CHAPTER 8 . Physical Characteristics and Processes of the Rio de la Plata Estuary
171
flow into two branches. One branch flows along the northern coast up to Punta del
Este and the other turns to the south, into Samboromb6n Bay and reaches Cabo San
Antonio. They presented a distribution of surface and bottom salinity showing a clear
tendency for southward flow. Urien (1967,1972), showed vertical salinity sections for
February, March and October along the northern coast and for March and May in the
Samborombon Bay. Based on these sections, he suggested that the saline water movement is greater on the northern coast through the deep northern channels. In the
Samborombon Bay, he said, the saline water movement is quite restricted, due to the
shallowness of the area. Brandhorst and Castello (1971) and Brandhorst et al. (1971)
presented seasonal maps of temperature, salinity and other chemical properties on
the continental shelf. From the analysis of the distribution of properties on the shelf
adjacent to the estuary, they concluded that the influence of the Rio de la Plata is more
important to the north, and proposed the southern discharge as a "periodic" event.
But they did not explain the characteristics and the frequency of observation of these
southern extensions. The northward influence of the river plume was inferred by
Hubold (1980), Carreto et al.(1986) and Lusquifios and Figueroa (1982) working with
in situ data from hydrographic stations on the shelf. Nagy et al. (1987) also proposed
a northward flow based on observations at coastal stations. A southward discharge
pattern was reported by Brandhorst et al. (1971) and Carreto et al. (1982). The mean
surface distribution presented by CARP (1989) shows higher salinity values along the
northern coast, although they noted very low surface salinity in this area during periods of high river flow.
Based on this brief review, it is evident that previous studies provide information
to support the occurrence of both possible paths proposed initially by Ottman and
Urien (1965). The northward flow was the case more frequently observed and the hypothesis most likely accepted. It corresponds to the general picture for the dispersion
of a buoyant plume injected on an eastern coast in the Southern Hemisphere. The low
salinities along the southern coast have been explained in terms of a more complex
tidal regime and mixing processes associated with the shallowness of the Samborombon Bay (Urien 1972; CARP 1989). Both situations have been observed and documented. Nevertheless, no clear explanation of the mechanism that generates the bimodal discharge pattern was proposed, and the frequency of occurrence was not established.
Many rivers around the world exhibit a seasonal pattern for the river plume as the
result of seasonal variability of the forcing. The Columbia River plume off Oregon is
a well known example. During winter the river plume is a low temperature, low salinity feature against the coastline to the north. During summer, however, the plume extends over an area of about 105 km2 and is deflected southward by prevailing summer
winds and coastal currents (Barnes et al. 1972; Conomos et al. 1972; Bowman 1988;
Landry et al. 1989). Another example is the Mississippi-Atchafalaya River in the TexasLouisiana continental shelf. Essentially, there is downcoast (westward to southward)
nearshore flow, except during the summer months. In July and August the average wind
has an upcoast component and the nearshore flow is reversed (Cochrane and Kelly
1986; Li et al. 1997). During these months the influence of the Mississippi-Atchafalaya
River freshwater discharge is observed in the salinity field to the east. In the case of
the Rio de la Plata, the shallowness of the estuary and the adjacent shelf might increase
the role of the wind in the force balance.
171
flow into two branches. One branch flows along the northern coast up to Punta del
Este and the other turns to the south, into Samboromb6n Bay and reaches Cabo San
Antonio. They presented a distribution of surface and bottom salinity showing a clear
tendency for southward flow. Urien (1967,1972), showed vertical salinity sections for
February, March and October along the northern coast and for March and May in the
Samborombon Bay. Based on these sections, he suggested that the saline water movement is greater on the northern coast through the deep northern channels. In the
Samborombon Bay, he said, the saline water movement is quite restricted, due to the
shallowness of the area. Brandhorst and Castello (1971) and Brandhorst et al. (1971)
presented seasonal maps of temperature, salinity and other chemical properties on
the continental shelf. From the analysis of the distribution of properties on the shelf
adjacent to the estuary, they concluded that the influence of the Rio de la Plata is more
important to the north, and proposed the southern discharge as a "periodic" event.
But they did not explain the characteristics and the frequency of observation of these
southern extensions. The northward influence of the river plume was inferred by
Hubold (1980), Carreto et al.(1986) and Lusquifios and Figueroa (1982) working with
in situ data from hydrographic stations on the shelf. Nagy et al. (1987) also proposed
a northward flow based on observations at coastal stations. A southward discharge
pattern was reported by Brandhorst et al. (1971) and Carreto et al. (1982). The mean
surface distribution presented by CARP (1989) shows higher salinity values along the
northern coast, although they noted very low surface salinity in this area during periods of high river flow.
Based on this brief review, it is evident that previous studies provide information
to support the occurrence of both possible paths proposed initially by Ottman and
Urien (1965). The northward flow was the case more frequently observed and the hypothesis most likely accepted. It corresponds to the general picture for the dispersion
of a buoyant plume injected on an eastern coast in the Southern Hemisphere. The low
salinities along the southern coast have been explained in terms of a more complex
tidal regime and mixing processes associated with the shallowness of the Samborombon Bay (Urien 1972; CARP 1989). Both situations have been observed and documented. Nevertheless, no clear explanation of the mechanism that generates the bimodal discharge pattern was proposed, and the frequency of occurrence was not established.
Many rivers around the world exhibit a seasonal pattern for the river plume as the
result of seasonal variability of the forcing. The Columbia River plume off Oregon is
a well known example. During winter the river plume is a low temperature, low salinity feature against the coastline to the north. During summer, however, the plume extends over an area of about 105 km2 and is deflected southward by prevailing summer
winds and coastal currents (Barnes et al. 1972; Conomos et al. 1972; Bowman 1988;
Landry et al. 1989). Another example is the Mississippi-Atchafalaya River in the TexasLouisiana continental shelf. Essentially, there is downcoast (westward to southward)
nearshore flow, except during the summer months. In July and August the average wind
has an upcoast component and the nearshore flow is reversed (Cochrane and Kelly
1986; Li et al. 1997). During these months the influence of the Mississippi-Atchafalaya
River freshwater discharge is observed in the salinity field to the east. In the case of
the Rio de la Plata, the shallowness of the estuary and the adjacent shelf might increase
the role of the wind in the force balance.