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{{short description|Longest river on the Iberian Peninsula}}
{{About|a river in the Iberian Peninsula|the ancient Thessalian title|Tagus (title)|the ghost town in North Dakota|Tagus, North Dakota}}
{{Infobox river
| name               = Tagus
| name_other         = {{native name|es|Tajo}}<br />{{native name|pt|Tejo}}
| name_etymology     = Vulgar Latin ''taliāre'', "to cut through"
| image              = Tagus River Panorama - Toledo, Spain - Dec 2006.jpg
| image_size         = 300
| image_caption      = View of Tagus River in [[Toledo, Spain]]
| map                = Tagus Watershed.jpg
| map_size           = 300
| map_caption        = Tagus River basin ([https://mghydro.com/watersheds/shared/9AA7ED.html Interactive map])
| mapframe           = yes
| mapframe-zoom      = 5
| mapframe-frame-width = 300
| subdivision_type1  = Country
| subdivision_name1  = [[Spain]], [[Portugal]]
| source1            = Fuente de García, [[Montes Universales]]
| source1_location   = [[Frías de Albarracín]], Spain
| source1_coordinates= {{Coord|40.321|-1.697|format=dms|display=i}}
| source1_elevation  = {{convert|1593|m|abbr=on}}
| mouth              = Estuary of the Tagus, [[Atlantic Ocean]]
| mouth_location     = [[Lisbon]], Portugal
| mouth_coordinates  = {{Coord|38.691|-9.171|format=dms|display=i}}
| mouth_elevation    = {{convert|0|m|abbr=on}}
| length             = {{convert|1007|km|mi|abbr=on}} 
| discharge1_avg     = {{convert|500|m3/s|cuft/s|abbr=on}}
| basin_size         = {{convert|80100|km2|abbr=on}}
| tributaries_left   = [[Guadiela]], [[Algodor River|Algodor]], [[Gévalo]], [[Ibor]], [[Almonte (river)|Almonte]], [[Salor River|Salor]], [[Sever (river)|Sever]]
| tributaries_right  = [[Gallo River|Gallo]], [[Jarama]], [[Guadarrama (river)|Guadarrama]], [[Alberche]], [[Tiétar (river)|Tiétar]], [[Alagón (river)|Alagón]], [[Zêzere River|Zêzere]]
| extra              = 
}}

The '''Tagus''' ({{IPAc-en|ˈ|t|eɪ|ɡ|ə|s}} {{respell|TAY|gəs}}; {{langx|es|Tajo}} {{IPA|es|ˈtaxo||Pronunciation of Tajo in Spanish.ogg}}; {{langx|pt|Tejo}} {{IPA|pt|ˈtɛʒu|}}) is the longest river in the [[Iberian Peninsula]]. The river rises in the [[Montes Universales]] between [[Province of Cuenca|Cuenca]] and [[Province of Teruel|Teruel]], in mid-eastern [[Spain]], flows {{convert|1007|km|mi|0|abbr=on}}, generally westward, and empties into the [[Atlantic Ocean]] at [[Lisbon]].<ref name="RNF">{{cite book |lang=en |last1=Riera Aragay |first1=Ana |last2=Sánchez |first2=Javier |date=15 March 2023 |orig-date=2022:GeoPlaneta |title=Guía de las Reservas Naturales Fluviales de la cuenca del Tajo |trans-title=Guía de las Reservas Naturales Fluviales de la cuenca del Tajo |publisher=GeoPlaneta |location=Madrid |isbn=9788408268888 |oclc=1376251354 |url=https://books.google.com/books?id=qNivEAAAQBAJ&pg=PT19}}</ref>

== Name ==
The river's Latin name is ''Tagus''. While the etymology is unclear, the most probable etymological origin for the [[hydronym]] ''Tagus'' is [[Indo-European]] *(s)tag- ('to drip').<ref>{{Cite journal|url=https://digitalis-dsp.uc.pt/bitstream/10316.2/37749/3/Toponyms%20of%20Lusitania.pdf|page=151|first=Leonard A.|last=Curchin|title=Toponyms of Lusitania: a re-assessment of their origins|doi=10.14195/1647-8657_46_7|journal=Conímbriga|volume=XLVI|year=2007}}</ref>

