. Urien (1972), Gagliardini et al. (1984) and
Boschi (1988) divided the Rio de la Plata into three different zones:
i. the upper region (Playa Honda), from the head of the estuary to an imaginary line
between Colonia (Uruguay) to La Plata (Argentina) cities (the narrowest cross-section, 40 km), with fluvial characteristics and affected by tides;
ii. an intermediate zone that extends up to the Montevideo-Punta Piedras line dominated by several shallow banks and estuarine fronts;
iii. an the wide outer region with typical marine features.
Sediment supply from the tributary rivers consist mainly of silt and clay. In the
upper estuary sand is scarce and localised in banks, bars and beaches on the northern coast as it is provided mostly by the Uruguay River. This coast is rocky and sandy
with many pocket beaches In the outer estuary a sand carpet extends from the inner
continental shelf into the estuary. These are relict sands of the last Holocene transgression which invaded the estuary between 7000 and 3000 years B.P. The southern
coast is nearly flat, with its maximum development in Samborombon Bay. The regional
estuarine environment is mostly fluvial, but the mixing of ocean waters creates a
gradual change from fluvial in the upper river to fluvio-marine and marine outer river
(Urien 1972).
At the head of the Rio de la Plata Estuary there is a very active delta. The geomorphological extension of the delta of the Parana River exceeds its long-accepted limits as
shown by sedimentological, morphological and stratigraphic studies (Parker and
Marcolini 1992). The recognition of a subaqueous plain, a delta front and a prodelta,
suggests that the delta extends to the outer zone of the Rio de la Plata Estuary. Estimated growing rates for the delta is between 40 and 70 m yr- 1 •
Tides are tlIe physical processes most investigated in tlIe estuary. The dimensions of
tlIe Rio de la Plata are such tlIat its natural period of oscillation is nearly that of tlIe semidiurnal tide. The principal tide affecting the estuary is the semidiurnallunar (M 2 ) witlI
a period of 12.42 h. Due to Coriolis, tlIe incoming tide is deflected towards the soutlIern
shore of the estuary (Balay 1961), yielding greater tidal amplitudes than along tlIe nortlIern shore. Maximum range is 1 m on the Argentina coast (Balay 1956; O'Connor 1991).
Much larger variations in sea level are induced by meteorological forcing. Winds
that flow along the estuary produce the largest storm surges. Specially with winds from
the S-SE during high tide, associated with stationary cyclones over the Rio de la Plata
can originate storm surges greater than 4 m with disastrous flooding on the coasts
(Balay 1958; 1961). Several numerical models of tidal prediction in the Rio de la Plata
have been developed (Mazio 1990, 1991; O'Connor 1991) and Gagliardini et al. (1984)
have studied the interaction between tides and the river flow using remote sensing
images. Figure 6.3 shows the tidal amplitude and currents at two hours for high and
low tide (O'Connor 1991).These results agree with those of Balay (1956; 1961) and
Lanfredi et al. (1979). They also show that in SamborombOn Bay there is a counterclockwise system of residual currents which flow seaward following the coastline together
with fluvial drainage.
Surface salinity patterns are controlled by the wind field and, to a lesser extent by
river discharge (Fig. 6.4). Both forces act upon the upper layer. Diluted shelf waters
105
22000 m3 s-" governs the movements and mixture of river and marine waters being
also controlled by wind and tidal action. Urien (1972), Gagliardini et al. (1984) and
Boschi (1988) divided the Rio de la Plata into three different zones:
i. the upper region (Playa Honda), from the head of the estuary to an imaginary line
between Colonia (Uruguay) to La Plata (Argentina) cities (the narrowest cross-section, 40 km), with fluvial characteristics and affected by tides;
ii. an intermediate zone that extends up to the Montevideo-Punta Piedras line dominated by several shallow banks and estuarine fronts;
iii. an the wide outer region with typical marine features.
Sediment supply from the tributary rivers consist mainly of silt and clay. In the
upper estuary sand is scarce and localised in banks, bars and beaches on the northern coast as it is provided mostly by the Uruguay River. This coast is rocky and sandy
with many pocket beaches In the outer estuary a sand carpet extends from the inner
continental shelf into the estuary. These are relict sands of the last Holocene transgression which invaded the estuary between 7000 and 3000 years B.P. The southern
coast is nearly flat, with its maximum development in Samborombon Bay. The regional
estuarine environment is mostly fluvial, but the mixing of ocean waters creates a
gradual change from fluvial in the upper river to fluvio-marine and marine outer river
(Urien 1972).
At the head of the Rio de la Plata Estuary there is a very active delta. The geomorphological extension of the delta of the Parana River exceeds its long-accepted limits as
shown by sedimentological, morphological and stratigraphic studies (Parker and
Marcolini 1992). The recognition of a subaqueous plain, a delta front and a prodelta,
suggests that the delta extends to the outer zone of the Rio de la Plata Estuary. Estimated growing rates for the delta is between 40 and 70 m yr- 1 •
Tides are tlIe physical processes most investigated in tlIe estuary. The dimensions of
tlIe Rio de la Plata are such tlIat its natural period of oscillation is nearly that of tlIe semidiurnal tide. The principal tide affecting the estuary is the semidiurnallunar (M 2 ) witlI
a period of 12.42 h. Due to Coriolis, tlIe incoming tide is deflected towards the soutlIern
shore of the estuary (Balay 1961), yielding greater tidal amplitudes than along tlIe nortlIern shore. Maximum range is 1 m on the Argentina coast (Balay 1956; O'Connor 1991).
Much larger variations in sea level are induced by meteorological forcing. Winds
that flow along the estuary produce the largest storm surges. Specially with winds from
the S-SE during high tide, associated with stationary cyclones over the Rio de la Plata
can originate storm surges greater than 4 m with disastrous flooding on the coasts
(Balay 1958; 1961). Several numerical models of tidal prediction in the Rio de la Plata
have been developed (Mazio 1990, 1991; O'Connor 1991) and Gagliardini et al. (1984)
have studied the interaction between tides and the river flow using remote sensing
images. Figure 6.3 shows the tidal amplitude and currents at two hours for high and
low tide (O'Connor 1991).These results agree with those of Balay (1956; 1961) and
Lanfredi et al. (1979). They also show that in SamborombOn Bay there is a counterclockwise system of residual currents which flow seaward following the coastline together
with fluvial drainage.
Surface salinity patterns are controlled by the wind field and, to a lesser extent by
river discharge (Fig. 6.4). Both forces act upon the upper layer. Diluted shelf waters