Journal of the Geological Society, London, Vol. 162, 2005, pp. 897–908. Printed in Great Britain.

Recent evolution of a Mediterranean deltaic coastal zone: human impacts on the Inner Gulf, NW Aegean Sea

VASILIOS KAPSIMALIS1, SERAFIM E. POULOS2, ARISTOMENIS P. KARAGEORGIS1, PETROS PAVLAKIS1 & MICHAEL COLLINS3 1Hellenic Centre for Marine Research, Institute of Oceanography, P.O. Box 712, 19013, Anavyssos, (e-mail: [email protected]) 2University of Athens, Faculty of Geology & Geoenvironment, Department of Geography & Climatology, Panepistimiopolis, Zografou 157 84, Athens, Greece 3School of Ocean and Earth Science, Southampton Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK

Abstract: The Inner Thermaikos Gulf is located in the northwestern Aegean Sea, receiving water and sediment fluxes from the Axios, Aliakmon, 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 3 106 m3 a1. 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 turn, 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 3 106 m3 a1); 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 reaches. The former situation has been documented by Wood- continental and marine environments, with high levels of spatial ward (1995), in the case of the Mediterranean rivers, where and temporal variability in the prevailing natural processes human impact upon the Ebro, Po and Axios rivers, for example, (Wright & Coleman 1973). Superimposed upon the natural extended back to 2500 years bp (Milliman 2001). The latter evolution, deltaic coasts have undergone considerable develop- situation has been reported for the River Axios (Poulos et al. ment, such as intensive agriculture, industrialization and urbani- 1996). Since the beginning of the 20th century, a dramatic zation. Thus, human interference incorporates: (1) changes in reduction in sediment supply to the coastal zone has occurred fluvial freshwater and sediment fluxes; (2) canalization and flood globally, following the construction of dams for irrigation and control works; (3) land reclamation schemes; (4) channel dred- hydroelectric power schemes (Vo¨ro¨smarty et al. 2003). In addi- ging for aggregates; (5) more recently, structures for coastal tion, a further reduction in sediment supply has been attributed protection. Within this context and against a background of to sand and gravel extraction along river channels (Marchetti minimal tidal range (in the case of the Mediterranean Basin), 2002). This dramatic reduction has not only stopped delta-front within the Holocene, the two principal factors controlling the progradation, but has initiated coastal erosion, which includes morphological and sedimentary evolution of any deltaic coastal coastline retreat and/or the removal of nearshore sediments system are the fluvial water and sediment supply and the wave- (McManus 2002; Yang et al. 2003), together with the landward derived longshore drift (McManus 2002). intrusion of saline groundwaters (Oueslati 1995; Zalidis 1998). Globally, riverine sediment fluxes have been modified by In the case of the Mediterranean Basin, it has been estimated various anthropogenic activities (Vo¨ro¨smarty et al. 2003). Since that the potential (natural) sediment supply has been reduced by late prehistoric times, long periods (centuries to millennia) of almost 50% since the middle of the 20th century (Poulos & increased sediment delivery to the coast have been attributed to Collins 2002). Such sediment reduction, exceeding 90% in the changes in land use of the catchment area (mainly as a result of case of the larger river systems, has already initiated annual rates deforestation and crop farming) (Van Andel et al. 1990; Walling of coastal erosion ranging from tens of metres (e.g. Ebro River, 1999), whereas short periods (over the order of a few decades) of Jimenez et al. 1997; Rhoˆne River, Bird 1988; Po River, Simeoni significant increase in the detrital sediment discharge have been & Bondesan 1997), to hundreds of metres (e.g. Nile River, Fanos attributed to the canalization and straightening of lower river 1995; Stanley 1996).

897 898 V. KAPSIMALIS ET AL.