== Geography ==

=== Source ===
The Tagus River originates at an elevation of 1,593 meters above sea level in a place known as Fuente García, within the municipality of [[Frías de Albarracín]] in Teruel, Spain. Its source is located between the Muela de San Juan (1,830 m) and Cerro de San Felipe (1,839 m), in the [[Sierra de Albarracín]], which belongs to the [[Montes Universales]] in the western branch of the Iberian System.<ref>{{cite web  |author=Redaragón  |title=Nacimiento del Tajo  |url=http://www.redaragon.com/turismo/que_hacer/default.asp?accion=pagina&CosaQueHacer_ID=1664  |website=Redaragón  |access-date=2024-06-30  |language=es  |archive-url=  |archive-date= }}</ref>

=== Course ===
The river flows through Spain for {{Convert|816|km|abbr=on}}, passing through four autonomous communities ([[Aragón]], [[Castilla-La Mancha]], [[Madrid]], and [[Extremadura]]) and a total of six provinces (Teruel, Guadalajara, Cuenca, Madrid, Toledo, and Cáceres). After forming a {{Convert|47|km|abbr=on|adj=on}} border between Spain and Portugal, it enters Portugal.<ref>The 47 km stretch where the river forms the border between Spain and Portugal is known as the "International Tagus".</ref> In Portugal, it flows for {{Convert|145|km|abbr=on}} through the traditional regions of Beira Baixa, Alto Alentejo, Ribatejo, and Estremadura, which include the districts of [[Castelo Branco (district)|Castelo Branco]], [[Portalegre District|Portalegre]], [[Santarém District|Santarém]], [[Lisbon]], and [[Setúbal]].

The most important cities along its course are [[Aranjuez]], [[Toledo, Spain|Toledo]], and [[Talavera de la Reina]] in Spain; and [[Abrantes]], [[Santarém, Portugal|Santarém]], and [[Lisbon]] in Portugal.

=== Basin ===
The Tagus basin has a total area of {{Convert|80,600|km2|abbr=on}}. It is the most populated basin in the Iberian Peninsula, with more than ten million inhabitants. It includes the Madrid metropolitan area and the Lisbon region. The Tagus basin has a total reservoir capacity of around {{Convert|14,500|hm3|e9cuft|abbr=unit}}.

=== Geology ===
The lower Tagus region in Portugal is a seismically active area. Major [[earthquake]]s in the Lower Tagus include those of 1309, [[1531 Lisbon earthquake|1531]], [[1755 Lisbon earthquake| 1755]], and [[1909 Benavente earthquake|1909]].<ref>{{cite book|first=William Herbert|last=Hobbs|author-link=William Herbert Hobbs|title=Earthquakes: An Introduction to Seismic Geology|publisher=D. Appleton and Company|location=New York|year=1907|pages=142–144}} [https://archive.org/details/earthquakesanin00hobbgoog/page/n200 <!-- pg=142 quote=Tagus. --> Downloadable Internet Archive]</ref><ref name="Diário de Notícias">{{cite web |title=Sismo sentido em Lisboa na mesma zona dos grandes abalos de 1531 e 1909|trans-title=Earthquake felt in Lisbon in the same area as the great earthquakes of 1531 and 1909 |url=https://www.dn.pt/sociedade/sismo-sentido-em-lisboa-ocorreu-na-mesma-zona-dos-grandes-abalos-de-1531-e-1909--13472236.html |publisher=[[Diário de Notícias]] |access-date=29 October 2021 |language=Portuguese |date=18 March 2021}}</ref>

=== Estuary ===
{{unreferenced section|date=September 2024}}
The Tagus river is one of the few rivers in the world to have an [[Inverted river delta|inverted delta]]. Its delta is wider at the beginning and narrows down as it approaches the sea, contrary to a typical [[River delta|delta]]. This is because it flows into the sea through a small opening in a valley. Although due to sedimentation, this delta is now only very partially inverted, with the valley now mostly filled with sediment. The delta is about {{Convert|15|km|4=0|abbr=on}} wide and {{Convert|25|km|abbr=on}} long, but its exit into the sea is only {{Convert|2|km|abbr=on}} wide. It thus forms a large lagoon with large and very shallow sand banks which go uncovered during low tides. The delta used to be even bigger thousands of years ago.

