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Recent Evolution of a Mediterranean Deltaic Coastal Zone: Human Impacts on the Inner Thermaikos Gulf, NW Aegean Sea

Posted on: Wednesday, 16 November 2005, 06:00 CST

By Kapsimalis, Vasilios; Poulos, Serafim E; Karageorgis, Aristomenis P; Pavlakis, Petros; Collins, Michael

Abstract:

The Inner Thermaikos Gulf is located in the northwestern Aegean Sea, receiving water and sediment fluxes from the Axios, Aliakmon, Gallikos and Loudias Rivers. The geomorphological and sedimentological evolution of the system is reconstructed for the last 150 years (1850-2000), on the basis of detailed analysis of historical bathymetric charts. Late Holocene history is considered within the context of changing riverine sediment supply and human activities. Three evolutionary stages are identified. Stage I (1850- 1916) corresponds to a natural phase of rapid deltaic progradation and sea-floor deposition, with an average sediment accumulation rate of 6.5 10^sup 6^ m^sup 3^ a^sup -1^. During Stage II (1916-1956), human interference (e.g. artificial changes in river delta plains, realignment of channels and land reclamation schemes) to the deltaic system increased sediment delivery to the coastal waters by a factor of three; this, in rum, enhanced the progradation of the active river mouth areas. In contrast, Stage III (1956-2000) is characterized by significant coastline (deltaic) retreat and erosion of the adjacent sea floor (net loss of 2.5 106 m^sup 3^ a^sup - 1^); this was as a result of extensive river damming, which trapped a significant part of the sediment moving seaward. Furthermore, these human impacts have affected the character of the surficial sea- bed sediments of the Gulf, by reducing the proportion of mud. The response of the deltaic margin of the Inner Thermaikos Gulf to various anthropogenic interventions seems to be analogous to that of other deltas in the Mediterranean region where large drainage projects, the development of irrigation networks and dam construction have taken place within their river basins.

Keywords: Thermaikos Gulf, Aegean Sea, delta evolution, human impact, sediment budget.

Deltaic coastal systems form a complex interface between continental and marine environments, with high levels of spatial and temporal variability in the prevailing natural processes (Wright & Coleman 1973). Superimposed upon the natural evolution, deltaic coasts have undergone considerable development, such as intensive agriculture, industrialization and urbanization. Thus, human interference incorporates: (1) changes in fluvial freshwater and sediment fluxes; (2) canalization and flood control works; (3) land reclamation schemes; (4) channel dredging for aggregates; (5) more recently, structures for coastal protection. Within this context and against a background of minimal tidal range (in the case of the Mediterranean Basin), within the Holocene, the two principal factors controlling the morphological and sedimentary evolution of any deltaic coastal system are the fluvial water and sediment supply and the wavederived longshore drift (McManus 2002).

Globally, riverine sediment fluxes have been modified by various anthropogenic activities (Vorosmarty et al. 2003). Since late prehistoric times, long periods (centuries to millennia) of increased sediment delivery to the coast have been attributed to changes in land use of the catchment area (mainly as a result of deforestation and crop farming) (Van Andel et al. 1990; Walling 1999), whereas short periods (over the order of a few decades) of significant increase in the detrital sediment discharge have been attributed to the canalization and straightening of lower river reaches. The former situation has been documented by Woodward (1995), in the case of the Mediterranean rivers, where human impact upon the Ebro, Po and Axios rivers, for example, extended back to 2500 years BP (Milliman 2001). The latter situation has been reported for the River Axios (Poulos et al. 1996). Since the beginning of the 20th century, a dramatic reduction in sediment supply to the coastal zone has occurred globally, following the construction of dams for irrigation and hydroelectric power schemes (Vorosmarty et al. 2003). In addition, a further reduction in sediment supply has been attributed to sand and gravel extraction along river channels (Marchetti 2002). This dramatic reduction has not only stopped delta-front progradation, but has initiated coastal erosion, which includes coastline retreat and/or the removal of nearshore sediments (McManus 2002; Yang et al. 2003), together with the landward intrusion of saline groundwaters (Oueslati 1995; Zalidis 1998). In the case of the Mediterranean Basin, it has been estimated that the potential (natural) sediment supply has been reduced by almost 50% since the middle of the 20th century (Poulos & Collins 2002). Such sediment reduction, exceeding 90% in the case of the larger river systems, has already initiated annual rates of coastal erosion ranging from tens of metres (e.g. Ebro River, Jimenez et al. 1997; Rhne River, Bird 1988; Po River, Simeoni & Bondesan 1997), to hundreds of metres (e.g. Nile River, Fanos 1995; Stanley 1996).

This study examines the geomorphological and sedimentological evolution of the deltaic coastal zone of the Inner Thermaikos Gulf, NW Aegean Sea, located between 4025' and 4040'N and 2235' and 2300'E. Although a preliminary analysis of changes in the deltaic coastline has been presented previously by Poulos et al. (1994, 2000), the present study, with the use of appropriate geographic information system (GIS) tools, not only examines coastal changes, but extends this investigation to the adjacent sea bed. Such an approach aims to provide: (1) a detailed description (qualitatively and quantitatively) of the overall pattern of deltaic coastal zone evolution during historical times (from 1850 to 2000) in relation to human-induced changes in freshwater and sediment supply, in response to various anthropogenic constructions and interventions either within the upstream basin (i.e. dam construction) and/or on the coastal deltaic plain (canalization, land reclamation etc); (2) an estimation of nearshore and offshore (extending to the southern limit of the Inner Thermaikos Gulf) changes in bathymetry, defining areas of accretion or erosion; (3) a volumetric assessment of sediments that have entered the Gulf over the last 150 years; (4) the identification of changes in the sedimentary (grain-size) character of the surficial sea-bed sediments, providing an indication of their mobility.