This study examines the geomorphological and sedimentologi- Table 1. General characteristics of the main rivers discharging along the cal evolution of the deltaic coastal zone of the Inner Thermaikos coastline of the Inner Thermaikos Gulf deltaic coast (after Poulos et al. Gulf, NW Aegean Sea, located between 408259 and 408409N and 2000) 228359 and 238009E. Although a preliminary analysis of changes Axios Aliakmon Gallikos in the deltaic coastline has been presented previously by Poulos et al. (1994, 2000), the present study, with the use of appropriate Geomorphology geographic information system (GIS) tools, not only examines Catchment area (km2) 23747 9250 1230 coastal changes, but extends this investigation to the adjacent sea Maximum relief (m) 2800 2200 2180 Relief ratio (102) 1.15 1.7 0.25 bed. Such an approach aims to provide: (1) a detailed description Deltaic plain (km2) ~1500 (qualitatively and quantitatively) of the overall pattern of deltaic Lithology coastal zone evolution during historical times (from 1850 to Acid þ mafic (%) 51.2 23.7 57.5 2000) in relation to human-induced changes in freshwater and Carbonates (%) 11.3 15.7 1.0 sediment supply, in response to various anthropogenic construc- Clastic deposits (%) 37.5 60.6 41.5 tions and interventions either within the upstream basin (i.e. dam Climate Mean annual temperature (8C) 14.5 16.5 16.5 construction) and/or on the coastal deltaic plain (canalization, Mean annual rainfall (mm) 650 750 480 land reclamation etc); (2) an estimation of nearshore and off- Type Terrestrial Mediterranean shore (extending to the southern limit of the Inner Thermaikos Land use Gulf) changes in bathymetry, defining areas of accretion or Forest (%) 16.9 19.0 3.4 erosion; (3) a volumetric assessment of sediments that have Uncultivated area (%) 5.9 40.0 38.9 entered the Gulf over the last 150 years; (4) the identification of Arable land (%) 75.4 39.3 55.6 Urban area (%) 1.7 1.3 1.7 changes in the sedimentary (grain-size) character of the surficial Wetland (%) 0.1 0.4 0.4 sea-bed sediments, providing an indication of their mobility.

The study area: the Inner Thermaikos Gulf canal for the Yiannitsa Lake), form the Plain of , c. 1500 km2 in area (Fig. 1b). Today, the Plain hosts a population Physical geographical setting in excess of 1 million; it is one of the most important socio- The Inner Thermaikos Gulf is located in the southern Balkan economic areas of the southern Balkans and the second most Peninsula and incorporates the mouths of the Axios, Aliakmon important zone in Greece. The seaward boundary of the Inner and Gallikos Rivers (Fig. 1a). The subaerial deltaic plains of Thermaikos Gulf is taken here as an imaginary west–east line, these rivers (Table 1), together with the artificially drained lake some 20 km in length, connecting the promontories of Atheris of Yiannitsa and the River Loudias (operating now as a drainage and Epanomi (Fig. 1b). Water depths vary from a few metres,

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 . HUMAN IMPACT ON A DELTAIC COASTAL ZONE 899 adjacent to the delta fronts, to c. 35 m at the southerly limit (Fig. Table 2. Freshwater and sediment fluxes of the main rivers discharging 2). At the northeastern extension of the Gulf lies the Bay of into the Inner Thermaikos Gulf Thessaloniki, where the harbour of Thessaloniki is located (Fig. Axios Aliakmon Gallikos 1b). The morphology of the deltaic coastline, in the essentially Freshwater discharge (106 m3 a1)* tideless environment of Greek waters (Tsimplis 1994), results Mean annual 5030 2292 1.2† mainly from interaction between the fluvial water and sediment Monthly maximum 8800 4320 ? discharge and the prevailing wave activity; the latter is relatively Monthly minimum 1545 662 ? Overall total 7323 low (mean monthly wave power approaching the Inner Gulf 2 High water period (months) Dec.– Jan. Dec.– May ? is ,30 W m ), when compared with other Greek deltas (i.e. Low water period (months) Jul.– Nov. Jun.– Nov. ? 70–1454 W m2 for the Pinios River (Poulos et al. 1993) and Sediment fluxes (106 ta1) .25 000 W m2 for the Alfios River (Poulos et al. 2002)). This a. Suspended load 13.2‡ 6.2§ 1.0‡ † pattern is related to the restricted wave fetches, caused by the b. Dissolved load 1.7 1.2 0.6 k semi-enclosed and shallow character of the Inner Thermaikos c. Bed load 2.6 1.3 0.3 Total sediment load (a þ b þ c) 17.5 8.7 1.9 Gulf, leading to the formation of a birdfoot-type delta (Galloway Overall total 28.1 1975). *Measured values (after Therianos 1974). †Measured values (after Skoulikidis 1993). ‡Estimated value (after Poulos et al. 1996). §Extrapolated value, from the total catchment area and based upon in situ measurements over part of the drainage basin. Fluvial water and sediment fluxes kEstimated value, based upon the assumption that bed load represents some 15% of the total sediment load (after Poulos & Chronis 1997). 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 Delta evolution overall annual mean discharge of 276 m3 s1, between 1926 and 1970 (Therianos 1974). However, recent flow measurements The rapid progradation of the deltaic complex since the 5th bc (1997-1998) have identified a substantial decrease in the fresh- century (Fig. 3) has been established (Stu¨rck, 1908) from water supply (to some 130 m3 s1), attributed mainly to the archaeological data and historical records from ancient travellers. increasing consumption of freshwater because of agricultural and At that time, the ancient towns of Skydra (to the west) and urban development (Karageorgis & Anagnostou 2001). (to the north) were located adjacent to the sea; today they are The principal rivers (Axios, Aliakmon and Gallikos) supply located some 30 km from the coast. During the second half of annually some 28 3 106 tonnes of total sediment load (Table 2). the 20th century, the evolution of the deltaic coast of the Inner This sediment flux, associated with high sediment yields Thermaikos Gulf has been investigated by Evmorphopoulos (.500 t km2), results from intensive weathering processes (1961), Kotoulas (1984), Poulos et al. (1994) and Albanakis et al. (1993). An assessment of the observed subsidence rates of the induced by: (1) highly variable climatic conditions (i.e. large 1 daily and seasonal air temperature fluctuations and relatively plain (10 cm a ), for the period 1960–1999, has been provided high precipitation levels); (2) erodible lithologies; (3) the by Stiros (2001). 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 Marine sediments have not reached the coastal zone, as they accumulate behind a Modern sedimentary processes in the Thermaikos Gulf have been number of irrigation and hydroelectric dams (Poulos & Collins studied extensively in recent decades by Lykousis et al. (1981), 2002; Karageorgis et al. 2004). Lykousis & Chronis (1989), Poulos et al. (1996) and Karageorgis & Anagnostou (2001, 2003). Similarly, Georgas & Perissoratis

