Allondon

Pont Moulin Fabry

The Allondon is a 18 km long right tributary of the Rhône. It rises in France, in the municipality of Échenevex. The mouth is located at La Plaine, in the municipality of Dardagny in Switzerland west of Geneva. The Allondon drained a section of the Pays de Gex with a catchment area of 148 km ², of which 11 km ² are located on Swiss soil.

Geography

The source of the Allondon lies in a wooded valley at 580 m above sea level. M. of the Jura foothills below the Colomby de Gex near the hamlet of Naz Dessus the community Echenevex in the French department of Ain. The Allondon flows southward through a slightly incised in the slopes of the Jura foothills V-shaped valley before he steps out into the basin characterized by Quaternary deposits of Geneva.

Shortly before the Allondon reached the Swiss border, joins from the left of the Lion, the most important tributary. The two watercourses generally result in a similar amount of water with it. The Lion with its source Bach Journans ( origin in the high Jura also on Colomby de Gex ) has thus far but with 20 km already a much longer distance than the Allondon (8.5 km).

Below Saint- Genis- Pouilly, the Allondon gradually deepened into the surroundings and first forms at 2 km of the border between France and Switzerland, before he enters on Swiss territory ( Canton of Geneva ). Here it flows into a up to 500 m wide and 50 m sunken in the plateaus of the Geneva basin valley before it empties in La Plaine (municipality Dardagny ) in the Rhône.

Major tributaries are beside the Lion and the Allemogne the Roulavaz, both of which drain the steep south-east slopes of the Jura in the area whose highest elevations of Cret de la Neige and Reculet.

Hydrology

The Allondon has a nivopluviales flow regime with strongly fluctuating water level. Flood can in the spring during snowmelt in the Jurassic and later in the year when severe thunderstorms or prolonged rainfall occur. When heavy flooding (caused, for example, in February 1990 by heavy snow melt in the Jura in combination with heavy rainfall ) runoff of sometimes more than 100 m³ / s can be achieved. In contrast, the average discharge amount is at low tide only, just 0.6 m³ / s

Conservation

The 6 km long section of the Swiss Allondon Valley is a nature reserve since 1968 and is one of the protected areas of national importance. The course of the Allondon is in its natural state. The entire valley is filled with alluvial material of the river, which is partially secured by bush and forest vegetation. Elsewhere there are large gravel areas. The river, which can move unhindered throughout the valley floor, shifted after each flood his run, pours new gravel banks and carries it elsewhere again.

The valley of the Allondon is a popular tourist destination and a recreational area for the city of Geneva. Various hiking trails invite you to explore the nature.

Flora and Fauna

The mild climate and the optimum orientation of the valley ( north-south) have enabled the development of a partially Mediterranean vegetation. In the valley there are numerous dry meadows with rare species of plants, 20 different species of orchids, many chestnut and black locust. The Allondon itself is rich in fish, the trout and grayling make up the largest populations. The natural river valley provides habitat and haven for rare birds ( kingfishers, dippers, gray wagtails, warblers, Willow Tit ) and reptiles.

History

The first mention of Allondon was in 1397 under the name Aqua de Alandons; in the 18th century appeared on some maps the name La London, and today the river on French card works is sometimes referred to as Allandon. The name comes from pre-Celtic times and means living water.

In the 18th century, the water power of the Allondon for the operation of a paper mill has been used. At the confluence of the Roulavaz in the 19th century an asphalt mine was exploited. The project of a reservoir between Dardagny and Russian in 1938 abandoned.