Growth patterns of the acheloos and evinos deltas,Western Greece

The coastal plain complex on the north side of the Gulf of Patras has been built by the overlapping Acheloos and small Evinos deltas. Upstream river channels are braided; on the lower part of the delta they are meandering. Classical writings indicate that the present Acheloos channel has been occupied for at least 2300 years; air photographs show three former major distributary channels. The present annual sediment discharge (suspended: 3–4 million tons; bedload: 2 million tons) is sufficient to build all the modern delta plain by progradation of these four distributaries in the last 5000–7000 years of relatively stable sea level.Old maps and air photographs show that river mouths prograde by periodic plugging of sub-distributaries within 1 km of the mouth. Where the river progrades into deep water (> 20 m) a steep sandy pro-delta slope forms and a shoal mouth bar is developed by wave reworking. The freshwater muddy plume of the Acheloos extends up to 15 km offshore. Analysis of about 30 bottom samples shows that very silty sediment settles within a few kilometers of the river mouth, becoming progressively more clayey offshore. Marine reworking of abandoned distributaries takes place rapidly (0.5 km in the first ten years, and a further 0.5 km in the next 30 years). Sand is redistributed to form parallel accretionary spits and barrier beaches. A shallow pro-delta platform up to 2 km wide has formed by such reworking along the southern margin of both deltas. Gradual subsidence (less than 0.5 mm/year) leads to flooding of shallow lagoons behind barrier beach systems.Subaerial delta facies have been mapped in the field, and extensively sampled in artificial channel cuts. Normal tidal range is only 16 cm, but storm tides up to 1 m high flood low supratidal flats that locally extend several kilometres inland.Broad immediately subtidal silt flats are found on the inner part of the western Evinos pro-delta platform. Silt is drifted in suspension by strong winter easterly winds from the Evinos mouth, and the pro-delta platform is sufficiently wide and shallow to damp down most large waves that would subsequently remove the silt.Where sand supply is limited, the coastline is marked by a sandy low-tide terrace, and a berm or storm ridge built of variable proportions of sand and reeds. Only near delta mouths is sand supply sufficient for broad barrier beach-dune systems to form.