150
Jorge Lopez Laborde . Gustavo J. Nagy
conditions (wind direction and permanence). Mixing evolution diagrams (according
to Dyer and New 1986), in agreement with the previous ones, showed:
a conditions with no mixing and maintenance of the previous stratification patterns,
b increasing mixing conditions allowing the break of the previous stratification patterns, and
c decreasing mixing conditions allowing the development of stratification patterns.
7.3.5
Suspended Sediments
There are numerous publications on suspended sediment distribution and behaviour.
For instance, Ottmann and Urien (1966) divided the Rio de la Plata into three geographical zones, which corresponded to three sedimentological features:
1. an upper zone, to the NW of the Colonia-Buenos Aires transverse section, comprises
the Parana River Delta front and its subaqueous extension (Playa Honda), it is a
typical sedimentation area of a fluvial delta;
2. a middle or intermediate zone, which comprises the area between Colonia-Buenos
Aires transverse section up to Oyarvide, a fluvial zone where transport of suspended
sediments predominates;
3. an outer zone, up to Punta del Este-Cabo San Antonio transverse section, which is
the area of greater salinity variation where actual sedimentation processes take place.
These authors pointed out the increasing suspended sediment concentration to the
SE and its asymmetrical distribution along the Uruguayan and Argentinean middle
Rio de la Plata coasts, showing greater values over the southern than over the northern
region. This fact was related to the flocculation processes, the lag between suspended
sediment and water transport, the opposing river discharge and incoming tide, and
the wave and tidal current resuspension processes. They concluded that a "bouchon
vaseux" (fluid muds) existed in the outer zone. Table 7.5 presents information on suspended sediment concentrations that has been compiled from various sources.
Studies of composition by Ottmann and Urien (1966) revelled organisms, organic
fibers, diatoms, quartz and irregular shaped silt, volcanic ashes, ferruginous clays and
heavy minerals (augite, epidote, hornblende, hypersthene and garnet). Ruhstaller and
Maihle (1968) stated that Rio de la Plata suspended sediments were mainly montmorillonite and illite.
According to Urien (1972) the suspended load delivered to the upper Rio de la Plata
consists of 75% coarse to medium silt, 15% fine to very fine silt and 10% clay, and has
a mean diameter of about 5.8 phi (0.017 mm), with an annual average suspended load
of 72.8 million t yr- 1 for the Parana River complex and of 7.0 million t yr- 1 for the Uruguay River.
Also Urien (1972) stated that the actual sedimentary pattern is primary controlled
by the estuarine environment. In the upper and middle river, fluvial fresh water conditions exist. There, tides and waves from the East control the fluvial discharge and
sediment dispersion. In the outer river the fluvial discharge has influence only along
the coast, aided by the bottom topography. The bulk of fluvial discharge flows out more
Jorge Lopez Laborde . Gustavo J. Nagy
conditions (wind direction and permanence). Mixing evolution diagrams (according
to Dyer and New 1986), in agreement with the previous ones, showed:
a conditions with no mixing and maintenance of the previous stratification patterns,
b increasing mixing conditions allowing the break of the previous stratification patterns, and
c decreasing mixing conditions allowing the development of stratification patterns.
7.3.5
Suspended Sediments
There are numerous publications on suspended sediment distribution and behaviour.
For instance, Ottmann and Urien (1966) divided the Rio de la Plata into three geographical zones, which corresponded to three sedimentological features:
1. an upper zone, to the NW of the Colonia-Buenos Aires transverse section, comprises
the Parana River Delta front and its subaqueous extension (Playa Honda), it is a
typical sedimentation area of a fluvial delta;
2. a middle or intermediate zone, which comprises the area between Colonia-Buenos
Aires transverse section up to Oyarvide, a fluvial zone where transport of suspended
sediments predominates;
3. an outer zone, up to Punta del Este-Cabo San Antonio transverse section, which is
the area of greater salinity variation where actual sedimentation processes take place.
These authors pointed out the increasing suspended sediment concentration to the
SE and its asymmetrical distribution along the Uruguayan and Argentinean middle
Rio de la Plata coasts, showing greater values over the southern than over the northern
region. This fact was related to the flocculation processes, the lag between suspended
sediment and water transport, the opposing river discharge and incoming tide, and
the wave and tidal current resuspension processes. They concluded that a "bouchon
vaseux" (fluid muds) existed in the outer zone. Table 7.5 presents information on suspended sediment concentrations that has been compiled from various sources.
Studies of composition by Ottmann and Urien (1966) revelled organisms, organic
fibers, diatoms, quartz and irregular shaped silt, volcanic ashes, ferruginous clays and
heavy minerals (augite, epidote, hornblende, hypersthene and garnet). Ruhstaller and
Maihle (1968) stated that Rio de la Plata suspended sediments were mainly montmorillonite and illite.
According to Urien (1972) the suspended load delivered to the upper Rio de la Plata
consists of 75% coarse to medium silt, 15% fine to very fine silt and 10% clay, and has
a mean diameter of about 5.8 phi (0.017 mm), with an annual average suspended load
of 72.8 million t yr- 1 for the Parana River complex and of 7.0 million t yr- 1 for the Uruguay River.
Also Urien (1972) stated that the actual sedimentary pattern is primary controlled
by the estuarine environment. In the upper and middle river, fluvial fresh water conditions exist. There, tides and waves from the East control the fluvial discharge and
sediment dispersion. In the outer river the fluvial discharge has influence only along
the coast, aided by the bottom topography. The bulk of fluvial discharge flows out more