Fig. 2. Recent bathymetric chart of the Thermaikos Gulf (Hydrographic Fig. 3. Palaeogeographical evolution of the Thessaloniki Plain during Service of the Greek Navy 2000), based upon the Greek Geodetic historical time, based upon information provided by ancient Greek Reference System (1987). Water depths in metres. writers (modified from Stu¨rck 1908). 900 V. KAPSIMALIS ET AL.

(1992) and Poulos et al. (1994) have provided a socio-economic associated bathymetry of the Inner Thermaikos Gulf. These appraisal of the ranges of human activities over the area. In include significant spatial trends in erosion and accretion general, the bottom sediments consist mainly of silt- and clay- dynamics over the past 150 years. Three evolutionary stages have sized particles, with increasing amounts of clay occurring off- been identified: an early stage, from 1850 to 1916 (Stage I); an shore. Sand is abundant near the coastlines, adjacent to the river intermediate stage, from 1916 to 1956 (Stage II); and a late mouths and, particularly, over the eastern part of the Gulf (as stage, from 1956 to 2000 (Stage III). Spatial changes in the sea- relict deposits). Furthermore, Lykousis & Chronis (1989), inter- bed elevations, accompanied by a volumetric estimation of the preting high-resolution (3.5 kHz) seismic profiles taken from the sediment deposited and/or eroded from the sea floor of the Inner Inner Thermaikos Gulf, have indicated that the thickness of the Thermaikos Gulf, are presented in Table 3. The thickness of the Holocene sedimentary cover exceeds 20 m near the river mouths deposits and the areas affected by river mouth evolution are and decreases to ,4 m over the eastern part of the Gulf (Fig. 4). summarized, on the basis of the earlier evolutionary stages (see above), in Table 4. Materials and methods Stage I A series of bathymetric charts were obtained from the UK Hydrographic Office (Taunton), dated 1850, 1916, 1947 and 1956, and the Hydrogra- The coastal and bathymetric changes shown between the 1850 phical Service of the Greek Navy (Athens, Greece), published in 2000. and 1916 charts reveal progradation of the deltaic coast and, in All the charts were projected onto the Greek Geodetic Reference System particular, the area incorporating the active mouths of the Axios (1987) and processed with the use of the ArcView GIS (Version 3.2). and Aliakmon Rivers. Over this period, the position of the active Quantitative estimates of the volumetric changes in the offshore water mouth of the Axios River varied. In particular, before the 1890s, depths between the time periods of the available charts were derived using the SURFER# software package. The weight of the sediments the river mouth was located at the Kavoura Cape, opposite the accumulated in front of the river mouths was extrapolated from the Megalo Emvolo Cape. Subsequently and until 1934, the mouth sediment volume and bulk density; the latter is equal to c. 1.5 g cm3, was located near Palaiomana, opposite the Mikro Emvolo Cape, according to Lykousis et al. (2002). some 10 km to the NE (Konstantinidis 1989). However, it can be Comparison between the various charts requires consideration of assumed that these mouths (Kavoura and Palaiomana) were various potential errors, which include: (1) vertical and horizontal datum active concurrently, especially during high freshwater discharges. inconsistencies; (2) the absence or use of different georeference systems; During high river discharges, overbank flows flooded extensive (3) computer-gridding errors. To minimize such errors, standard terres- areas of the delta plain, creating swamps, coastal lakes and trial reference points (such as churches, capes, lighthouses), were ephemeral channels. Conversely, under the influence of southerly selected; these were used for an intercomparison of shoreline position and offshore bathymetric contours, for all of the charts. Furthermore, a