Sedimentary characterization of bed types along the Guadiana estuary (SW Europe) before the construction of the Alqueva dam

Abstract

The Guadiana River estuary is one of the most important mesotidal fluvio-marine systems of the south-western Iberian Peninsula. The river mouth was formed as a narrow channel that was excavated by fluvial incision during the Pleistocene lowstand and was then flooded 6500 years ago. The estuary is in an advanced state of sediment infilling in its proximal part, due to its narrow morphology, which prevents passage of sediment through to the open coast. Consequently, sediment is accumulating in the river mouth, which causes progradation. The analysis of the sediments into the estuarine channel (grain size, organic carbon content) and bedform distribution (Side-Scan Sonar) allows a distinction between two types of sedimentation to be made: a fluvial–marine sedimentation (favoured by tidal action) and an autochthonous sedimentation that is related to water mixing. The net transport of sediments is towards the sea because of the tidal current asymmetry. The autochthonous deposits and the sediments from the extreme fluvial floods that tides are unable to rework are preserved on the meandering convex margins under low tidal velocities. These areas acquire a lateral tidal-bar morphology with cohesive beds because the narrow geometry of the estuary inhibits the presence of longitudinal tidal bars, as may be expected in tide-dominated systems. The higher energy zones (deeper zones of the channel) become bypassing channels where the flocculated material cannot be settled on the bottom; this part of the channel then develops sandy beds with mesoforms as ebb-oriented two-dimensional and three-dimensional dunes (sand waves and megaripples).