The study area: the Inner Thermaikos Gulf

Physical geographical setting

The Inner Thermaikos Gulf is located in the southern Balkan Peninsula and incorporates the mouths of the Axios, Aliakmon and Gallikos Rivers (Fig. 1a). The subaerial deltaic plains of these rivers (Table 1), together with the artificially drained lake of Yiannitsa and the River Loudias (operating now as a drainage canal for the Yiannitsa Lake), form the Plain of Thessaloniki, c. 1500km^sup 2^ in area (Fig. 1b). Today, the Plain hosts a population in excess of 1 million; it is one of the most important socioeconomic areas of the southern Balkans and the second most important zone in Greece. The seaward boundary of the Inner Thermaikos Gulf is taken here as an imaginary west-east line, some 20 km in length, connecting the promontories of Athens and Epanomi (Fig. 1b). Water depths vary from a few metres, adjacent to the delta fronts, to c. 35 m at the southerly limit (Fig. 2). At the northeastern extension of the Gulf lies the Bay of Thessaloniki, where the harbour of Thessaloniki is located (Fig. 1b).

Fig. 1. Location maps: (a) the Inner Thermaikos Gulf catchment; (b) the Yiannitsa-Thessaloniki Plain and its coastal system. F.Y.R.O.M., Former Yugoslav Republic of Macedonia.

Table 1. General characteristics of the main rivers discharging along the coastline of the Inner Thermaikos Gulf deltaic coast (after Poulos et al. 2000)

The morphology of the deltaic coastline, in the essentially tideless environment of Greek waters (Tsimplis 1994), results mainly from interaction between the fluvial water and sediment discharge and the prevailing wave activity; the latter is relatively low (mean monthly wave power approaching the Inner Gulf is <30 W m^sup -2^), when compared with other Greek deltas (i.e. 70-1454Wm^sup -2^ for the Pinios River (Poulos et al. 1993) and >25 000 W m^sup -2^ for the Alfios River (Poulos et al. 2002)). This pattern is related to the restricted wave fetches, caused by the semi-enclosed and shallow character of the Inner Thermaikos Gulf, leading to the formation of a birdfoot-type delta (Galloway 1975).

Fluvial water and sediment fluxes

Most of the freshwater inputs to the Inner Thermaikos Gulf coastline originate from the Axios, Aliakmon and Gallikos Rivers (Table 2), which together have been shown to represent an overall annual mean discharge of 276 m^sup 3^ s^sup -1^, between 1926 and 1970 (Therianos 1974). However, recent flow measurements (1997- 1998) have identified a substantial decrease in the freshwater supply (to some 130 m^sup 3^ s^sup -1^), attributed mainly to the increasing consumption of freshwater because of agricultural and urban development (Karageorgis & Anagnostou 2001).

The principal rivers (Axios, Aliakmon and Gallikos) supply annually some 28 10^sup 6^ tonnes of total sediment load (Tab\le 2). This sediment flux, associated with high sediment yields (>5001 km^sup -2^), results from intensive weathering processes induced by: (1) highly variable climatic conditions (i.e. large daily and seasonal air temperature fluctuations and relatively high precipitation levels); (2) erodible lithologies; (3) the mountainous terrain of the region, with high relief ratios (Table 1); (4) sparse vegetation cover (Woodward 1995). During the second half of the 20th century, such high sediment discharges have not reached the coastal zone, as they accumulate behind a number of irrigation and hydroelectric dams (Poulos & Collins 2002; Karageorgis et al. 2004).

Fig. 2. Recent bathymetrie chart of the Thermaikos Gulf (Hydrographie Service of the Greek Navy 2000), based upon the Greek Geodetic Reference System (1987). Water depths in metres.

Table 2. Freshwater and sediment fluxes of the main rivers discharging into the Inner Thermaikos Gulf

Delta evolution

The rapid progradation of the deltaic complex since the 5th century BC (Fig. 3) has been established (Sturck, 1908) from archaeological data and historical records from ancient travellers. At that time, the ancient towns of Skydra (to the west) and Pella (to the north) were located adjacent to the sea; today they are located some 30 km from the coast. During the second half of the 20th century, the evolution of the deltaic coast of the Inner Thermaikos Gulf has been investigated by Evmorphopoulos (1961), Kotoulas (1984), Poulos et al. (1994) and Albanakis et al. (1993). An assessment of the observed subsidence rates of the plain (10 cm a^sup -1^), for the period 1960-1999, has been provided by Stiros (2001).

Marine sediments

Modern sedimentary processes in the Thermaikos Gulf have been studied extensively in recent decades by Lykousis et al. (1981), Lykousis & Chronis (1989), Poulos et al. (1996) and Karageorgis & Anagnostou (2001, 2003). Similarly, Georgas & Perissoratis (1992) and Poulos et al. (1994) have provided a socio-economic appraisal of the ranges of human activities over the area. In general, the bottom sediments consist mainly of silt- and clay-sized particles, with increasing amounts of clay occurring offshore. Sand is abundant near the coastlines, adjacent to the river mouths and, particularly, over the eastern part of the Gulf (as relict deposits). Furthermore, Lykousis & Chronis (1989), interpreting high-resolution (3.5 kHz) seismic profiles taken from the Inner Thermaikos Gulf, have indicated that the thickness of the Holocene sedimentary cover exceeds 20 m near the river mouths and decreases to <4 m over the eastern part of the Gulf (Fig. 4).

Fig. 3. Palaeogeographical evolution of the Thessaloniki Plain during historical time, based upon information provided by ancient Greek writers (modified from Stiirck 1908).

Materials and methods

A series of bathymetrie charts were obtained from the UK Hydrographie Office (Taunton), dated 1850, 1916, 1947 and 1956, and the Hydrographical Service of the Greek Navy (Athens, Greece), published in 2000. All the charts were projected onto the Greek Geodetic Reference System (1987) and processed with the use of the ArcView GIS (Version 3.2). Quantitative estimates of the volumetric changes in the offshore water depths between the time periods of the available charts were derived using the SURFER software package. The weight of the sediments accumulated in front of the river mouths was extrapolated from the sediment volume and bulk density; the latter is equal to c. 1.5 g cm^sup -3^, according to Lykousis et al. (2002).