winds, the sea invaded the coastal zone of the deltaic platform, 0.5 m range in sea-floor change was designated as a ‘moderate’ degree infilling the salt marshes and lagoons. of error in the bathymetric comparisons; as such, this has been Although some of the sediment load transported by the Axios characterized as a zone of ‘no significant’ change (List et al. 1997). River was deposited on the deltaic plain during floods, the The datasets used for the study of the grain-size characteristics of the greater proportion was discharged into the Thermaikos Gulf. surficial sea-bed sediments were collected in 1978 (Chronis 1986) and Most of the coarse-grained riverine sediments are likely to have 1997–1998 (Karageorgis & Anagnostou 2003). 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 Results and discussion sea bed over the inner part of the Gulf (including the navigation The reconstruction of the historical charts (Fig. 5) has revealed channel to the port of Thessaloniki). major changes in the evolution of the coastline and the Sediment deposited during Stage I (Fig. 6), over the Kavoura mouth of the Axios River, covered an area of 25 3 106 m2, with a maximum thickness of 22 m; this corresponds to a sediment volume of 85 3 106 m3. Similarly, at the Palaiomana mouth, the new prodeltaic deposit covered an area of 23 3 106 m2, with a maximum thickness of 16 m and a total volume of 60 3 106 m3 (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 3 106 m3 a1 and 0.9 3 106 m3 a1, 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 accumula- tion rates for the Kavoura and Palaiomana mouths become 1.7 3 106 m3 a1 and 3.8 3 106 m3 a1, 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 km2). Fig. 4. Thickness of the Holocene sedimentary cover in the Inner Along the deltaic coast, between the Axios River mouths, no Thermaikos Gulf, in milliseconds (where 10 ms is equivalent to c. 8 m), significant deposition is observed (Fig. 6a), and this implies that based upon the interpretation of geophysical data (after Lykousis & the secondary drainage networks played only a minor role in Chronis 1989). sediment dispersion, except during extreme floods. HUMAN IMPACT ON A DELTAIC COASTAL ZONE 901

Fig. 5. Digital output of the various (dated) bathymetric charts provided by the UK Hydrographic 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

1850–1916 1916–1956 1956–2000

Spatial changes in the sea bed (106 m2)* Net gain 120 270 60 Unchanged 180 70 150 Net loss 80 40 170 Volumetric changes in the sea bed (106 m3) Net gain 560 1020 190 Net loss 130 120 300 Balance 430 900 110 Weight of deposited or eroded sea-bed sediment (106 t) Deposition 860 1570 300 Erosion 200 180 460 Net sediment supply 660 1390 160 Accretion or erosion rates (106 m3 a1) 6.5 18 2.5 (106 ta1)10284

*Areas of net gain (.þ0.5 m), no significant change (0.5 m) and net loss (, 0.5 m).

Over the same period of time (Stage I), the Aliakmon River (Table 4). Furthermore, a limited amount of the fine-grained was in a phase of rapid progradation, incorporating a meandering sediments (mostly muds) was transported by a cyclonic surface channel system. Some of the sediment load was trapped within circulation pattern (Poulos et al. 2000) to the west and deposited the meanders, forming extended point-bars. The remainder of the in the Methoni Bay (Figs 1b and 6). Overall, the derived rate of load reached the coast, contributing to the progradation of the deposition is 5.2 3 106 m3 a1, which is considerably higher than mouth and the offshore deposition of fine-grained sediments. either of those attributed to the Axios River mouths. During this period, some 340 3 106 m3 of sediments accumulated In general, the Axios River carried substantially less sediment over an area of 55 3 106 m2 with a maximum thickness of 20 m load than the Aliakmon River, although it has a much larger 902 V. KAPSIMALIS ET AL.

