Gier
The Gier is a Affluent the Rhone.
It takes its source with Jasserie (1299 m) in the Mount Pilat to join the Rhone with Givors after a course of 44 kilometers.
Its principal affluent is the Dorlay.
The word Gier derives from Latin Jaresis who also gave the term Jarez which indicates the slopes of the valley of Gier and which enters designation many communes: Holy-Cross-in-Jarez, Saint Paul in Jarez, Holy Romain in Jarez, etc
Its water was used by the Romans to feed Lyon out of water thanks to the Aqueduc of Gier.
To the 19th century and 20th century, water of Gier brought energy necessary to many industries distributed of the buttresses of Pilat until in the valley of Gier.
Hydrology
The flow of Gier was observed over one 44 years period (1964-2007), with Givors, city of the Département of the Rhone located at its confluence with the Rhone. The catchment area of the river is of 406 km ².
The interannual medium flow or module of the river is of 3,19 m ³ a second.
Gier presents seasonal of medium flow and typical fluctuations of the rivers of the French Massif Central with a partially nival food. The high waters are located in winter and in spring, and carry the average monthly flow on a level from 3,75 to 4,45 m ³ a second, from November to May included (with an in November maximum followed by a second top in May), and low waters of summer, July to September, with a fall of the monthly medium flow up to the level of 1,13 m ³ in August.
The VCN3 can fall until 0,3 m ³, in the event of dry quinquennial period. Let us recall that the VCN3 is the past minimal quantity or minimal flow over three days consecutive.
In addition the risings can be important and are rather frequent. The recorded maximum instantaneous flow was of 338 m ³ a second on December 2nd, 2003, while the value maximum day laborer was of 208 m ³ a second at the same date. The QIX 2 and QIX 5 or flows calculated for a biennial and quinquennial rising are worth respectively 59 and 100 m ³. The QIX 10, or calculated flow of decennial rising, is of 130 m ³ a second. As for the QIX 20, it is worth 160 m ³ a second, while the QIX 50 is of 190 m ³ (see note).
As comparison, let us recall that the QIX 10 of the the Eure to the Cailly-on-Eure is worth 90 m ³ a second, while its QIX 50 is of 120 m ³. One can thus notice that the QIX 10 like the QIX 50 of Gier, small river equipped with a small basin of 406 km ², exceeds those of the Eure by far, whereas the catchment area of this last is more than eleven times wider.
The Lame of water past in the basin of Gier is of 248 millimetres annually, which is poor, definitely lower than the overall average of France. The specific Débit (or Qsp) is assembled at 7,9 liters a second and per square kilometer of basin.
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