Grafito

See also: Tarn (homonymy)

The Tarn is a river south of the France. If one regards the Dordogne as a river, the Tarn is the most important affluent of the the Garonne.

Name

Tarnis or Tanara, of the roots ligure tan (cliff) + rear (river).

Geography

It takes its source on the Mont Lozere, in the Lozere (48), and is thrown in the the Garonne close to Castelsarrasin, in Tarn-et-Garonne (82).

The Tarn is famous for the throats which it dug upstream of Millau and which makes a high tourist place of it.

Departments and main cities crossed

Principal affluents

Hydrology

  • the Tarn is subjected to a Mediterranean and oceanic pluvio-nival mode

  • extreme Débits: from 8 with 6  000 m ³ /seconde at the time of the Raw of March 1930

Flows at the end of the course with Moissac

The flow of the Tarn was observed over one 57 years period (1923-1979), with Moissac, city of the Département of Tarn-et-Garonne, located at little distance from its confluence with the Garonne. The catchment area of the river is there of 15  400 km ², are almost the totality of this one (98%) which makes 15  700 km ².

The interannual medium flow or module of the river with Moissac is of 233 m ³ a second.

Tarn present of the seasonal fluctuations of flow very important, with risings of winter-spring, carrying the average flow monthly between 269 and 396 m ³ a second, from December to April included, with an in February maximum - March (snow melt), and low waters of summer, July to September, with a fall of the monthly medium flow up to the level of 55 m ³ in August, which remains in spite of very comfortable.

The VCN3 can however fall until 8,8 m ³, in the event of dry period quinquennial, which is very low. The VCN3 is the past minimal quantity or minimal flow over three days consecutive.

In addition the risings can be extremely important. Indeed, the recorded maximum daily output was of 4  000 m ³ a second on March 4th, 1930. The maximum instantaneous flow of this day not having been recorded with precision this day, one estimates it between 6 and 8  000 m ³ a second.

The QIX 10, or calculated instantaneous flow of decennial rising, just as QIX 2, QIX 5, QIX 20 and QIX 50, was never calculated. On the other hand the QJ correpondants indeed were it (see note).

The QJ 2 and QJ 5 of the Tarn are worth 2  respectively; 000 and 2  700 m ³ a second. The QJ 10 is worth 3  200 m ³, the QJ 20 are worth 3  of it; 600, while the QJ 50 is assembled to 4  200 m ³ a second.

The Lame of water past in the basin of the Tarn is of 478 millimetres annually, which is very high, largely higher than the overall average of France, following the example majority of the rivers resulting from the French Massif Central and especially from the area of the Cevennes. The specific Débit (or Qsp) is assembled at 15,1 liters a second and per square kilometer of basin.

Flows on the level of the gorges of the Tarn

Quelques préliminaires definitions for better including/understanding the following table:

the module is the interannual medium flow of the river, i.e. the average of the annual medium flows recorded during a certain period (more than 10 years) .

the VCN3 is a measurement of low water level of the rivers. It is the past minimal quantity or minimal flow over three days consecutive .

The gorges of the Tarn are located between the localities of Quézac and Rozier over a 53 km length. The DIREN the Midday-Pyrenees does not have a station hydrometric at these precise places. However it is nevertheless possible for us to know the average monthly flows of the river on the levels of the upstream and the downstream of its throats thanks to the stations of the established DIREN in the vicinity: Montbrun is with 6 kilometers downstream from Quézac, and Mostuéjouls with five kilometers downstream from Rozier. It is also on the level of Rozier that the confluence of Jonte with the Tarn is located.

The values of Dourbie and the Tarn in Millau are mentioned on a purely complementary basis, in order to have an good idea of the hydrology of the area.

It is interesting to notice how much the Tarn was regularized between the stations of Montbrun and Mostuéjouls, i.e. throughout its course in the throats. The VCN3 passes indeed from 0,36 to 4,80 m ³ a second, that is to say a multiplication by twelve, whereas the Tarn receives one affluent, Jonte, which contributes of nothing to regularize its flow since its VCN3 is itself extremely weak. The phenomenon is due to numerous the source S and Résurgence S which brings to the Tarn substantial water complements, at the beginning of the subsoil water masses stored in the karstic networks located under neighbouring causses (Causse of Sauveterre in north - Right Bank - and Causse Méjean in the south - left bank). Let us note that it is a phenomenon similar to that which one observes out of Champagne chalky (see the article concerning the Vesle, affluent of Aisne.)

See also: Aquiferous

Tourist activities

River navigation

The Tarn was navigable of Saint-Juéry to his Confluent with the Garonne with Moissac

References

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