Characterization of the Ebre and Rhone estuaries: A basis for defining and classifying salt‐wedge estuaries

Abstract

River discharge is the main factor controlling the hydrologic dynamics of the Ebre and Rhone estuaries. The topography of the estuary bed also influences the extent, advance, and retreat of the salt wedge. Tides have little influence because of the low tidal range. Mean annual river 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 river flow, the salt wedge is established and the estuary becomes a depositional environment. With higher flows the salt wedge is washed away and erosive conditions prevail. On the basis of the conditions in which the salt wedge is formed, two types of highly stratified estuaries can 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 corresponds to the “salt‐wedge estuary” or type 4 of the Hansen‐Rattray classification, 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 pattern corresponds to types 3b and 2b of the Hansen‐Rattray classification.