Comparison between the various charts requires consideration of various potential errors, which include: (1) vertical and horizontal datum inconsistencies; (2) the absence or use of different georeference systems; (3) computer-gridding errors. To minimize such errors, standard terrestrial reference points (such as churches, capes, lighthouses), were selected; these were used for an intercomparison of shoreline position and offshore bathymetrie contours, for all of the charts. Furthermore, a 0.5 m range in sea- floor change was designated as a 'moderate' degree of error in the bathymetrie comparisons; as such, this has been characterized as a zone of 'no significant' change (List et al. 1997).

The datasets used for the study of the grain-size characteristics of the surficial sea-bed sediments were collected in 1978 (Chronis 1986) and 1997-1998 (Karageorgis & Anagnostou 2003).

Results and discussion

The reconstruction of the historical charts (Fig. 5) has revealed major changes in the evolution of the coastline and the associated bathymetry of the Inner Thermaikos Gulf. These include significant spatial trends in erosion and accretion dynamics over the past 150 years. Three evolutionary stages have been identified: an early stage, from 1850 to 1916 (Stage I); an intermediate stage, from 1916 to 1956 (Stage II); and a late stage, from 1956 to 2000 (Stage III). Spatial changes in the seabed elevations, accompanied by a volumetric estimation of the sediment deposited and/or eroded from the sea floor of the Inner Thermaikos Gulf, are presented in Table 3. The thickness of the deposits and the areas affected by river mouth evolution are summarized, on the basis of the earlier evolutionary stages (see above), in Table 4.

Fig. 4. Thickness of the Holocene sedimentary cover in the Inner Thermaikos Gulf, in milliseconds (where 10 ms is equivalent to c. 8m), based upon the interpretation of geophysical data (after Lykousis & Chronis 1989).

Stage I

The coastal and bathymetrie changes shown between the 1850 and 1916 charts reveal progradation of the deltaic coast and, in particular, the area incorporating the active mouths of the Axios and Aliakmon Rivers. Over this period, the position of the active mouth of the Axios River varied. In particular, before the 1890s, the river mouth was located at the Kavoura Cape, opposite the Megalo Emvolo Cape. Subsequently and until 1934, the mouth was located near Palaiomana, opposite the Mikro Emvolo Cape, some 10 km to the NE (Konstantinidis 1989). However, it can be assumed that these mouths (Kavoura and Palaiomana) were active concurrently, especially during high freshwater discharges. During high river discharges, overbank flows flooded extensive areas of the delta plain, creating swamps, coastal lakes and ephemeral channels. Conversely, under the influence of southerly winds, the sea invaded the coastal zone of the deltaic platform, infilling the salt marshes and lagoons.

Although some of the sediment load transported by the Axios River was deposited on the deltaic plain during floods, the greater proportion was discharged into the Thermaikos Gulf. Most of the coarse-grained riverine sediments are likely to have been trapped on the seaward side of the mouth, forming a prodeltaic lobe. The fine- grained suspended sediment load would have been dispersed offshore, covering almost the whole of the sea bed over the inner part of the Gulf (including the navigation channel to the port of Thessaloniki).

Sediment deposited during Stage I (Fig. 6), over the Kavoura mouth of the Axios River, covered an area of 25 10^sup 6^ m^sup 2^, with a maximum thickness of 22 m; this corresponds to a sediment volume of 85 10^sup 6^ m^sup 3^. Similarly, at the Palaiomana mouth, the new prodeltaic deposit covered an area of 23 10^sup 6^ m^sup 2^, with a maximum thickness of 16 m and a total volume of 60 10^sup 6^ m^sup 3^ (Table 4).

The two main river mouths were active simultaneously during Stage I (over 66 years), when the accumulation rates for the Kavoura and Palaiomana mouths were of the order of 1.3 10^sup 6^ m^sup 3^ a^sup -1^ and 0.9 10^sup 6^ m^sup 3^ a^sup -1^, respectively (Table 4). However, Konstantinidis (1989) has mentioned that the active mouth of the Axios River was located near the Kavoura Cape from 1850 to 1900 (50 years) and for the following 16 years (1900-1916) in Palaiomana. If this is correct, then the accumulation rates for the Kavoura and Palaiomana mouths become 1.7 10^sup 6^ m^sup 3^ a^sup - 1^ and 3.8 10^sup 6^ m^sup 3^ a^sup -1^, respectively (Fig. 6b). The much lower accumulation rate at the Kavoura mouth could be attributed to the entrapment of large water and sediment fluxes within the Loudias Swamps (some 80 km^sup 2^).

Along the deltaic coast, between the Axios River mouths, no significant deposition is observed (Fig. 6a), and this implies that the secondary drainage networks played only a minor role in sediment dispersion, except during extreme floods.

Fig. 5. Digital output of the various (dated) bathymetrie charts provided by the UK Hydrographie Office (Taunton). The projection system used is the Greek Geodetic Reference System (1987). Water depths in metres.

Table 3. Estimation of morphological changes of the sea bed of the Inner Thermaikos Gulf over various time intervals

Over the same period of time (Stage I), the Aliakmon River was in a phase of rapid progradation, incorporating a meandering channel system. Some of the sediment load was trapped within the meanders, forming extended point-bars. The remainder of the load reached the coast, contributing to the progradation of the mouth and the offshore deposition of fine-grained sediments. During this period, some 340 10^sup 6^ m^sup 3^ of sediments accumulated over an area of 55 10^sup 6^ m^sup 2^ with a maximum thickness of 20 m (Table 4). Furthermore, a limited amount of the fine-grained sediments (mostly muds) was transported by a cyclonic surface circulation pattern (Poulos et al. 2000) to the west and deposited in the Methoni Bay (Figs Ib and 6). Overall, the derived rate of deposition is 5.2 10^sup 6^ m^sup 3^ a^sup -1^, which is considerably higher than either of those attributed to the Axios River mouths.

