The Rio de !a Plata Estuary, Argentina-Uruguay
187
(about 70%) during the spring and summer. Mean monthly wind velocities
in the watershed are moderate, with maxima in spring and summer. Extreme winds with velocities above 11m s- 1 (11-13 %) are evenly distributed
along the year. The large size and shallow waters make the estuary highly
susceptible to atmospheric forcing. Air and water temperatures are in a
quasi-permanent state of equilibrium, ranging from 10-11 °C in winter to
22-23 oc in summer (Guerrero et al.1997a,b ). Significant horizontal temperature gradients are absent. However, under the influence of NNE winds in
summer, upwelling between Punta del Este and Polonio Cape may cause
temperatures 4-5 °C below those of the surrounding shelf waters (Framifian
et al. 1999). Since the surrounding shelf surface waters and riverine water
have a similar heat content, the effect of temperature on vertical stratification is negligible (Guerrero et al. 1997a). Mean vertical temperature differences ( < 1 oq in the water column change between seasons, with cooler surface waters in winter. Maximum vertical stratification occurs during spring,
when surface-bottom temperature differences vary from +2 to +5 °C.
13.2.2 Estuarine Dynamics
The freshwater discharge (annual mean 22,000 m 3 s- 1 ; Framifian and
Brown 1996) from the Parana and Uruguay rivers into the estuary exhibits
low seasonality, with a mean maximum of 26,000 m 3 s- 1 in winter and a
mean minimum of 19,000 m 3 s- 1 in summer. The tidal wave originates on
the outer shelf and enters the estuary from the southeast, with amplitudes
ranging from 30 to 100 em (Balay 1961). Tidal currents are typically below
45 em s- 1 (CARP 1989; Framifian et al. 1999), thus friction energy between
tidal and bottom currents is insufficient to erode vertical stratification.
Between the head of the estuary and Barra del Indio shoal a fluvial regime
with vertically mixed river waters dominates. After the shoal, the cross section surface area of the estuary almost duplicates (Fig.13.1) and denser shelf
water intrudes along the bottom to maintain a gravitational balance, taking
the shape of a salt wedge. The estuary therefore becomes a typically twolayer system with strong vertical stratification that gradually weakens seaward. The salt wedge regime is characterized by pronounced vertical differences in salinity, a constant halocline depth (5 m), and gradually sloping
bottom topography. The boundary between the riverine regime and the salt
wedge regime defines the bottom salinity front over the Barra del Indio
shoal (Figs.l3.1, 13.2). Flocculation of suspended matter at the tip of the salt
wedge and re-suspension of sediment due to tidal current friction at the
bottom form a turbidity front (Fig.13.2A), which extends from Montevideo
along the 5-m isobath over the Barra del Indio shoal to Samboromb6n Bay
187
(about 70%) during the spring and summer. Mean monthly wind velocities
in the watershed are moderate, with maxima in spring and summer. Extreme winds with velocities above 11m s- 1 (11-13 %) are evenly distributed
along the year. The large size and shallow waters make the estuary highly
susceptible to atmospheric forcing. Air and water temperatures are in a
quasi-permanent state of equilibrium, ranging from 10-11 °C in winter to
22-23 oc in summer (Guerrero et al.1997a,b ). Significant horizontal temperature gradients are absent. However, under the influence of NNE winds in
summer, upwelling between Punta del Este and Polonio Cape may cause
temperatures 4-5 °C below those of the surrounding shelf waters (Framifian
et al. 1999). Since the surrounding shelf surface waters and riverine water
have a similar heat content, the effect of temperature on vertical stratification is negligible (Guerrero et al. 1997a). Mean vertical temperature differences ( < 1 oq in the water column change between seasons, with cooler surface waters in winter. Maximum vertical stratification occurs during spring,
when surface-bottom temperature differences vary from +2 to +5 °C.
13.2.2 Estuarine Dynamics
The freshwater discharge (annual mean 22,000 m 3 s- 1 ; Framifian and
Brown 1996) from the Parana and Uruguay rivers into the estuary exhibits
low seasonality, with a mean maximum of 26,000 m 3 s- 1 in winter and a
mean minimum of 19,000 m 3 s- 1 in summer. The tidal wave originates on
the outer shelf and enters the estuary from the southeast, with amplitudes
ranging from 30 to 100 em (Balay 1961). Tidal currents are typically below
45 em s- 1 (CARP 1989; Framifian et al. 1999), thus friction energy between
tidal and bottom currents is insufficient to erode vertical stratification.
Between the head of the estuary and Barra del Indio shoal a fluvial regime
with vertically mixed river waters dominates. After the shoal, the cross section surface area of the estuary almost duplicates (Fig.13.1) and denser shelf
water intrudes along the bottom to maintain a gravitational balance, taking
the shape of a salt wedge. The estuary therefore becomes a typically twolayer system with strong vertical stratification that gradually weakens seaward. The salt wedge regime is characterized by pronounced vertical differences in salinity, a constant halocline depth (5 m), and gradually sloping
bottom topography. The boundary between the riverine regime and the salt
wedge regime defines the bottom salinity front over the Barra del Indio
shoal (Figs.l3.1, 13.2). Flocculation of suspended matter at the tip of the salt
wedge and re-suspension of sediment due to tidal current friction at the
bottom form a turbidity front (Fig.13.2A), which extends from Montevideo
along the 5-m isobath over the Barra del Indio shoal to Samboromb6n Bay