The Thermaic gulf is a closed, relatively shallow gulf which is very important as a place for fish reproduction on a national level and a region of crucial importance for the northern Aegean.
It is characterized by an even bed with low declivities and depths ranging from 2 to 200 m, at its southern side. The accumulation of sediment from the three great rivers (Axios, Loudias, Aliakmon) which flow into the gulf has shaped the bed as well as the morphology of the western and northern coastline.
The Thermaic gulf receives fresh water from the three great rivers, which are estimated to jointly drain a region of about 3,500,000 hectares. The mean annual supply of fresh water that the Thermaic used to receive was, according to historic evidence, 276 cubic meters per second, however more recent measurements have revealed a dramatic decline in the quantity of fresh water that comes into the gulf.
There has been a similar drop in the quantity of sediment deposited in the Thermaic gulf, which is of vital importance to its ecosystem. Following the diversion of the Axios and the Aliakmon rivers in the first decades of the 20th century, an increase in the amount of alluvia was observed also due to the increased speed of water and the erosion of the banks.
With the construction of the Aliakmon dams, almost all alluvia are retained, resulting in the drastic reduction of the deltaic evolution, the salinisation of the ground water and soil, the penetration of sea water (especially during the astronomical and meteorological tide) and the general degrading of the wetlands.
Human activity affects considerably the coastal ecosystem of the Thermaic gulf and its natural evolution. The construction of dams in the main rivers has drastically decreased the supply of water and alluvia, resulting in the retreat of the deltaic formation of rivers, due to sea erosion and the presence of brackish water in the underground aquifers.
Karageorgis Α.P., M.S. Skourtos, V. Kapsimalis, A.D. Kontogianni, N.Th. Skoulikidis, K. Pagou, N. P. Nikolaidis, P. Drakopoulou, B. Zanou, H. Karamanos, Z. Levkov, Ch. Anagnostou, 2005. An integrated approach to watershed management within framework: Axios River catchment and Thermaikos Gulf. Regional Environmental Change (2005) 5: 138–160.