In general, the Axios River carried substantially less sediment load than the Aliakmon River, although it has a much larger drainage bas\in (Table 2). According to Poulos et al. (2000), this difference in sediment yield could be attributed to: (1) differences in lithology, with the Axios River draining mostly resistant mafic and acidic rocks whereas the Aliakmon River drains mostly weaker clastic formations (Table 1); (2) the geomorphological characteristics of the two catchments, with the Axios River having a much lower relief ratio (1.1) than the Aliakmon River (1.7), incorporating a drainage network with a large number of primary and secondary branches and meanders, together with a more gentle slope to its deltaic plain; (3) soil degradation, associated with land use changes.

Table 4. Estimates of morphological changes of the subaqueous deltaic wedges in front of the active river mouths

Fig. 6. (a) Spatial distribution of accretional and erosional patterns in the Inner Thermaikos Gulf during Stage I (1850-1916), derived from the comparison of bathymetrie charts; (b) schematic representation of the estimated sediment influx, at the active river mouths.

Over this period, the Loudias River drained the Yiannitsa Lake and Loudias Swamps to the west of the Kavoura mouth (Fig. 1b). These systems discharged into the Thermaikos Gulf either directly, as an independent river with its mouth located between those of the Axios (Kavoura) and Aliakmon, or indirectly, as a tributary of the western channel of the Axios River (Konstantinidis 1989). Finally, the Gallikos River, although its mouth was located very close to the Palaiomana mouth of the Axios River (Fig. 6), was active independently. The Gallikos River drains a much smaller catchment area (1230 km^sup 2^) and is characterized by ephemeral (seasonal) flows.

Fig. 7. (a) Spatial distribution of accretional and erosional patterns in the Inner Thermaikos Gulf during Stage II (1916-1956), derived on the basis of the comparison of bathymetrie charts; (b) schematic representation of the estimated sediment influx, at the active river mouths.

During Stage I, the sea floor of the Inner Thermaikos Gulf, excluding the areas of active deltaic progradation, does not appear to have undergone any significant changes (Fig. 6a). Limited sediment accumulation can be observed over the northeastern part of the Gulf and close to the port of Thessaloniki; this could be associated with the Palaiomana mouth of the Axios River. Over some parts of the central area of the Gulf, the sea bed appears to have undergone erosion (>0.5 m) and this is probably related to near-bed current activity.

Stage II

Within this period (1916-1956), two substages can be identified: the first (1916-1930) is characterized by natural coastal evolution, and the second (1931-1956) by increasing human interventions on the deltaic plain in the form of canalization and land reclamation.

During the first sub-period (1916-1930), the deltaic plain was evolving naturally. The active mouth of the Axios River was that of Palaiomana (Fig. 7a), causing siltation of the port of Thessaloniki. Such siltation led to rerouting of the Axios River, diverting its active mouth to its previous location near the Kavoura Cape (Fig. 7a). Construction commenced in 1930 and was finalized in 1934. Accompanying works, undertaken within the deltaic plain, were: (1) the artificial drainage of the Yiannitsa Lake; (2) associated construction of the Loudias drainage channel (39 km long, 50 m wide and 5 m deep, along the former route of the Loudias River); (3) land reclamation works on the Loudias Swamps (1933-1935), located to the NW of the Kavoura mouth (Fig. 1b). Following diversion of the main tributary of the Axios River (1934), an extended birdfoot delta developed near the Kavoura mouth. During the second subperiod (1931- 1956), the Axios River delta developed farther to the south and SW. Erosion occurred over the eastern part of its active mouth, affecting the earlier deposits of the abandoned Kaboura mouth (Fig. 7). At this time, the old route of the Axios River, ending at the Palaiomana mouth, functioned episodically during floods.

Over the same sub-period, the lower meandering course of the Aliakmon River was aligned artificially; this decreased its length from 52.2km to 30.4km (Konstantinidis 1989), leading to further progradation of its mouth area to the NE and to the development of an extended birdfoot delta. Similarly, works were carried out along the lower course of the Gallikos River, focusing upon the protection of its subaerial delta from flood events (Konstantinidis 1989). Overall, Stage II (1916-1956) was characterized by substantial sediment deposition within the Inner Thermaikos Gulf and this is estimated to be of the order of 900 10^sup 6^ m^sup 3^ (Table 3). Although sediments were deposited over the adjacent sea floor, the highest rates of accretion occurred in front of the active river mouths (Fig. 7b). Thus, at the Palaiomana mouth, some 140 10^sup 6^ m^sup 3^ of sediments settled over an area of 30 10^sup 6^ m^sup 2^ corresponding to a maximum thickness of 21 m and an accumulation rate of 3.5 10^sup 6^ m^sup 3^ a^sup -1^ (Table 4). However, on the basis of the assumption that the majority of these sediments had been deposited before the artificial diversion of the main Axios River channel (i.e. from 1916 to 1934), the accumulation rate could be increased to 7.8 10^sup 6^ m^sup 3^ a^sup -1^. At the Kavoura mouth during Stage II, if only a small amount of sediment was deposited prior to 1934, then sediment volume of 220 10^sup 6^ m^sup 3^ was associated with an accumulation rate of 10.0 10^sup 6^ m^sup 3^ a^sup -1^ (between 1934 and 1956).

A general increase in sediment supply can be attributed, mainly, to alignment of the Axios River and straightening of its lower course; this caused elevated flow velocities and (possible) erosion of older terrestrial deposits (Konstantinidis 1989). Likewise, the bulk of the sediment load was transported directly to the sea, limiting the deposition of sediment on the deltaic plain. Furthermore, such high sediment discharge rates also reflect the absence of any major dams along the river channel. However, a small reservoir (capacity 3.6 10^sup 6^ m^sup 3^) was constructed, in 1938, on a tributary of the Axios River (the Treska River) some 300 km upstream of the river mouth.