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

1850–1916 1916–1956 1956–2000

Maximum thickness of the subaqueous deltaic wedges (m) Axios (Kavoura) mouth 22 20 16 Axios (Palaiomana) mouth 16 21 5 Aliakmon mouth 20 22 10 Accretional areas, in front of the active river mouths (106 m2) Axios (Kavoura) mouth 25 39 18 Axios (Palaiomana) mouth 23 30 7 Aliakmon mouth 55 59 24 Volume of deposits at the active river mouth (106 m3) Axios (Kavoura) mouth 85 220 60 Axios (Palaiomana) mouth 60 140 10 Aliakmon mouth 340 270 35 Accretion rates of the subaqueous deltaic wedges (106 m3 a1) Axios (Kavoura) mouth 1.3 5.5 1.4 Axios (Palaiomana) mouth 0.9 3.5 0.2 Aliakmon mouth 5.2 6.8 0.8 Sediment budget, in front of the active river mouth (106 t) Axios (Kavoura) mouth 130 340 90 Axios (Palaiomana) mouth 90 215 15 Aliakmon mouth 520 415 55 Accretion rates of the subaqueous deltaic wedges (106 ta1) Axios (Kavoura) mouth 2.0 6.8 1.3 Axios (Palaiomana) mouth 1.4 4.3 0.2 Aliakmon mouth 7.8 8.3 1.2

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 bathymetric charts; (b) schematic representation of the estimated sediment influx, at the active river mouths. HUMAN IMPACT ON A DELTAIC COASTAL ZONE 903 drainage basin (Table 2). According to Poulos et al. (2000), this During Stage I, the sea floor of the Inner Thermaikos Gulf, difference in sediment yield could be attributed to: (1) differ- excluding the areas of active deltaic progradation, does not ences in lithology, with the Axios River draining mostly resistant appear to have undergone any significant changes (Fig. 6a). mafic and acidic rocks whereas the Aliakmon River drains Limited sediment accumulation can be observed over the north- mostly weaker clastic formations (Table 1); (2) the geomorpholo- eastern part of the Gulf and close to the port of Thessaloniki; gical characteristics of the two catchments, with the Axios River this could be associated with the Palaiomana mouth of the Axios having a much lower relief ratio (1.1) than the Aliakmon River River. Over some parts of the central area of the Gulf, the sea (1.7), incorporating a drainage network with a large number of bed appears to have undergone erosion (.0.5 m) and this is primary and secondary branches and meanders, together with a probably related to near-bed current activity. more gentle slope to its deltaic plain; (3) soil degradation, associated with land use changes. Over this period, the Loudias River drained the Yiannitsa Lake Stage II and Loudias Swamps to the west of the Kavoura mouth (Fig. 1b). Within this period (1916–1956), two substages can be identified: These systems discharged into the Thermaikos Gulf either the first (1916–1930) is characterized by natural coastal evolu- directly, as an independent river with its mouth located between tion, and the second (1931–1956) by increasing human interven- those of the Axios (Kavoura) and Aliakmon, or indirectly, as a tions on the deltaic plain in the form of canalization and land tributary of the western channel of the Axios River (Konstantini- reclamation. dis 1989). Finally, the Gallikos River, although its mouth was During the first sub-period (1916–1930), the deltaic plain was located very close to the Palaiomana mouth of the Axios River evolving naturally. The active mouth of the Axios River was that (Fig. 6), was active independently. The Gallikos River drains a of Palaiomana (Fig. 7a), causing siltation of the port of much smaller catchment area (1230 km2) and is characterized by Thessaloniki. Such siltation led to rerouting of the Axios River, ephemeral (seasonal) flows. diverting its active mouth to its previous location near the

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 bathymetric charts; (b) schematic representation of the estimated sediment influx, at the active river mouths. 904 V. KAPSIMALIS ET AL.

