Yerre

Etymology

Yerre is documented in the forms Era (1031), Edera (1045), Erra (about 1140), Hière (1384), probably of old a Atura . The current name was subject to the influence of former French yerre > “Lierre”.

Geography

Yerre takes its source within the forest of Montmirail, in the Perche, on the territory of the commune of Vault-Guillaume, in the extreme South-west of the department of Eure-et-Loir. It runs then towards the east and south-east on a little less than 50 km and is thrown finally in the Dormouse with Saint-Hilaire-on-Yerre.

Principal affluents

Yerre particularly has some affluents a length which does not exceed a few kilometers, on the level of Bazoche-Gouët and before Saint-Hilaire-on-Yerre.

Crossed cities

Hydrology

The flow of Yerre was observed for one 15 years period (1993-2007), with Saint-Hilaire-on-Yerre, locality of the department of Eure-et-Loir, located at the level of the confluence with the Dormouse. The catchment area of the river is of 297 km ².

The interannual medium flow or module of the river with Saint-Hilaire-on-Yerre is of 1,63 m ³ a second.

Yerre present of the seasonal fluctuations of flow rather important, with high waters of winter carrying the average flow monthly on a level located between 2,81 and 4,45 m ³ a second, from December to March included (maximum in January), and of low waters of summer from May to October, with a fall of the monthly medium flow until 0,397 m ³ in August (397 liters a second), which remains however rather consistent.

The VCN3 can fall until 0,10 m ³, in the event of dry quinquennial period, that is to say 100 liters a second. The VCN3 is the past minimal quantity or minimal flow over three days consecutive.

In addition the risings can be rather important. The QIX 2 and QIX 5 are worth respectively 37 and 58 m ³. The QIX 10 or calculated flow of decennial rising is of 72 m ³ a second, while the QIX 20 is assembled to 86 m ³. The QIX 50 was not calculated fault one duration of sufficient observation (see note). The recorded maximum instantaneous flow was of 90 m ³ a second on January 22nd, 1995, while the value maximum day laborer was of 65,5 m ³ a second on February 26th, 1997. By comparing the first of these figures with the values of the various QIX of the river, it appears that this rising was of a vicennial nature and by no means exceptional, and thus statistically intended to repeat itself every approximately 20 years.

The Lame of water past in the basin of Yerre is of 174 millimetres annually, which is poor, very definitely lower than the overall average of France, but also than that of the catchment whole of the area of the Loire (244 millimetres), but however largely higher than the whole of the basin of the Dormouse (128 millimetres with Durtal). The specific Débit (or Qsp) is assembled consequently at 5,5 liters a second and per square kilometer of basin.

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