Artificial cut-off of meanders in the Aliakmon River deltaic plain would also have increased sediment supply into Methoni Bay (Fig. 1a). The volume of the deposits that accumulated in front of the Aliakmon River mouth was 270 10^sup 6^ m^sup 3^, corresponding to an average accumulation rate of 5.4 10^sup 6^ m^sup 3^ a^sup - 1^. Over the same period, it proved impossible to estimate the progradation rate of the Gallikos River, especially following the engineering works. This is due primarily to its proximity to the Paliomana mouth of the Axios River, but also to its low sediment discharge.

Earlier investigations of sediment supply, based upon changes in the coastline (from 1850 to 1952), have revealed an annual sediment input of 16 10^sup 6^ m^sup 3^ a^sup -1^ (Evmorphopoulos 1961) or 16.4 10^sup 6^ m^sup 3^ a^sup -1^ (Kotoulas 1984). Calculations undertaken here, on the basis of comparison of the 1850 and 1956 bathymetrie charts, show a net sediment accumulation of 1350 10^sup 6^ m^sup 3^ or 12.8 10^sup 6^ m^sup 3^ a^sup -1^. This difference between the results of the present study and the abovementioned investigations can be attributed, mainly, to: (1) the different approaches used, and especially to the fact that the present investigation takes into account not only coastline displacements but also changes in offshore water depths; (2) the escape seawards of part of the very fine-grained suspended and dissolved material (c. 10-15% of the total sediment load), which does not contribute to long-term coastal change.

In general, sea-floor changes in the Inner Thermaikos Gulf reveal an accretional pattern, associated with high river sediment discharges. Net sediment deposition is of the order of 900 10^sup 6^ m^sup 3^ (Table 3), with the impact of such sedimentation being most pronounced at the river mouths. In some places erosion has also taken place, including, for example, to the east of the Kavoura Cape, from both sides of the Mikro Emvolo Cape, and to the south of the Megalo Emvolo Cape. These erosional areas are related to near- bed currents and the overall water circulation pattern within the Inner Thermaikos Gulf (Balopoulos et al. 1986).

For the whole of the Inner Thermaikos Gulf, the overall accretion of sediment (1020 10^sup 6^ m^sup 3^) (Table 3), in comparison with that deposited in front of the river mouths (630 10^sup 6^ m^sup 3^), implies that some 40% of the terrestrial sediment inputs were dispersed seawards.

Stage III

The period between 1956 and 2000 witnessed an intensification of human intervention, resulting in substantial alterations to the natural systems. This followed, in turn, the construction of dams in the various catchments, for irrigation and/or hydroelectric power generation. Two irrigation dams were constructed along main channels of the rivers Axios (Prochoma) and Aliakmon (near Verroia), in 1958; these were located some 30 km and 50 km inland from their river mouths, respectively (Fig. 1b). Such construction was followed by the development of an extensive irrigation network over the Thessaloniki Plain, from 1958 to the end of the 1960s, associated with intensive agricultural activities such as rice cultivation. This development caused a significant reduction in the area of wetlands, together with massive consumption of river water. Furthermore, 11 dams have been constructed over the last 50 years along the Axios River, within the part of the catchment that belongs to FYROM (Former Yugoslav Republic of Macedonia), with a total capacity of 803 10^sup 6^ m^sup 3^ (Psilovikos & Psilivikos 1997). This development has resulted in a considerable reduction inthe volumes of freshwater reaching the Gulf. Mean annual water discharge, estimated from historical data, is 5.0 10^sup 9^ m^sup 3^ a^sup -1^ (Therianos 1974). This contrasts with recent (1995- 2000) measurements, which have revealed a considerable decrease to around 3.4 10^sup 9^ m^sup 3^ a^sup -1^ (Karageorgis et al. 2004). In addition, three more hydroelectric dams were constructed along the Aliakmon River between 1974 and 1989.

Entrapment of the bed load and the majority of the suspended material, within the reservoirs, has resulted in a dramatic reduction in sediment supply to the deltaic coastal plain. The mean annual sediment discharges were estimated from historical data (1970- 1987) to be (1-2) 10^sup 6^ t a^sup -1^. Recent (1995-2000) estimates are 10- to 20-fold lower (0.1 10^sup 6^ t a^sup -1^) (Karageorgis & Anagnostou 2001). Initially, such a reduction in sediment supply inhibited deltaic progradation, causing coastline retreat. Furthermore, reduction in the natural freshwater flow levels, through the retention of large quantities of water within the reservoirs and/or increasing consumption, has caused: (1) saline intrusion into the coastal deltaic areas (Stiros 2001); (2) deterioration of the wetlands; (3) seawater intrusion along the lower courses of the rivers, during storm surges (Konstatinidis 1989).

To control coastal erosion, especially along the abandoned Palaiomana mouth of the Axios River, coastal protection measures have been undertaken (from 1960 to 1974). These include the construction of 22 km of seawall (2.5 m in height) and five pumping stations that drain the agricultural plain. Parts of the seawall have been destroyed by rapid subsidence of the deltaic plain, which in some places reaches 10 cm a"1 (Stiros 2001). This subsidence is related to the decomposition of organic matter and compaction of fine-grained material accompanied by synsedimentary deformation. Such an effect has been enhanced by the extended, often illegal, overpumping of the deltaic aquifers (Andronopoulos et al. 1991). Coastal erosion has also been caused by the extraction (not always appropriately licensed) of aggregates (sand and gravel), from the lower courses of the rivers. Such activity has reduced bed load transport rates and, consequently, the process of deltaic infilling and progradation. This is a common situation in many Greek rivers and deltas, such as has been reported for the lower reaches Alfios River in the Peloponnese (Nicholas et al. 1999).

Fig. 8. (a) Spatial distribution of accretional and erosional patterns in the Inner Thermaikos Gulf during Stage III (1956-2000), derived on the basis of the comparison of bathymetric charts; (b) schematic presentation of the estimated sediment influx at the active river mouths.

Since 1956, the Inner Thermaikos Gulf has entered into an erosional phase, at an annual rate of c. 2.5 10^sup 6^ m^sup 3^ (Table 3). However, in front of the active river mouths and near the port of Thessaloniki, limited amounts of sediment are still deposited (Fig. 8).