Kavoura Cape (Fig. 7a). Construction commenced in 1930 and 16.4 3 106 m3 a1 (Kotoulas 1984). Calculations undertaken was finalized in 1934. Accompanying works, undertaken within here, on the basis of comparison of the 1850 and 1956 the deltaic plain, were: (1) the artificial drainage of the Yiannitsa bathymetric charts, show a net sediment accumulation of Lake; (2) associated construction of the Loudias drainage 1350 3 106 m3 or 12.8 3 106 m3 a1. This difference between channel (39 km long, 50 m wide and 5 m deep, along the former the results of the present study and the abovementioned investi- route of the Loudias River); (3) land reclamation works on the gations can be attributed, mainly, to: (1) the different approaches Loudias Swamps (1933–1935), located to the NW of the used, and especially to the fact that the present investigation Kavoura mouth (Fig. 1b). Following diversion of the main takes into account not only coastline displacements but also tributary of the Axios River (1934), an extended birdfoot delta changes in offshore water depths; (2) the escape seawards of part developed near the Kavoura mouth. During the second sub- of the very fine-grained suspended and dissolved material (c. period (1931–1956), the Axios River delta developed farther to 10–15% of the total sediment load), which does not contribute the south and SW. Erosion occurred over the eastern part of its to long-term coastal change. active mouth, affecting the earlier deposits of the abandoned In general, sea-floor changes in the Inner Thermaikos Gulf Kaboura mouth (Fig. 7). At this time, the old route of the Axios reveal an accretional pattern, associated with high river sediment River, ending at the Palaiomana mouth, functioned episodically discharges. Net sediment deposition is of the order of during floods. 900 3 106 m3 (Table 3), with the impact of such sedimentation Over the same sub-period, the lower meandering course of the being most pronounced at the river mouths. In some places Aliakmon River was aligned artificially; this decreased its length erosion has also taken place, including, for example, to the east from 52.2 km to 30.4 km (Konstantinidis 1989), leading to of the Kavoura Cape, from both sides of the Mikro Emvolo further progradation of its mouth area to the NE and to the Cape, and to the south of the Megalo Emvolo Cape. These development of an extended birdfoot delta. Similarly, works were erosional areas are related to near-bed currents and the overall carried out along the lower course of the Gallikos River, focusing water circulation pattern within the Inner Thermaikos Gulf upon the protection of its subaerial delta from flood events (Balopoulos et al. 1986). (Konstantinidis 1989). Overall, Stage II (1916–1956) was char- For the whole of the Inner Thermaikos Gulf, the overall acterized by substantial sediment deposition within the Inner accretion of sediment (1020 3 106 m3) (Table 3), in comparison Thermaikos Gulf and this is estimated to be of the order of with that deposited in front of the river mouths (630 3 106 m3), 900 3 106 m3 (Table 3). Although sediments were deposited over implies that some 40% of the terrestrial sediment inputs were the adjacent sea floor, the highest rates of accretion occurred in dispersed seawards. front of the active river mouths (Fig. 7b). Thus, at the Palaiomana mouth, some 140 3 106 m3 of sediments settled over Stage III an area of 30 3 106 m2 corresponding to a maximum thickness of 21 m and an accumulation rate of 3.5 3 106 m3 a1 (Table 4). The period between 1956 and 2000 witnessed an intensification However, on the basis of the assumption that the majority of of human intervention, resulting in substantial alterations to the these sediments had been deposited before the artificial diversion natural systems. This followed, in turn, the construction of dams of the main Axios River channel (i.e. from 1916 to 1934), the in the various catchments, for irrigation and/or hydroelectric accumulation rate could be increased to 7.8 3 106 m3 a1. At the power generation. Two irrigation dams were constructed along Kavoura mouth during Stage II, if only a small amount of main channels of the rivers Axios (Prochoma) and Aliakmon sediment was deposited prior to 1934, then sediment volume (near Verroia), in 1958; these were located some 30 km and of 220 3 106 m3 was associated with an accumulation rate of 50 km inland from their river mouths, respectively (Fig. 1b). 