== Hydrology ==

The hydrological regime of the Tagus is determined by the rain and snow variations typical of the central region of the Iberian Peninsula, especially in reference to the mountain formations integrated here. The river's major floods usually occur from January to April, with an absolute maximum in March (when thawing occurs), while the lowest flows occur between July and October, with a minimum in September.

This results in a very irregular course, with strong flow oscillations. As it passes through Alcántara (Cáceres), these range from {{Convert|350|m3/s|abbr=on}} in February and March to {{Convert|11|m3/s|abbr=on}} in August and September.

This regime has been altered in the second half of the 20th century as a result of the construction of different engineering works, aimed at regulating its basin for five main uses: drinking water supply, irrigation, water diversion to an external basin (that of the Segura River), electricity production, and cooling of nuclear power plants.

== Ecology ==

The banks and areas of influence of the Tagus are home to relevant flora and fauna, representative of the central region of the Iberian Peninsula. The high degree of depopulation in some areas integrated into its basin, such as its upper and middle-lower course, has allowed the preservation of places of great ecological interest. Some of them have been legally protected, such as the [[Alto Tajo Nature Park]] (Guadalajara and Cuenca), the [[Monfragüe National Park]] (Cáceres), and the [[Tagus Estuary Natural Reserve]], near Lisbon. Around the course of the river, four endangered animal species live: the Iberian lynx, the Spanish imperial eagle, the black stork, and the black vulture.

== Historical importance ==
[[File:David Roberts (1796-1864) - A View of Toledo and the River Tagus - RCIN 405042 - Royal Collection.jpg|thumb|''[[A View of Toledo and the River Tagus]]'' by [[David Roberts (painter)|David Roberts]], 1841]]
In addition to its geographical relevance, the Tagus has great historical importance, resulting from its connection with Toledo and Lisbon, two of the cities with the greatest weight and historical journey in the Iberian Peninsula. The first was built on a hill about 100 m high, around the meander known as Torno del Tajo, which preserved it from possible attacks and incursions.

From the Late Middle Ages, the mouth of the Tagus articulated an intense commercial activity between Northern Europe and the Mediterranean. From the Renaissance, it was the main communications hub of the Portuguese Empire, which extended through America, Africa, and Asia.

== Recreational uses ==

The nearly {{Convert|100|km|4=-1|abbr=on}} of the Alto Tajo canyon as it passes through the natural park of the same name offer one of the best options for practicing whitewater canoeing in the central peninsula. Several adventure companies offer this activity in the vicinity of Poveda de la Sierra and Ocentejo. In the flat water modality, canoeing can be practiced in the Entrepeñas and Bolarque reservoirs. There are also important flat water canoeing clubs in Aranjuez and Talavera de la Reina.

== See also ==
{{columns-list|colwidth=30em|
* [[Tagus basin]]
* [[Rivers of Spain]]
}}

== References ==
{{reflist}}

== External links ==
{{commonscat|Tagus River}}
* [http://www.chtajo.es/ Website of the Tagus Hydrographic Confederation]
* [https://saihtajo.chtajo.es/index.php#H8 Website of the Automatic Hydrological Information System of the Tagus Hydrographic Basin] (real-time data)
* [http://www.embalses.net/cuenca-3-tajo.html Status of the reservoirs in the Spanish Tagus basin]

{{authority control}}

[[Category:Tagus| ]]
[[Category:Belém (Lisbon)]]
[[Category:Geography of Lisbon]]
[[Category:International rivers of Europe]]
[[Category:Portugal–Spain border]]
[[Category:Rivers of Castilla–La Mancha]]
[[Category:Rivers of Extremadura]]
[[Category:Rivers of Portugal]]
[[Category:Rivers of Spain]]
[[Category:Rivers of the Community of Madrid]]