Sediment deposited seawards of the Axios River mouth has accumulated at a rate of 1.4 10^sup 6^ m^sup 3^ a^sup -1^; this compares with an estimated sediment load for the Aliakmon River of 0.8 10^sup 6^ m^sup 3^ a^sup -1^ (Table 4). Such rates are approximately four (Axios River) and eight (Aliakmon River) times less than those estimated during Stage II (Tables 3 and 4). Most of the 'missing' sediment load has been retained behind the dams. Moreover, the remainder of the load that eventually reaches the sea could represent: (1) dissolved load; (2) very fine-grained suspended sediment; (3) material originating from the erosion of the deltaic plain. Furthermore, it is likely that a significant amount of suspended paniculate matter and the majority of the dissolved load are transported farther offshore, extending even to the continental shelf edge of the Outer Thermaikos Gulf (Lykousis & Chronis 1989; Karageorgis & Anagnostou, 2001, 2003). Sediment discharges associated with the Loudias drainage channel (with average discharges ranging from O to <10 m^sup 3^ s^sup -1^) and the Gallikos River (dry during most of the year, but undergoing subsidence over the coastal part of the deltaic plain) are considered to be minimal. A comparison of the surficial sediment distributions within the Inner Thermaikos Gulf in 1978 (Fig. 9a) and 1997-1998 (Fig. 9b) reveals that they are essentially similar, but that there is a distinct change in the northeastern part of the Gulf, where muds have been replaced by sandy muds and muddy sands. Hence, the percentage of sand has increased in places up to 15%, where it was previously minimal (<1%). Such a change is likely to be the result of: (1) a decline in sediment supply (i.e. eventual cessation of supply from the Axios River mouth (Palaiomana) and the Gallikos River); (2) sea-bed erosion, induced by local near-bed current activity, and probably enhanced by the navigation activities related to the port of Thessaloniki. This change in the sedimentary facies of the sea bed has also led to alteration of the associated benthic habitat (Voutsinou-Taliadouri & Varnavas 1995).

Conclusions

The recent sedimentary evolution of the Inner Thermaikos Gulf reflects anthropogenic impact on the natural evolution of the coastal system, through human-induced modifications to the sediment supply and water discharge (Fig. 10). This influence is most pronounced along the deltaic coastal zone, where two main rivers (Axios and Aliakmon) and two smaller rivers (Gallikos and Loudias) discharge into the marine waters.

In general terms, over the past 150 years, the Gulf has accumulated a net sediment volume of 1230 10^sup 6^ m^sup 3^, at an average rate of accretion of 8 10^sup 6^ m^sup 3^ a^sup -1^ or 12 10^sup 6^ a^sup -1^. Some 85% of this load has been deposited around the active river mouths, with approximately 15% being dispersed offshore.

Over this period of time, three evolutionary 'stages' have been identified on the basis of changes in sediment supply and associated human interference.

(1) From the middle of the 19th century to the early 20th century (Stage I: 1850-1916), the coastal system of the Gulf was controlled naturally, with a net marine sediment supply of some 430 10^sup 6^ m^sup 3^, at an average fluvial discharge of 6.5 10^sup 6^ m^sup 3^ a^sup -1^ or 10 10^sup 6^ t a^sup -1^. This input resulted in progradation of the delta complex, especially near the active river mouths.

Fig. 10. A summary of the human activities affecting the evolution of the deltaic system of the Inner Thermaikos Gulf in relation to sediment supply between 1850 and 2000. Horizontal bars indicate the duration of each activity; lighter and darker shading indicates lower and higher impact of human activities, respectively. (For details, see text.)

Fig. 9. Distribution patterns of the sedimentary facies on the sea bed of the Inner Thermaikos Gulf: (a) in 1978 (modified from Chronis 1986); (b) in 1997-1998 (modified from Karageorgis & Anagnostou 2001). Sampling stations are shown in both parts of the figure. Sedimentary facies are defined according to the classification of Folk (1974).

(2) Subsequently, during Stage II (1916-1956), especially in its second part (1934-1956), human intervention to the natural system of the deltaic plain was at a maximum through: (a) artificial realignment of the main river channels (1934); (b) drainage of the Yiannitsa Lake and the Loudias Swamps (1935); (c) the instigation of other land reclamation projects (Fig. 10). The net riverine sediment supply increased considerably to 900 10^sup 6^ m^sup 3^; this corresponds to an average input of 18 10^sup 6^ m^sup 3^ a^sup -1^ or 28 10^sup 6^ t a^sup -1^. 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.

(3) Stage III (1959-2000) is strongly characterized by a further 'cycle' of human interference. This incorporates the construction of irrigation reservoirs and hydroelectric dams. Such structures have caused a significant reduction in sediment supply, leading to an overall erosional phase over the Gulf and a mean sediment loss of 2.5 10^sup 6^ m^sup 3^ a^sup -1^ or 4 10^sup 6^ t a^sup -1^. During this period, the river mouths underwent rapid retreat. The 'active' rivers prograded only minimally, with the lower reaches of the deltaic plain being subjected to coastal flooding. Furthermore, human activities have affected the texture of the offshore sea-bed sediments by reducing the proportion of the mud fraction.

The authors gratefully acknowledge the helpful and critical revision of an earlier draft of the manuscript by J.C. Woodward (University of Manchester). We also appreciate the constructive comments and suggestions made by two anonymous reviewers.

References

ALBANAKIS, K., VAVLIAKIS, E., PSILOVIKOS, A. & SOTIRIADIS, L. 1993. Mechanisms and evolution of the delta of Axios River during the 20th century. In: Proceedings, 3rd National Geography Conference. Hellenic Geographical Society, Athens, 311-325.

ANDRONOPOULOS, V., Rozos, D. & HADZINAKOS, I. 1991. Subsidence phenomena in the industrial area of Thessaloniki, Greece. In: JOHNSON. A. (ed.) Land Subsidence. International Association of Hydrological Sciences Publication, 100, 59-69.

