Jouanne

The Jouanne is a French Rivière which runs in the department of the Mayenne (53). It is an affluent of the Mayenne out of left bank, therefore a tributary of a tributary of the the Loire by Mayenne and the Maine.

Geography

It takes its source in the commune of Bais, in the North-East of the department. It is formed by several brooks of the escarpées hills which are drawn up in the south of Bais; one of these brooks sprinkles the territory of Evron. Jouanne passes to Neau, and, grown bigger river of the Deux-Évailles , bathes Montsûrs, Argentré, Forcé, and will be thrown in the Mayenne (left bank) downstream from Entrammes.

Its overall length is approximately 49 kilometers.

It is fed by the brook of the Rock of Mézangers, the brook of Saint-Christophe-of-Luat the and the brook of Deux-Évailles.

At the end of the 19th century the Jouanne actuated twenty mills with grains and a pillery of Trèfle.

Etymology

It is probable that the name of JOUANNE goes back to Gallic form DIV-ONNA " divine-eau". The evolution of DIVONNA is very exactly the same one as that of DIVRNUS which generated DAY in modern French.

Hydrology

Jouanne is a fairly abundant river. Its flow was observed over one 40 years period (1968-2007), with Forcé, locality of the department of the Mayenne, located little before its confluence with the Mayenne. The catchment area of the river is there of 410 km ², i.e. its quasi totality.

The interannual medium flow or module of the river with Forcé is of 2,99 m ³ a second.

Jouanne presents seasonal fluctuations of flow very marked, like very often in the Western part of the basin of the Loire located on the old Armorican base. The high waters are held in winter and carry the average monthly flow on a level located between 5,0 and 7,7 m ³ a second, from January to March included (with an in January maximum and February). As of at the end of March, the flow falls quickly to the low waters of summer which take place from June to October, involving a fall of the monthly medium flow going up to 0,26 m ³ in August. But the fluctuations are much more marked over moreover short periods.

With the low water level, the VCN3 can fall until 0,035 m ³, in the event of dry period quinquennial, that is to say only 35 liters a second, which must be regarded as severe even for a small river, but rather normal in this area where the impermeability of the basement does not allow a prolonged retention of the water of the estival downpours. Let us recall that the VCN3 is a measurement of the severity of the low water levels which gives the past minimal quantity or minimal flow over three days consecutive.

As for the risings, they can be extremely important taking into account the rather average size of the basin. The QIX 2 and QIX 5 are worth indeed respectively 40 and 59 m ³. The QIX 10 or calculated flow of decennial rising is of 72 m ³ a second, the QIX 20 of 84 m ³. As for the QIX 50, it is assembled to 100 m ³ a second (see note).

The recorded maximum instantaneous flow with Forcé was of 79,7 m ³ a second on February 26th, 1996, while the value maximum day laborer was of 75,4 m ³ a second the same day. If one compares the first of these values on a QIX scale exposed higher, one notes that this rising was hardly of a vicennial nature, and thus by no means exceptional. On the whole, Jouanne is an irregular and fairly abundant river in the context of the rivers of plain of the basin of the Loire. The Lame of water past in its basin is of 231 millimetres annually, which is certainly lower than the overall average of France all confused basins, but approaches the average of the basin of the the Loire (more or less 244 millimetres). The specific Débit of the river (or Qsp) is assembled consequently at 7,3 liters a second and per square kilometer of basin.

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