Lake St. Cassian is a reservoir located in French Provence in the southeastern department of Var, in the canton or "country" of Fayence.
It is bordered to the north and east by the municipalities of Montauroux and Callian, east by the municipalities of Tanneron and Adrets-of-the Esterel, and south by the esterets-of-LAC1 (neighborhood the common Montauroux), located just below the line of watershed between the basin and that of Reyran Vallon des Vaux (tributary of Biançon2).
Its length is 7 km (north-south) of 3 km (east-west). The dam holds 60 million cubic meters of water. With 430 ha in area, is the largest lake of the Esterel.