CHAPTER 9 . Geomorphological and Physical Characteristics of the Bahia Blanca Estuary
201
From Puerto Galvan headward, the channel has not been ever dredged and maintains the original shape. The channel narrows considerably and keeps an average width
of about 500 m almost all the way upstream (Gomez et al. 1997). Also it becomes more
V-shaped with the asymmetry following the meandering pattern that is more pronounced in the inner reach. This meandering pattern, together with a steady reduction of depth and width, follows up to its disappearance into the Salitral de la Vidriera.
The latter is a backestuary salt flat that becomes partly inundated during very strong
storm surges occurring simultaneously with spring high tides.
There are only two freshwater tributaries into the estuary both entering from the
northern shore. The most important is the Sauce Chico River that connects with the
Principal Channel some 3 km downstream from the head. The Naposta Grande Creek
reaches the estuary at about 1 km downstream of Ingeniero White. There are series of
small tributaries in between that may input minor quantities of runoff only during
local rainfalls. The rest of the time they behave as tidal channels.
The most important tributaries (none of them carry freshwater) occur on the southern margin of the Principal Channel. They range in size from small grooves to major
tributaries that can be as large as the Principal Channel itself at their mouth. The
geomorphology and time evolution of the largest ones (Cabeza de Buey, La Lista and
EI Embudo) were described by Ginsberg (1993) and Ginsberg and Perillo (1997).
All these channels have a funnel shape at their mouth becoming strongly meandering headward. The level of meandering has prompted Perillo et al. (1996) to analyse two of them from both fractal and spatial spectral procedures to define the main
modes of the meander wavelengths. In all cases, the mouth of the channels at their
confluence with the Principal Channel are turned seaward demonstrating the strong
ebb dominance that occurs in it. This is also very well seen because of the disposition
of the shoals that forms only along the borders of the channels.
A very interesting feature in these channels and that has only been described before by Kjerfve et al. (1979), is the presence of deep scour holes at the confluence of
channels. Normally the large channels have maximum depths on the order of 10 m;
however, at the holes they reach up to 25 m (Cabeza de Buey hole) (Fig. 9.4). The holes
resemble those formed at the confluence of rivers. That is they have elliptical plan view,
with the long axis approximately parallel to the main stream, and asymmetrical in
cross section. However, deep scour holes on tidal channels differ because they have
the steeper side (in the long axis direction) pointing into the tributaries and the gentler away from them; whereas in rivers the sides are reversed. Ginsberg and Perillo
(1997) have proposed that even though both types may form first on the imaginary
continuation of the tributary channel on the main one, the difference arises because
of the effect of the reversing condition of the tidal currents and the headward erosion
that occurs at the head going into the tributary.
All tidal channels form a very complex drainage system that in many cases have
conflicting channel numbers since the trunk channel may change over the course of
a tidal period. The situation arise because several channels go all across the tidal flats
from one major channel to the other. The best example is the channel known as
Laborde Creek. It connect the Principal Channel with the Bermejo Channel. As the
tidal wave moves from south to north in the inner shelf, the higher levels occur first
in the Bermejo Channel, then currents in Laborde Creek flow northward. But as the
tide rises in the Principal Channel, because of the much larger tidal prism, the cur-
201
From Puerto Galvan headward, the channel has not been ever dredged and maintains the original shape. The channel narrows considerably and keeps an average width
of about 500 m almost all the way upstream (Gomez et al. 1997). Also it becomes more
V-shaped with the asymmetry following the meandering pattern that is more pronounced in the inner reach. This meandering pattern, together with a steady reduction of depth and width, follows up to its disappearance into the Salitral de la Vidriera.
The latter is a backestuary salt flat that becomes partly inundated during very strong
storm surges occurring simultaneously with spring high tides.
There are only two freshwater tributaries into the estuary both entering from the
northern shore. The most important is the Sauce Chico River that connects with the
Principal Channel some 3 km downstream from the head. The Naposta Grande Creek
reaches the estuary at about 1 km downstream of Ingeniero White. There are series of
small tributaries in between that may input minor quantities of runoff only during
local rainfalls. The rest of the time they behave as tidal channels.
The most important tributaries (none of them carry freshwater) occur on the southern margin of the Principal Channel. They range in size from small grooves to major
tributaries that can be as large as the Principal Channel itself at their mouth. The
geomorphology and time evolution of the largest ones (Cabeza de Buey, La Lista and
EI Embudo) were described by Ginsberg (1993) and Ginsberg and Perillo (1997).
All these channels have a funnel shape at their mouth becoming strongly meandering headward. The level of meandering has prompted Perillo et al. (1996) to analyse two of them from both fractal and spatial spectral procedures to define the main
modes of the meander wavelengths. In all cases, the mouth of the channels at their
confluence with the Principal Channel are turned seaward demonstrating the strong
ebb dominance that occurs in it. This is also very well seen because of the disposition
of the shoals that forms only along the borders of the channels.
A very interesting feature in these channels and that has only been described before by Kjerfve et al. (1979), is the presence of deep scour holes at the confluence of
channels. Normally the large channels have maximum depths on the order of 10 m;
however, at the holes they reach up to 25 m (Cabeza de Buey hole) (Fig. 9.4). The holes
resemble those formed at the confluence of rivers. That is they have elliptical plan view,
with the long axis approximately parallel to the main stream, and asymmetrical in
cross section. However, deep scour holes on tidal channels differ because they have
the steeper side (in the long axis direction) pointing into the tributaries and the gentler away from them; whereas in rivers the sides are reversed. Ginsberg and Perillo
(1997) have proposed that even though both types may form first on the imaginary
continuation of the tributary channel on the main one, the difference arises because
of the effect of the reversing condition of the tidal currents and the headward erosion
that occurs at the head going into the tributary.
All tidal channels form a very complex drainage system that in many cases have
conflicting channel numbers since the trunk channel may change over the course of
a tidal period. The situation arise because several channels go all across the tidal flats
from one major channel to the other. The best example is the channel known as
Laborde Creek. It connect the Principal Channel with the Bermejo Channel. As the
tidal wave moves from south to north in the inner shelf, the higher levels occur first
in the Bermejo Channel, then currents in Laborde Creek flow northward. But as the
tide rises in the Principal Channel, because of the much larger tidal prism, the cur-