214
Gerardo M.E. Perillo . M. Cintia Piccolo
in the outer reaches of the channels. As the Flandrian transgression retreated in a much
slower pace than the transgression, the coast moved progressively leaving sand bars
and spits. Although there are no definitive paleontological features that can provide
hard evidence, we consider that the climatic conditions during the retreating period
were already as dryas the present ones. Therefore, very little input of sediment was
provided by the rivers coming into the receding bay.
This supposition is derived from a comparative geomorphological analysis of
Anegada Bay. After its formation, this southern portion of the Colorado Delta had very
few fluvial (if any) tributaries coming into it, thus its evolution was dependent only
on the input of sediments from the shelf. However, both Anegada and Bahia Blanca
have exactly the same geomorphological properties. Normally, sediment from the shelf
is not input into Bahia Blanca Estuary (most probably occurs the same with Anegada
Bay although there are no studies dealing with this subject there).
Based on the salinity and temperature distribution alone, the Bahia Blanca Estuary is divided in two. The inner one, from the mouth of the Sauce Chico River to Ingeniero White, is classified as a partially mixed estuary during normal runoff conditions, but with a strong tendency to become sectionally homogeneous during low
runoff. The outer reach is sectionally homogeneous. The boundary between both
reaches is transitional depending on the river discharge. According to the classification of Hansen and Rattray (1966), the estuary may be characterized as Type 1a.
The mixing regime is characterized by Ri < 2 and it is associated to strong turbulent processes during maximum ebb and flood conditions, in particular on the northern flank of the estuary. Therefore, full developed mixing is found almost throughout
the tidal period. Stratification is established at the beginning of ebb cycle being generated by the combined outflow of river and ebbing tide, producing a seaward tilt of
the isohalines. The process is accompanied by low turbulence levels due to the small
currents. As the current velocity increases, turbulent mixing also increases. During
the flooding stage, the possibilities of developing any stratification are very low since
the flood is driving well-mixed water from down estuary. Turbulent mixing at midflood
is high and practically constant resulting in low Ri.
The residual circulation shows a significant difference in the direction of the mass
transport. On the deeper parts of the sections (northern flank) the flow reverses with
depth, being headward near the bottom. The net transport is completely landward on
the shallower parts. This behaviour, added to the washing of the backestuary saline,
produce a concentration of salt in the inner portion of the estuary, resulting in salinities
larger than those observed in the inner continental shelf. Analysis by Piccolo and Perillo
(1990) of the current stations located on the southern flank of the Principal Channel
may indicate that the asymmetry of the tidal current is mainly originated by the presence of the extensive tidal flats bordering the channel.
Acknowledgements
Funds for this study have been provided by grants from Consejo Nacional de
Investigaciones Cientificas y Tecnicas (CONICET) de la Republica Argentina (period 1983-1998), and Universidad Nacional del Sur (period 1994-1998). The authors
thank Lie Walter D. Melo for drawing some of the figures.
Gerardo M.E. Perillo . M. Cintia Piccolo
in the outer reaches of the channels. As the Flandrian transgression retreated in a much
slower pace than the transgression, the coast moved progressively leaving sand bars
and spits. Although there are no definitive paleontological features that can provide
hard evidence, we consider that the climatic conditions during the retreating period
were already as dryas the present ones. Therefore, very little input of sediment was
provided by the rivers coming into the receding bay.
This supposition is derived from a comparative geomorphological analysis of
Anegada Bay. After its formation, this southern portion of the Colorado Delta had very
few fluvial (if any) tributaries coming into it, thus its evolution was dependent only
on the input of sediments from the shelf. However, both Anegada and Bahia Blanca
have exactly the same geomorphological properties. Normally, sediment from the shelf
is not input into Bahia Blanca Estuary (most probably occurs the same with Anegada
Bay although there are no studies dealing with this subject there).
Based on the salinity and temperature distribution alone, the Bahia Blanca Estuary is divided in two. The inner one, from the mouth of the Sauce Chico River to Ingeniero White, is classified as a partially mixed estuary during normal runoff conditions, but with a strong tendency to become sectionally homogeneous during low
runoff. The outer reach is sectionally homogeneous. The boundary between both
reaches is transitional depending on the river discharge. According to the classification of Hansen and Rattray (1966), the estuary may be characterized as Type 1a.
The mixing regime is characterized by Ri < 2 and it is associated to strong turbulent processes during maximum ebb and flood conditions, in particular on the northern flank of the estuary. Therefore, full developed mixing is found almost throughout
the tidal period. Stratification is established at the beginning of ebb cycle being generated by the combined outflow of river and ebbing tide, producing a seaward tilt of
the isohalines. The process is accompanied by low turbulence levels due to the small
currents. As the current velocity increases, turbulent mixing also increases. During
the flooding stage, the possibilities of developing any stratification are very low since
the flood is driving well-mixed water from down estuary. Turbulent mixing at midflood
is high and practically constant resulting in low Ri.
The residual circulation shows a significant difference in the direction of the mass
transport. On the deeper parts of the sections (northern flank) the flow reverses with
depth, being headward near the bottom. The net transport is completely landward on
the shallower parts. This behaviour, added to the washing of the backestuary saline,
produce a concentration of salt in the inner portion of the estuary, resulting in salinities
larger than those observed in the inner continental shelf. Analysis by Piccolo and Perillo
(1990) of the current stations located on the southern flank of the Principal Channel
may indicate that the asymmetry of the tidal current is mainly originated by the presence of the extensive tidal flats bordering the channel.
Acknowledgements
Funds for this study have been provided by grants from Consejo Nacional de
Investigaciones Cientificas y Tecnicas (CONICET) de la Republica Argentina (period 1983-1998), and Universidad Nacional del Sur (period 1994-1998). The authors
thank Lie Walter D. Melo for drawing some of the figures.