Up until January 1848, date when the Génissiat dam was filled, the Rhône, flowing from Bellegarde, used to disappear during the dry season into a 60m-deep geological fault, known as the "loss" of the Rhône. The site was turned into a 23km-long reservoir, held back by a dam-weight that is 104m high and 100m wide at its base. To control the variations in the Rhône's flow, two evacuating canals were built on either side of the dam on each of its banks.