The Douro River drains a large part of the Iberian Peninsula before it flows into the Atlantic Ocean in northern Portugal. Although heavily dammed throughout its course, the Douro forms a mesotidal estuary in the last 22 km. No data had ever been collected in this estuary prior to 1994. An analysis of salinity data measured once a month in 1994 and current data obtained in the fall of 1994 revealed a salt wedge estuary highly dependent on river discharge: strong vertical salinity stratification under conditions of low runoff, whereas during high river flows the whole estuary became a river. The tide in the fall of 1994 was very close to a standing wave throughout the estuary; the tidal range, as high as 2.8 m at spring tides, was not powerful enough to destroy the vertical stratification. Tidal straining prevented mixing within the salt wedge during ebb; however, the column became vertically mixed at the end of flood. The salt wedge became arrested at three favored positions determined by the bathymetry. The salt intrusion, more sensitive to freshwater inflow than to tidal action, penetrated as far as 2/3 of the estuary's length. Residence times were estimated with a box model and were dependent on river discharge; they varied from 8 hours with early spring (March) inflows to 16 days under summer runoff conditions. © 2000 Ifremer/CNRS/IRD/É ditions scientifiques et médicales Elsevier SAS Résumé-L'estuaire du Douro (Portugal) : coin salé mésotidal. Le Douro est un fleuve dont le bassin versant est le plus important de la Péninsule ibérique car il draine 17 % de son territoire. Avec un débit liquide moyen de 480 m 3 s-1 , le Douro se jette dans l'océan Atlantique à Porto. Le long du cours principal du fleuve, long de 930 km, et de ses tributaires, environ soixante grands barrages hydroélectriques contrô lent le flux d'eau douce arrivant à l'estuaire. L'estuaire mésotidal est actuellement confiné aux derniers 22 km. L'analyse des données (i) de salinité, obtenues entre janvier et décembre 1994 en marée de vive eau, et (ii) de courants durant l'automne 1994, a démontré l'existence d'une intrusion saline liée au débit du fleuve. Une situation de faible débit conduit à une stratification verticale prononcée. Quand le débit augmente, l'eau douce domine dans l'estuaire. Durant l'automne 1994, la marée était proche d'une onde stationnaire avec une amplitude de 2,8 mètres. Cependant, elle n'était pas suffisamment puissante pour perturber la stratification verticale de la colonne d'eau. La friction provoquée par le courant est le mécanisme de mélange le plus important, très active en amont de l'intrusion saline. Trois points nodaux principaux d'arrêt des eaux de mer, conditionnés par la bathymétrie, se localisent à 2,6; 7,5 et 15 km de l'embouchure. L'intrusion saline est donc plus sensible au débit du fleuve qu'à l'action tidale. La pénétration d'eau de mer vers l'amont couvre environ deux tiers de l'estuaire. Le temps de résidence de la masse d'eau dans l'estuaire, estimé a partir d'un modèle de boîtes, dépend fortement d...

Near-inertial motion on the shelf-slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near-inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near-inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near-inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf-slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near-inertial energy in the front.

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