Estimation of fluxes to the ocean from mega-tidal estuaries under moderate climates and the problems they present

Abstract

Evaluation of the quantity of estuarine flux material carried into the ocean, is difficult. High tides produce major oscillations in ocean waters and suspended matter (S.M.) near river mouths and, therefore, direct measurements at the river outlet do not represent real expulsions of S.M. The temperate climate, characterised by important seasonal variations throughout the year, induces significant hydrologic variations and consequently material fluxes into the ocean. Hence, it is difficult to represent measurements with respect to the time, season and especially the year, during which they take place.

The Gironde estuary is a good example of this type of estuary, and shows that the evaluation of flux material can only be made from the following equation: expulsion = fluviatile inputs-deposits. Measurements are made from the fluxes of S.M. and from a few metals such as Zn, Cu, and Pb.

It took three consecutive years (1976–1979) to carry out our measurements, which helped us to develop a general idea of the chief components of the system and to determine the flux variations.

According to the results, important variations occur from year to year, particularly as far as S. M. is concerned. The estuary, thanks to its significant oscillating stock of S.M. (turbidity maximum), behaves as a regulator for the metals. In a surplus year it acts as a sink, and in a deficit year it acts as a source.