10.0 3 106 m3 a1 (between 1934 and 1956). Such construction was followed by the development of an A general increase in sediment supply can be attributed, extensive irrigation network over the Thessaloniki Plain, from mainly, to alignment of the Axios River and straightening of its 1958 to the end of the 1960s, associated with intensive lower course; this caused elevated flow velocities and (possible) agricultural activities such as rice cultivation. This development erosion of older terrestrial deposits (Konstantinidis 1989). Like- caused a significant reduction in the area of wetlands, together wise, the bulk of the sediment load was transported directly to with massive consumption of river water. Furthermore, 11 dams the sea, limiting the deposition of sediment on the deltaic plain. have been constructed over the last 50 years along the Axios Furthermore, such high sediment discharge rates also reflect the River, within the part of the catchment that belongs to FYROM absence of any major dams along the river channel. However, a (Former Yugoslav Republic of Macedonia), with a total capacity small reservoir (capacity 3.6 3 106 m3) was constructed, in of 803 3 106 m3 (Psilovikos & Psilivikos 1997). This develop- 1938, on a tributary of the Axios River (the Treska River) some ment has resulted in a considerable reduction in the volumes of 300 km upstream of the river mouth. freshwater reaching the Gulf. Mean annual water discharge, Artificial cut-off of meanders in the Aliakmon River deltaic estimated from historical data, is 5.0 3 109 m3 a1 (Therianos plain would also have increased sediment supply into Methoni 1974). This contrasts with recent (1995–2000) measurements, Bay (Fig. 1a). The volume of the deposits that accumulated in which have revealed a considerable decrease to around front of the Aliakmon River mouth was 270 3 106 m3, corre- 3.4 3 109 m3 a1 (Karageorgis et al. 2004). In addition, three sponding to an average accumulation rate of 5.4 3 106 m3 a1. more hydroelectric dams were constructed along the Aliakmon Over the same period, it proved impossible to estimate the River between 1974 and 1989. progradation rate of the Gallikos River, especially following Entrapment of the bed load and the majority of the suspended the engineering works. This is due primarily to its proximity to material, within the reservoirs, has resulted in a dramatic the Paliomana mouth of the Axios River, but also to its low reduction in sediment supply to the deltaic coastal plain. The sediment discharge. mean annual sediment discharges were estimated from historical Earlier investigations of sediment supply, based upon changes data (1970–1987) to be (1–2) 3 106 ta1. Recent (1995–2000) in the coastline (from 1850 to 1952), have revealed an annual estimates are 10- to 20-fold lower (0.1 3 106 ta1) (Karageorgis sediment input of 16 3 106 m3 a1 (Evmorphopoulos 1961) or & Anagnostou 2001). Initially, such a reduction in sediment HUMAN IMPACT ON A DELTAIC COASTAL ZONE 905 supply inhibited deltaic progradation, causing coastline retreat. consequently, the process of deltaic infilling and progradation. Furthermore, reduction in the natural freshwater flow levels, This is a common situation in many Greek rivers and deltas, through the retention of large quantities of water within the such as has been reported for the lower reaches Alfios River in reservoirs and/or increasing consumption, has caused: (1) saline the Peloponnese (Nicholas et al. 1999). intrusion into the coastal deltaic areas (Stiros 2001); (2) dete- Since 1956, the Inner Thermaikos Gulf has entered into an rioration of the wetlands; (3) seawater intrusion along the lower erosional phase, at an annual rate of c. 2.5 3 106 m3 (Table 3). courses of the rivers, during storm surges (Konstatinidis 1989). However, in front of the active river mouths and near the port of To control coastal erosion, especially along the abandoned Thessaloniki, limited amounts of sediment are still deposited Palaiomana mouth of the Axios River, coastal protection meas- (Fig. 8). ures have been undertaken (from 1960 to 1974). These include Sediment deposited seawards of the Axios River mouth has the construction of 22 km of seawall (2.5 m in height) and five accumulated at a rate of 1.4 3 106 m3 a1; this compares with pumping stations that drain the agricultural plain. Parts of the an estimated sediment load for the Aliakmon River of seawall have been destroyed by rapid subsidence of the deltaic 0.8 3 106 m3 a1 (Table 4). Such rates are approximately four plain, which in some places reaches 10 cm a1 (Stiros 2001). (Axios River) and eight (Aliakmon River) times less than those This subsidence is related to the decomposition of organic matter estimated during Stage II (Tables 3 and 4). Most of the ‘missing’ and compaction of fine-grained material accompanied by syn- sediment load has been retained behind the dams. Moreover, the sedimentary deformation. Such an effect has been enhanced by remainder of the load that eventually reaches the sea could the extended, often illegal, overpumping of the deltaic aquifers represent: (1) dissolved load; (2) very fine-grained suspended (Andronopoulos et al. 1991). Coastal erosion has also been sediment; (3) material originating from the erosion of the deltaic caused by the extraction (not always appropriately licensed) of plain. Furthermore, it is likely that a significant amount of aggregates (sand and gravel), from the lower courses of the suspended particulate matter and the majority of the dissolved rivers. Such activity has reduced bed load transport rates and, load are transported farther offshore, extending even to the

