The Bahia Blanca Estuary, Argentina
207
ward orientation of the ebb channel and associated shoals in the Principal
Channel are still preserved. Despite strong marine dynamics, which constantly modify the tidal delta channels and shoals, the position and shape
of the delta has been conserved, due to its stable connection to the northern shore and its position on top of a sill (Chasico Formation), which has
reduced sediment transport and served as an anchor (Cuadrado and Perillo 1997). The mobility of delta shoals depends on the approach angle of
tidal currents (Perillo and Cuadrado 1991), while 3D dunes (sand waves) in
the Principal Channel and tributary channels are formed due to high current velocity and geomorphologic "traps" which favor sedimentation
(Aliotta and Perillo 1987; Gomez et al. 1997). Except for bed forms on
flood-dominated channels, all 3D dunes and shoals in the Bahia Blanca
Estuary have ebb dominance. Since sediment runoff from rivers is virtually absent and ebb delta characteristics impede sediment input from the
shelf, the high concentration of suspended sediments in the estuary is due
to erosion of tidal flats and island shores (Ginsberg and Perillo 1990). The
southern coast of the Principal Channel across Ingeniero White Port has
retreated up to 50 m between 1980 and 1986 and 1.5 million m 3 were exported from an 8-km stretch at the mid-reach of the channel (Perillo and
Sequeira 1989). Because most sediments in the estuary are silt and clay,
strong currents and short slack water intervals impede their deposition in
channels and on the tidal flats, and waves erode old sediments and prevent
any accumulation of new ones. These conditions explain the erosional
stage of most of the estuary and the prevalence of sediments from the deltaic deposition period.
14.3 Physical Processes
14.3.1 Freshwater Input
Two freshwater tributaries enter the estuary from the northern shore. The
Sauce Chico River (drainage area of 1,600km 2 ) discharges into the Principal Channel about 3 km downstream from the head of the estuary and the
Naposta Grande Creek (drainage area of 920km 2 ) reaches the estuary
about 1 km downstream of Ingeniero White Port (Fig. 14.1). Both tributaries behave similarly in spring and summer during maximum mean rainfall but they are out of phase in autumn when the Sauce Chico River has a
secondary mode. Although mean annual runoff for the Sauce Chico River
and the Naposta Grande Creek are low (1.9 and 0.8 m 3 s- 1 , respectively),
207
ward orientation of the ebb channel and associated shoals in the Principal
Channel are still preserved. Despite strong marine dynamics, which constantly modify the tidal delta channels and shoals, the position and shape
of the delta has been conserved, due to its stable connection to the northern shore and its position on top of a sill (Chasico Formation), which has
reduced sediment transport and served as an anchor (Cuadrado and Perillo 1997). The mobility of delta shoals depends on the approach angle of
tidal currents (Perillo and Cuadrado 1991), while 3D dunes (sand waves) in
the Principal Channel and tributary channels are formed due to high current velocity and geomorphologic "traps" which favor sedimentation
(Aliotta and Perillo 1987; Gomez et al. 1997). Except for bed forms on
flood-dominated channels, all 3D dunes and shoals in the Bahia Blanca
Estuary have ebb dominance. Since sediment runoff from rivers is virtually absent and ebb delta characteristics impede sediment input from the
shelf, the high concentration of suspended sediments in the estuary is due
to erosion of tidal flats and island shores (Ginsberg and Perillo 1990). The
southern coast of the Principal Channel across Ingeniero White Port has
retreated up to 50 m between 1980 and 1986 and 1.5 million m 3 were exported from an 8-km stretch at the mid-reach of the channel (Perillo and
Sequeira 1989). Because most sediments in the estuary are silt and clay,
strong currents and short slack water intervals impede their deposition in
channels and on the tidal flats, and waves erode old sediments and prevent
any accumulation of new ones. These conditions explain the erosional
stage of most of the estuary and the prevalence of sediments from the deltaic deposition period.
14.3 Physical Processes
14.3.1 Freshwater Input
Two freshwater tributaries enter the estuary from the northern shore. The
Sauce Chico River (drainage area of 1,600km 2 ) discharges into the Principal Channel about 3 km downstream from the head of the estuary and the
Naposta Grande Creek (drainage area of 920km 2 ) reaches the estuary
about 1 km downstream of Ingeniero White Port (Fig. 14.1). Both tributaries behave similarly in spring and summer during maximum mean rainfall but they are out of phase in autumn when the Sauce Chico River has a
secondary mode. Although mean annual runoff for the Sauce Chico River
and the Naposta Grande Creek are low (1.9 and 0.8 m 3 s- 1 , respectively),