BALOPOULOS, E.T., COLLINS. M.B. & JAMES, A.E. 1986. Satellite images and their use in the numerical modelling of coastal processes, International Journal of Remote Sensing, 7, 905-919.

BIRD, E.G. 1988. Coastline Changes: a Global Review. Wiley. Chichester.

CHRONIS, G. 1986. Recent dynamic and Holocene sedimentation of Thermaikos Bay. PhD thesis. University of Athens.

EVMORPHOPOULOS, L. 1961. The changes in Thessaloniki Bay. Technical Annul of Greece, 205-208, 51-76.

FANOS, A.M. 1995. \The impact of human activities on the erosion and accretion of the Nile Delta coast. Journal of Coastal Research, 11. 821-833.

FOLK, R. 1974. Petrology of Sedimentary Rocks. Hemphill. Austin, TX.

GALLOWAY, W.E. 1975. Process framework for describing the morphologic and stratigraphie evolution of deltaic depositional systems. In: BROUSSARD, M.L. (ed.) Deltas. Models of Exploration. Houston Geological Society, Society of the Economic Paleontologists and Mineralogists Foundation, Houston, TX, 87-98.

GEORGAS, D. & PERISSORATIS, C. 1992. Implications of future climatic changes on the Inner Thermaikos Gulf. In: JEFTIC. L., MILLIMAN, J. & SESTINL G. (eds) Climatic Change and the Mediterranean. UNEP, E. Arnold, London, 495-534.

JIMENEZ, J.A., SANCHEZ-ARCILLA. A. & MALDOLADO, A. 1997. Long to short term coastal changes and sediment transport in the Ebro delta; a multi-scale approach. In: BRIAND. F. & MALDOLADO. A. (eds) Transformations and Evolution of the Mediterranean Coastline. Bulletin de l'Institut Ocanographique, CIESM Science Series 3, Special Publication. 18. 169-185.

KARAGEORGIS, A.P. & ANAGNOSTOU. C.L. 2001. Paniculate matter spatialtemporal distribution and associated sediment properties: Thermaikos Gulf and Sporades Basin, NW Aegean Sea. Continental Shelf Research. 21. 2141-2153.

KARAGEORGIS, A.P. & ANAGNOSTOU. C.L. 2003. Seasonal variation in the distribution of suspended paniculate matter in the northwest Aegean Sea. Journal of Geophysical Research. 108(C8). 3274, doi: 10.1029/ 2002 JCOO1672.

KARAGEORGIS, A.P., Skourtos, M.S., KAPSIMALIS. V., et al. 2004. An integrated approach to watershed management within the DPSIR framework: Axios River catchment and Thermaikos Gulf. Regional Environmental Change, doi, 10.1007/SlOl 13-004-0078-7.

KONSTANTINIDIS. K. A. 1989. Land Reclamation Project of the Thessaloniki Plain. Geothechnical Chamber of Greece, Thessaloniki.

KOTOULAS, D. 1984. Bodenbtrag und -ab/agenmgen in Griechenland am Beispiel des Gebirgslandes und der Edene von Thessaloniki. Interpravent, Villach.

LIST, J.H., JAFFE, B.E.. SALLENGER, A.H. & HANSEN. M.E. 1997. Bathymetrie comparisons adjacent to the Lousiana Barrier Islands: processes of large-scale change. Journal of Coastal Research, 13. 670-678.

LYKOUSIS. V. & CHRONIS. G. 1989. Mechanisms of sediment transport and deposition: sediment sequences and accumulation during Holocene on the Thermaikos plateau, the continental slope and basin (Sporades Basin), northwestern Aegean Sea, Greece. Marine Geology: 87, 15-26.

LYKOUSIS. V., COLLINS, M.B. & FERENTINOS. G. 1981. Modem sedimentation in the NW Aegean Sea. Marine Geology: 43, 111-130.

LYKOUSIS, V.. ROUSSAKIS, G.. ALEXANDRI. M.. PAVLAKIS, P. & PAPOULIA. I. 2002. Sliding and regional slope stability in active margins: North Aegean Trough (Mediterranean). Marine Geology. 186, 281-298.

MARCHETTI. M. 2002. Environmental changes in the central Po Plain (northern Italy) due to fluvial modifications and anthropogenic activities. Geomorphology, 44, 361-373.

MCMANUS. J. 2002. Deltaic responses to changes in river regimes. Marine Chemistry. 79. 155-170.

MILLIMAN, J.D. 2001. Delivery and fate of fluvial water and sediment to the sea: a marine geologist's view of European rivers. Scientia Marina. 65(2), 121-132.

NICHOLAS, A.P., WOODWARD. J.C., CHRISTOPOULOS, G. & MACKLIN. M.G. 1999. Modelling and monitoring the impact of dam construction and gravel extraction on rates of bank erosion in the Alfios River. Peloponnese, western Greece. In: BROWN, A.G. & QUINE, T.A. (eds) Fluvial Processes and Environmental Change. Wiley. Chichester, 117- 137.

OUESLATI. A. 1995. The evolution of low Tunisian coasts in historical times: from progradation to erosion and salinization. Quaternary International. 29-30, 41-47.

POULOS, S.E. & CHRONIS, G. 1997. The importance of the Greek river systems in the evolution of the Greek coastline. In: BRIAND. F. & MALDOLADO, A. (eds) Transformation and Evolution of the Mediterranean Coastline. Bulletin de l'Institut Ocanographique, CIESM Science Series 3. Special Publication, 18, 75-96.

POULOS. S.E. & COLLINS, M.B. 2002. Fluviatile sediment fluxes to the Mediterranean Sea: a quantitative approach and the influence of dams. In: JONES, SJ. & FROSTICK, L-E. (eds) Sediment Flux to Basins: Causes, Controls and Consequences. Geological Society. London, Special Publications, 191, 227245.

