CHAPTER 6 . The Argentina Estuaries: A Review
121
the mouth of the estuary. Station 1 and 5 are 4 and 1 km from the mouth, respectively.
The salinity profiles are vertical homogeneous with the presence of the fresh river
waters only at station 1; however, a saline intrusion of marine waters was observed at
depths from 6-8 m at station 3. The steep vertical salinity gradient was of the order of
12 psu m- 1 • At stations 4 and 5, closer to the mouth of the estuary, although not at its
mouth, the profiles show significant vertical salinity gradients. Further measurements
in the estuary mouth would help to understand the penetration of the marine waters
into the estuary due to tidal action.
6.4.3
Chubut River Estuary
The Chubut River is the most important watercourse of the Chubut province; in the
Patagonia (Fig. 6.1). The river has been dammed at about 120 km from its mouth and
passes through several of the major cities of the province. River water is used for agricultural, industrial and human consumption. Several industries are installed along
the lower Chubut River, and a major fisheries-industrial park has being installed close
to Rawson Harbour (located only 600 m inland from the river mouth). Dredging of
the last 1000 m of the navigation channel to accommodate medium size fishing vessels has been made several times. The present day industrial and municipal sewages
are normally discharged into the river with very little or no treatment. Some work has
been done on the chemical characteristics of the estuary (Orfila et al. 1987). High gradients of CI-, Na+, SO~-, K+ and Mg2+ have been found in longitudinal surveys from
the mouth to about 2 km upstream. On the other hand the longitudinal concentration of Ca2+ and carbonates were constant in the water column. Waters are rich in silica.
Considering the period 1990-1994, the annual average river discharge is 35.1 m 3 s-"
and extreme mean daily values vary from 354-4.93 m3 S-1 (Evarsa 1995). The Chubut
River belong to a wide basin denominated by Soldano (1947) the Chubut-Senguer
basin. The meandering channel varies from 70-200 m in width, and averages about
2 m in depth. The river bed shows several sigmoidal bars constituted by medium to
coarse sand. The bars divide the river into two channels, which are connected intermittently by small oblique interbar channels. From the harbour to the mouth, the
bottom is mainly formed by very coarse sand and large patches of well-rounded gravels
(Perillo et al. 1989). Tides are semidiurnal and have mean spring and neap ranges of
3.8 and 2.3 m, respectively.
Perillo et al. (1989) investigated the hydrography and circulation of the Chubut River
under exceptionally low river discharge. Previous studies (Orfila and Scapini 1987;
Orfila et al. 1987) showed that the frontal zone formed by the entrance of the tide into
the estuary migrates with river discharge, but most of the time it is found at less than
2 km from the mouth. However, Perillo et al. (1989) observed it as far as 4.5 km from
the mouth showing that the salt intrusion due to the tidal effect reached further inland than during normal river discharge (Fig. 6.12). Based on the classification of
Hansen and Rattray (1966) the estuary pertains to the Type 1, with certain vertical
stratification observed on the seaward side of the frontal zone. The time-mean salinity over the tidal cycle was calculated at different levels in the Chubut River Estuary.
The station near the mouth shows a vertical profile characteristic of a partially mixed
estuary with an upper layer of about z / d = 0.25 and a mixing zone of similar thick-
121
the mouth of the estuary. Station 1 and 5 are 4 and 1 km from the mouth, respectively.
The salinity profiles are vertical homogeneous with the presence of the fresh river
waters only at station 1; however, a saline intrusion of marine waters was observed at
depths from 6-8 m at station 3. The steep vertical salinity gradient was of the order of
12 psu m- 1 • At stations 4 and 5, closer to the mouth of the estuary, although not at its
mouth, the profiles show significant vertical salinity gradients. Further measurements
in the estuary mouth would help to understand the penetration of the marine waters
into the estuary due to tidal action.
6.4.3
Chubut River Estuary
The Chubut River is the most important watercourse of the Chubut province; in the
Patagonia (Fig. 6.1). The river has been dammed at about 120 km from its mouth and
passes through several of the major cities of the province. River water is used for agricultural, industrial and human consumption. Several industries are installed along
the lower Chubut River, and a major fisheries-industrial park has being installed close
to Rawson Harbour (located only 600 m inland from the river mouth). Dredging of
the last 1000 m of the navigation channel to accommodate medium size fishing vessels has been made several times. The present day industrial and municipal sewages
are normally discharged into the river with very little or no treatment. Some work has
been done on the chemical characteristics of the estuary (Orfila et al. 1987). High gradients of CI-, Na+, SO~-, K+ and Mg2+ have been found in longitudinal surveys from
the mouth to about 2 km upstream. On the other hand the longitudinal concentration of Ca2+ and carbonates were constant in the water column. Waters are rich in silica.
Considering the period 1990-1994, the annual average river discharge is 35.1 m 3 s-"
and extreme mean daily values vary from 354-4.93 m3 S-1 (Evarsa 1995). The Chubut
River belong to a wide basin denominated by Soldano (1947) the Chubut-Senguer
basin. The meandering channel varies from 70-200 m in width, and averages about
2 m in depth. The river bed shows several sigmoidal bars constituted by medium to
coarse sand. The bars divide the river into two channels, which are connected intermittently by small oblique interbar channels. From the harbour to the mouth, the
bottom is mainly formed by very coarse sand and large patches of well-rounded gravels
(Perillo et al. 1989). Tides are semidiurnal and have mean spring and neap ranges of
3.8 and 2.3 m, respectively.
Perillo et al. (1989) investigated the hydrography and circulation of the Chubut River
under exceptionally low river discharge. Previous studies (Orfila and Scapini 1987;
Orfila et al. 1987) showed that the frontal zone formed by the entrance of the tide into
the estuary migrates with river discharge, but most of the time it is found at less than
2 km from the mouth. However, Perillo et al. (1989) observed it as far as 4.5 km from
the mouth showing that the salt intrusion due to the tidal effect reached further inland than during normal river discharge (Fig. 6.12). Based on the classification of
Hansen and Rattray (1966) the estuary pertains to the Type 1, with certain vertical
stratification observed on the seaward side of the frontal zone. The time-mean salinity over the tidal cycle was calculated at different levels in the Chubut River Estuary.
The station near the mouth shows a vertical profile characteristic of a partially mixed
estuary with an upper layer of about z / d = 0.25 and a mixing zone of similar thick-