Greek case studies: Geomorphological changes on the coastal area of the Inner Thermaikos Gulf over the past 150 years
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This article presents the geomorphological evolution of the deltaic coastal zone of the Inner Thermaikos Gulf, northern Greece, over the past 150 years (1850–2000). It should be read in conjunction with the article Greek case studies: Long term geomorphological changes in the coastal zone of the Thermaikos Gulf, Salonika Region, North Greece.
The reconstruction of the geomorphological evolution is based on a detailed analysis of historical bathymetric charts (Albanakis et al., 1993; Poulos et al., 1994; Kapsimalis et al., 2005). The evolution is mainly caused by human-induced modifications in the basins of two main rivers (Axios and Aliakmon) and two smaller rivers (Gallikos and Loudias), which discharge into the Gulf (Poulos et al., 2000; Karageorgis et al., 2005; Karageorgis et al., 2006). Over the past 150 years, the Gulf has accumulated a net sediment volume of 1230 x 106 m3, at an average rate of accretion of 8 x 106 m3 a-1 or 12 x 106 tons a-1. Some 85% of this load is deposited around the active river mouths, with approximately 15% being dispersed offshore (Kapsimalis et al., 2005). Over this period of time, three evolutionary ‘stages’ are identified on the basis of changes in sediment supply and associated human interference.
Stage I: 1850–1916
From the middle of the 19th century to the early 20th century, the coastal system of the Gulf evolved naturally. A net marine sediment supply of some 430 x 106 m3 resulted from fluvial discharge at an average rate of 6.5 x 106 m3 a-1 or 10 x 106 t a-1 (Kapsimalis et al., 2005; Karageorgis et al., 2006). This input resulted in the progradation of the delta complex, especially near the active river mouths.
Stage II: 1916–1956
Subsequently, human intervention to the natural system of the deltaic plain was at a maximum, especially in the second part of stage II (1934–1956). Major causes were:
- artificial realignment of the main river channels (1934);
- drainage of the Yiannitsa Lake and the Loudias Swamps (1935);
- the instigation of other land reclamation projects (Karageorgis et al., 2006).
The net riverine sediment supply increased considerably, up to 900 x 106 m3; this corresponds to an average input of 18 x 106 m3 a-1 or 28 x 106 t a-1 (Kapsimalis et al., 2005). Over this period, rapid progradation occurred at the active mouths of the Axios and Aliakmon Rivers. At the same time, some of the fine-grained components of the sediment load were dispersed over the Inner Thermaikos Gulf.
Stage III: 1959–2000
The last stage is strongly characterized by a further ‘cycle’ of human interference (Georgas and Perissoratis, 1992; Poulos et al., 2000; Karageorgis et al., 2005). This includes the construction of irrigation reservoirs and hydroelectric dams. Such structures caused a significant reduction in sediment supply, leading to an overall erosional phase over the Gulf and a mean sediment loss of 2.5 x 106 m3 a-1 or 4 x 106 t a-1 (Kapsimalis et al., 2005). During this period, the river mouths underwent rapid retreat. The ‘active’ rivers prograded only minimally, while the lower reaches of the deltaic plain were subjected to coastal flooding. Furthermore, human activities affected the circulation pattern of the Gulf (Nikolaidis et al., 2006) and the texture of the offshore sea-bed sediments (Kapsimalis et al., 2005). This resulted in the alteration of the associated benthic habitats (Voutsinou-Taliadouri and Varnavas, 1995).
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