CHARACTERIZATION OF THE EBRE AND RHONE ESTUARIES - A BASIS FOR DEFINING AND CLASSIFYING SALT-WEDGE ESTUARIES

River discharge is the main factor controlling the hydrologic dynamics of the Ebre and Rhone estuaries. The topography of the estuary bed al so influences the extent, advance, and retreat of the salt wedge. Tide s have little influence because of the low tidal range. Mean annual ri ver discharge is close to the critical value determining the formation and breakup of the salt wedge. This is related to the fact that river flow controls the sedimentary dynamics of the estuary, including the sandbar at the mouth. When discharge is lower than the mean annual riv er flow, the salt wedge is established and the estuary becomes a depos itional environment. With higher flows the salt wedge is washed away a nd erosive conditions prevail. On the basis of the conditions in which the salt wedge is formed, two types of highly stratified estuaries ca n be identified. In the first one, a salt-wedge regime is established during low river flows, whereas during high flows the wedge is washed away and the estuary becomes a river. This circulation pattern corresp onds to the ''salt-wedge estuary'' or type 4 of the Hansen-Rattray cla ssification, typical of river-dominated estuaries in microtidal seas. In the second one, a salt wedge is established during high river flows , whereas a partially mixed estuary occurs during low flows. This patt ern corresponds to types 3b and 2b of the Hansen-Rattray classificatio n.