The Lake of Geneva is located at the border between the "Préalpes de Chablais" and the "Plaine molassique Suisse" and has been often described as a lake due to erosion of the Rhône glacier. A seismic survey of the lake showed that a buried valley, dug in the molasse, exists under the bottom deposits of the lake. The molassic bedrock plunges towards the East or in other words this "prelemanic valley" was flowing in the opposite direction than the present Rhône river. Important subsidence of the eastern part of this old valley explains the formation of the lake and the establishment of the drainage towards the Mediterranean Sea. The main morphological characteristics of the lake depend on its geological history: its crescent shape follows very exactly the shape of the prelemanic valley. Its maximum depth, in the oriental part of the lake (309 m) is related to the very low altitude of the molassic bedrock which is 200 m below M. S. L. in this area. In this subsident area, important quantities of sediments have been brought mainly by the Rhône river. We shall give first some brief details about the present discharge of the Rhône and we shall study the bottom deposits of the lake.