The Tagus Estuary (Figure 1) is one of the largest estuaries in Europe occupying an area of 320 km2 (from the estuary’s mouth to Vila Franca de Xira, the upstream limit of saline water intrusion under normal hydrological conditions).
Figure 1. The Tagus Estuary location
Source: Own elaboration.
The North bank has a more accentuated orography and the South bank alluvial floodplains resulting from river erosion and deposition. The estuary presents an unusual morphology characterized by an extensive and shallow interior region with widths that can reach 15 km, and then follows a NNE-SSW direction into a narrow and deep channel with a minimum width of 1.8 km towards ENE-WSW (Cf. FREIRE, TABORDA & ANDRADE, 2006: 2-3) – Figure 2.
Figure 2. The Tagus Estuary
Source: Administration of the Port of Lisbon, S.A.
According to FREIRE, TABORDA & ANDRADE (2006: 3), the internal estuary is characterised by longitudinal sediment furrows, cut by tidal channels and extensive zones of tidal flats, mainly adjacent to the left bank, supporting the development of important areas of salt marshes.
The Tagus Estuary has several alluvial flats as a result of river erosion, transport, deposition and accumulation of matter transported by the river. “The estuary is subjected to constant silting, requiring occasional dredging to maintain the navigational channels (…) the estuary is navigable in all of its extension for vessels with draught to 2.5 m, and 20 m of mast, because of the bridges” (GOMES, 2008: 10).
According to GOMES (2008: 29-31), the estuary presents five distinct zones (Figure 3):
Figure 3. The Tagus Estuary Zones
Source: Own elaboration, based in GOMES, 2008.
The main physical characteristics of the estuary are presented in the following table:
Table 1. Main physical characteristics of the Tagus Estuary (under average hydrological conditions)
Source: FREIRE, TABORDA & ANDRADE, 2006.
The tide is an important factor in the Tagus Estuary given that the average tidal volume (600 x 106 m3) is significant in relation to the water volume below the low-tide level (1900 x 106 m3). This estuary corresponds to a “positive type” estuary, also being characterised as a partially stratified estuary (Cf. ICN, 2002: 5).
The tidal range observed and the geomorphological characteristics at its mouth, both at the upper and lower estuary, allow for the inclusion of the Tagus Estuary in the class of meso-tidal estuaries, subject to a tide with a semi-diurnal period and with the rising tide taking longer than the ebb tide (Cf. ICN, 2002: 5-6).
The resident population of the 12 municipalities that make up the margins of the Tagus Estuary is 1.762 million inhabitants. It is on the North bank that the greatest population concentration occurs (1.180 million inhabitants – 67.0% of the total), with particular concentration in the city of Lisbon, with 479 thousand inhabitants (40.7% of the resident population on the North bank of the estuary). In its turn, on the South bank of the Tagus Estuary, the greatest concentration occurs in the Almada-Seixal-Barreiro area with 421 thousand inhabitants (72.4% of the resident population of this bank of the estuary).
Table 2. Resident population per municipality (2001 and 2009)
Source: INE, XIV Recenseamento Geral da População, 2001 and INE, Anuário Estatístico da Região de Lisboa – 2009, 2010
Compared to 2001, the resident population of Tagus Estuary region increased by 9,747 inhabitants, which represents a population change of +0.6%. This overall change hides two different realities: the Northern bank losing -3.2% while the Southern bank grew by +9.3%.
Figure 4. Resident population per municipality (2001)
Source (Statistical): INE, XIV Recenseamento Geral da População, 2001.
Source (Map): Own elaboration.
Figure 5. Resident population per municipality (2009)
Source (Statistical): INE, Anuário Estatístico da Região de Lisboa – 2009, 2010.
Source (Map): Own elaboration.
On the other hand, the Tagus Estuary region occupies a strategic location (in the centre of the Lisbon Metropolitan Area) which allied to its natural characteristics has allowed it to become, through time, an important catalyst in the development of various economic activities related to the diversity of uses of the estuary (water plan) and adjacent areas. Among these, the following stand out: agricultural activities, fishing and aquaculture, forestry, recreation and leisure activities, shipping, naval construction, industrial uses and transportation (Figure 6).
Figure 6. Economic activities developed in the Tagus Estuary
Source: Own elaboration based in MONIZ, 2009 and APL, 2007.
APL – Administração do Porto de Lisboa (2007), “Plano Estratégico de Desenvolvimento do Porto de Lisboa – Sumário Executivo”. Lisboa: Administração do Porto de Lisboa, 25 p.
FREIRE, Paula; TABORDA, Rui; ANDRADE, César (2006), “Caracterização das praias estuarinas do Tejo”, in Actas do 8.º Congresso da Água. Figueira da Foz: Associação Portuguesa dos Recursos Hídricos, 12 p.
GOMES, José (2008), “Tagus Estuary Pilot – Routes & Destinations”. Linda-a-Velha: DG Edições, 196 p.
ICN – Instituto de Conservação da Natureza (2002), “Reserva Natural do Estuário do Tejo – Turismo de Natureza – Enquadramento Estratégico”. Alcochete: Instituto de Conservação da Natureza, 33 p.
INE – Instituto Nacional de Estatística (2010), “Anuário Estatístico da Região de Lisboa – 2009”. Lisboa: Instituto Nacional de Estatística, 372 p.
INE – Instituto Nacional de Estatística (2001), “XIV Recenseamento Geral da População”. Lisboa: Instituto Nacional de Estatística.
MONIZ, Gabriela (2009), “O Ordenamento do Estuário do Tejo”, in III Seminário Internacional de Náutica de Recreio e Desenvolvimento Local. Seixal: Câmara Municipal do Seixal, 30 p.