This reporl presents a summary of our knowledge on River Rhône, Switzerland, particularly downstream of lake Léman. lt shows that in the 1970's, the biological quality of the river water began deteriorating from Chalais in Valais. In the Geneva sector, the evaluation of the biological, chemical and bacteriological water quality placed the River Rhône in the "good to very good" category up to the waste water treatment plant. At this point, the discharge of waste water from the Aire plant together with the retention at the hydroelectric dam of Verbois (slowing down, silting up) sharply modifies the situation. Even, if there are some improvements in the biological activity downstream of the dam, the concentration of ammoniacal nitrogen and the bacterial count (parlicularly coliforms) pushes the River Rhône into the "highly polluted" water category up to Chancy (French border). On the other hand, triannual drainage of materials retained at the Verbois dam and the construction of the dam at Seujet (Geneva) have only limited impacts with respect to time, the effects of the overload resulting from the discharge of wastes are demonstrated from the water analysis surveys of 1984 and 1990-1991.