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. 906 V. KAPSIMALIS ET AL. continental shelf edge of the Outer Thermaikos Gulf (Lykousis & Chronis 1989; Karageorgis & Anagnostou, 2001, 2003). Sediment discharges associated with the Loudias drainage chan- nel (with average discharges ranging from 0 to ,10 m3 s1) and the Gallikos River (dry during most of the year, but undergoing subsidence over the coastal part of the deltaic plain) are consid- ered 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 en- hanced 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 3 106 m3,atan average rate of accretion of 8 3 106 m3 a1 or 12 3 106 ta1. 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 Fig. 9. Distribution patterns of the sedimentary facies on the sea bed of identified on the basis of changes in sediment supply and the Inner Thermaikos Gulf: (a) in 1978 (modified from Chronis 1986); associated human interference. (b) in 1997–1998 (modified from Karageorgis & Anagnostou 2001). (1) From the middle of the 19th century to the early 20th Sampling stations are shown in both parts of the figure. Sedimentary century (Stage I: 1850–1916), the coastal system of the Gulf was facies are defined according to the classification of Folk (1974).

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.) HUMAN IMPACT ON A DELTAIC COASTAL ZONE 907 controlled naturally, with a net marine sediment supply of some phique, CIESM Science Series 3, Special Publication, 18, 169–185. 430 3 106 m3, at an average fluvial discharge of 6.5 3 Karageorgis, A.P. & Anagnostou, C.L. 2001. Particulate matter spatial– 106 m3 a1 or 10 3 106 ta1. This input resulted in progradation temporal distribution and associated sediment properties: Thermaikos Gulf and Sporades Basin, NW Aegean Sea. Continental Shelf Research, 21, 2141– of the delta complex, especially near the active river mouths. 2153. (2) Subsequently, during Stage II (1916–1956), especially in Karageorgis, A.P. & Anagnostou, C.L. 2003. Seasonal variation in the its second part (1934–1956), human intervention to the natural distribution of suspended particulate matter in the northwest Aegean Sea. system of the deltaic plain was at a maximum through: (a) Journal of Geophysical Research, 108(C8), 3274, doi: 10.1029/ 2002JC001672. artificial realignment of the main river channels (1934); (b) Karageorgis, A.P., Skourtos, M.S., Kapsimalis, V., et al. 2004. An integrated drainage of the Yiannitsa Lake and the Loudias Swamps (1935); approach to watershed management within the DPSIR framework: Axios (c) the instigation of other land reclamation projects (Fig. 10). River catchment and Thermaikos Gulf. Regional Environmental Change,doi, The net riverine sediment supply increased considerably to 10.1007/S10113-004-0078-7. 6 3 Konstantinidis, K.A. 1989. Land Reclamation Project of the Thessaloniki Plain. 900 3 10 m ; this corresponds to an average input of 18 Geothechnical Chamber of Greece, Thessaloniki. 6 3 1 6 1 3 10 m a or 28 3 10 ta . Over this period, rapid progra- Kotoulas, D. 1984. Bodenbtrag und -ablagerungen in Griechenland am Beispiel dation occurred at the active mouths of the Axios and Aliakmon des Gebirgslandes und der Edene von Thessaloniki. Interpravent, Villach. Rivers. At the same time, some of the fine-grained components List, J.H., Jaffe, B.E., Sallenger, A.H. & Hansen, M.E. 1997. Bathymetric of the sediment load were dispersed over the Inner Thermaikos comparisons adjacent to the Lousiana Barrier Islands: processes of large-scale change. Journal of Coastal Research, 13, 670–678. Gulf. Lykousis, V. & Chronis, G. 1989. Mechanisms of sediment transport and (3) Stage III (1959–2000) is strongly characterized by a deposition: sediment sequences and accumulation during Holocene on the further ‘cycle’ of human interference. This incorporates the Thermaikos plateau, the continental slope and basin (Sporades Basin), construction of irrigation reservoirs and hydroelectric dams. northwestern Aegean Sea, Greece. Marine Geology, 87, 15–26. Lykousis, V., Collins, M.B. & Ferentinos, G. 1981. Modern sedimentation in Such structures have caused a significant reduction in sediment the NW Aegean Sea. Marine Geology, 43, 111–130. supply, leading to an overall erosional phase over the Gulf and a Lykousis, V., Roussakis, G., Alexandri, M., Pavlakis, P. & Papoulia, I. 2002. mean sediment loss of 2.5 3 106 m3 a1 or 4 3 106 ta1. During Sliding and regional slope stability in active margins: North Aegean Trough this period, the river mouths underwent rapid retreat. The ‘active’ (Mediterranean). 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Received 9 February 2004; revised typescript accepted 27 May 2005. Scientific editing by Jamie Woodward