An alluvial area of national importance for Switzerland, the Allondon valley is a remarkable natural area. The idea of protecting the valley germinated in 1945, after the publication of a project for the construction of a dam 25 m high and 300 m long. The delegates of about thirty companies decide to oppose the works. The dam project is abandoned. The Monuments and Sites Commission prepares the classification through consultation. In 1952, the Council of State classified the two banks upstream from the Pont des Granges. In 1955-56, plans were made to dam the river under the pretext of erosion. The works are limited to the protection of bridges and roads. The State of Geneva buys plots of forest, then fields, pastures and banks. In 1963, the valley as well as neighboring tributaries and woods were included in the Inventory of landscapes and sites of national importance worthy of protection. Composed of a rich mosaic of very diverse environments, enjoying moreover a particular climate, this site classified as a cantonal nature reserve in 1968 is a veritable lung of greenery. As far as insects are concerned, there is in particular the only recently observed population of the species Lachesilla rossica. However, toxic pollution affects the river on several occasions, such as during the summer of 1975 when a toxic spill in the Lion, a tributary of the Allondon, poisoned the river for several kilometres. In August 2001, a caustic soda spill directly into the river killed several thousand fish.