POULOS, S.E., COLLINS, M.B. & KE, X. 1993. Fluvial/wave interaction controls on delta formation for ephemeral rivers discharging into microtidal waters. Geo-Marine Letters. 13, 24-31.

POULOS, S.E.. PAPADOPOULOS, A. & COLLINS, M.B. 1994. Deltaic progradation in Thermaikos Bay, Northern Greece and its socio- economic implications. Ocean and Coastal Management, 22, 229-247.

POULOS. S.E., COLLINS, M.B. & EVANS, G. 1996. Water-sediment fluxes of Greek rivers, southeastern alpine Europe: annual yields, seasonal variability, delta formation and human impact. Zeitschrift fr Gomorphologie, 40. 243-261.

POULOS, S.E., CHRONIS, G.T., COLLINS, M.B. & LYKOUSIS. V. 2000. Thermaikos Gulf coastal system, NW Aegean Sea: an overview of water/ sediment fluxes in relation to air-land-ocean interactions and human activities. Journal of Marine Systems, 25, 47-76.

POULOS, S.E., VOULGARIS. G., KAPSIMALIS. V., COLLINS, M. & EVANS. G. 2002. Sediment fluxes and the evolution of a riverine-supplied tectonically active coastal system: Kyparissiakos Gulf, Ionian Sea (eastern Mediterranean). In: JONES, S.J. & FROSTICK, L.E. (eds) Sediment Flux to Basins: Causes. Controls and Consequences. Geological Society, London. Special Publications. 191, 247-266.

PSILOVIKOS, AN. & PSILIVIKOS. AR. 1997. Human versus natural processes. The case of Axios/Vardar River. In: MARINOS. P.O., KOUKIS. G.C., TSIAMBAOS, G.C. & STOURNARS. G.C. (eds) Engineering Geology and the Environment. Proceedings, International Symposium on Engineering Geology and the Environment of the international Association of Engineering Geologists, IAEG, 23-27 June 1997, Athens, Greece. Balkema, Rotterdam, 2859-2864.

SIMEONI, U. & BONDESAN, M. 1997. The role and responsibility of man in the evolution of the Italian Adriatic coast. In: BRIAND, F. & MALDOLADO, A. (eds) Transformations and Evolution of the Mediterranean Coastline. Bulletin de l'Institut Ocanographique, CIESM Science Series 3, Special Publication, 18, 75-96.

SKOULIKIDIS, N. 1993. Significance evaluation of factors controlling river water composition. Environmental Geology, 22, 178- 185.

STANLEY, D.J. 1996. Nile Delta: extreme case of sediment entrapment on a delta plain and consequent coastal land loss. Marine Geology, 129, 189-195.

STIROS, S.C. 2001. Subsidence of the Thessaloniki (northern Greece) coastal plain, 1960-1999. Engineering Geology, 61, 243-256.

STRCK, J. 1908. Mazedonisch Fahrten II, Die Mazedonischen Niederlaender. University of Serjevo, Serjevo.

THERIANOS, A.D. 1974. Rainfall and geographical distribution of river runoff in Greece. Bulletin of the Geological Society of Greece, 11, 28-58 (in Greek).

TSIMPLIS, M.N. 1994. Tidal oscillations in the Aegean and Ionian Seas. Estuarine Coastal and Shelf Science, 3, 201-208.

VAN ANDEL, T.H., ZANGGER, E. & DEMITRACK, A. 1990. Land use and soil erosion in prehistoric and historical Greece. Journal of Field Archaeology, 17, 379-396.

VRSMARTY, C.J., MEYBECK, M., FEKETE, B., SHARMA, K., GREEN, P. & SYVITSKI, P.M. 2003. Anthropogenic sediment retention: major global impact from registered river impoundments. Global and Planetary Change, 39, 169-190.

VOUTSINOU-TALIADOURI, F. & VARNAVAS, S.P. 1995. Geochemical and sedimentological patterns in the Thermaikos Gulf, North-west Aegean Sea, formed from a multisource of elements. Estuarine and Coastal Shelf Science, 40, 295-320.

WALLING, D.E. 1999. Linking land use, erosion and sediment yields in river basins. Hydrobiologia, 410, 223-240.

WOODWARD, J.C. 1995. Patterns of erosion and suspended sediment yield in Mediterranean river basins. In: FORSTER, I.D.L., GURNELL, A.M. & WEBB, B.W. (eds) Sediment and Water Quality in River Catchments. Wiley, Chichester, 365-389.

WRIGHT, L.D. & COLEMAN, J.M. 1973. Variations in morphology of major river deltas as functions of ocean wave and river discharge regimes. AAPG Bulletin, 57, 370-398.

YANG, S.L., BELKIN, I.M., BELKINA, A.I., ZHAO, Q.Y., ZHU, J. & DING, P.X. 2003. Delta response to decline in sediment supply from the Yangtze River: evidence of the recent four decades and expectations for the next half-century. Estuarine Coastal and Shelf Science, 57, 689-699.

ZALIDIS, G. 1998. Management of river water for irrigation to mitigate soil saiinization on a coastal wetland. Journal of Environmental Management, 54, 161-167.

Received 9 February 2004; revised typescript accepted 27 May 2005.

Scientific editing by Jamie Woodward

VASILIOS KAPSIMALIS1, SERAFIM E. POULOS2, ARISTOMENIS P. KARAGEORGIS1, PETROS PAVLAKIS1 & MICHAEL COLLINS3

1 Hellenic Centre for Marine Research, Institute of Oceanography, P.O. Box 712, 19013, Anavyssos, Greece (e-mail: kapsim@ath.hcmr.gr)

2 University of Athens, Faculty of Geology & Geoenvironment, Department of Geography & Climatology, Panepistimiopolis, Zografou 157 84, Athens, Greece

3 School of Ocean and Earth Science, Southampton Oceanography Centre, University of Southampton, European Way, Southampton SO 14 3ZH, UK

Copyright Geological Society Publishing House Nov 2005

Source: Journal of the Geological Society

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