Abstract

The Ebro River, draining approximately one-sixth of the Iberian peninsula, forms the fourth largest delta (350 km ²) in the Mediterranean. A detailed investigation based on aerial photographic and early map (to 16th century) analysis, coring and physical oceanographic and hydrographic surveys, shows that this delta has undergone remarkable modification during historic times.

The factors affecting recent delta evolution and morphology include fresh water discharge and short-term hydrologic fluctuations, and coastal processes. The latter involve the interaction of seiches plus tides (low range), wind generated waves, and occasional storm wave fronts. However, the most significant factor in recent years has been the notable decrease (by about 15%) of river discharge, as a result of damming and increase in irrigation.

The sediments of four major environments form the delta plain: fluvial, paludal, fluviomarine, and holomarine. The bay and shelf deposits are intimately related to the fluviomarine and holomarine. These six facies build up the delta complex and can be grouped as types of sequences: (1) transgressive, (2) coarsening-upward, and (3) fining-upward. The type and number of sequences change in each borehole depending on situation and distance to the inland limit of the delta plain. A sharp increase in the effective rate of sedimentation towards the top of these sequences, correlative to a slowing in the rate of sea level rise, has been detected.

The Ebro deltaic plain consists of three pronounced delta lobes extending 26 km seaward. The development of two of these lobes has notably increased the delta plain surface area during the past four centuries. A major flood in 1937 resulted in a fourth lobe when a new channel broke through one of the existing lobes. This channel diversion resulted in the abandonment of the older lobe in less than 20 years. By 1971, four generations of emerged and submerged mouth bars had formed.

The erosion of abandoned lobate deltas provides sediment for the development of beaches and nearshore spits, whose changes in dimension and form are extremely rapid; nearshore erosion also accounts for modification of the submerged prodelta zone.