Size: 280 km²
The Thau watershed is drained by numerous little streams (3?13 km) with intermittent flows. It is delimited by the Aumelas massif (altitude, ca. 300m above sea level) to the north, the Gardiole massif to its eastern rim and by the Hérault river basin to the west. Toward the south, a narrow sandy strip separates the lagoon from the sea. This area, called Lido, accounts for only 5% of total catchment area, with very low mean slope and sandy sediments, which renders the inputs extremely diffuse. Following Ben Othman et al. (1997) and Anon. (1997), two different zones can be differentiated on the catchment area, using geomorphologic criteria: on one hand the eastern part of the catchment area is composed of strongly karstic Jurassic limestone, overlaid by Miocene marls in its central part. This area corresponds more or less to the V`ene river watershed, which is fed by two karstic resurgences, the V`ene and Issanka springs. On the other hand, the western part is mainly composed of Eocene marls and Miocene molasses. This area concerns the Pallas river and all other little streams to the west. The river response delays to rainy events are supposed to be shorter in this area than in the karstic zone, where more water infiltrations are likely to occur.