The Vjosa River corridor: a model of natural hydro-morphodynamics and a hotspot of highly threatened ecosystems of European significance

Large near-natural rivers have become rare in Europe, a fact reflected in the high conservation status of many riverine ecosystems. While the Balkan still harbors several intact river corridors, most of these are under pressure from planned hydropower constructions. Unfortunately, there is little information available on the hydromorphodynamics and biota of Balkan rivers under threat. We present a synthesis of research on the Vjosa in Southern Albania. Here, longitudinal continuity in water flow, undisturbed sediment transport and intact fluvial dynamics are still maintained, but threatened by two large dams planned in its downstream section. We intend to provide a first multidisciplinary inventory of this river system as an example of the knowledge base required for sound water management decisions in the Balkans. Based on field work of a multidisciplinary consortium of scientists from Albania and other countries conducted from 2017 onwards, we summarize the most important findings on geomorphology of the riverine landscape, habitat turnover rates, vegetation ecology and selected animal taxa. We found evidence that significant areas (86%) of the river corridor are covered by habitats listed in Annex 1 of the European Union Habitats Directive. These are associated with a high number of threatened biota. Our findings underscore the value of the Vjosa as one of the few remaining reference sites for dynamic floodplains in Europe and as a natural laboratory for interdisciplinary research. We emphasize that such multidisciplinary studies are a prerequisite for informed evaluation of potential impacts caused by hydropower plants.

the riverine landscape, habitat turnover rates, vegetation ecology and selected animal taxa. Results We found evidence that significant areas (86%) of the river corridor are covered by habitats listed in Annex 1 of the European Union Habitats Directive. These are associated with a high number of threatened biota. Conclusions Our findings underscore the value of the Vjosa as one of the few remaining reference sites for dynamic floodplains in Europe and as a natural laboratory for interdisciplinary research. We emphasize that such multidisciplinary studies are a prerequisite for informed evaluation of potential impacts caused by hydropower plants.
Keywords Balkans rivers Á EU-regulations Á Habitat turnover Á Hydropower Á Stenotopic floodplain biota Á Vegetation succession The Balkans-the blue heart of Europe Stretching from Slovenia to northern Greece, the Balkans (Balkan Peninsula, see Fig. 1) are home to most of the remaining intact river systems in Europe with the exception of the arctic and boreal ecoregions. The majority of Balkan rivers are in an excellent hydromorphological state, a situation that is highly exceptional in Europe. This holds especially true for Montenegro and Albania (Fig. 1). The region in general, and its rivers in particular, are recognized as biodiversity hotspots with many endemic species (Griffith et al. 2004). However, recently, Balkan rivers have come under strong pressure from around 3000 planned hydropower projects, of which more than 1000 are located in protected areas such as national parks, nature reserves and Natura 2000 sites (see ''Eco-Masterplan for Balkan rivers'' 2018). In many cases, the conservation value of rivers impacted by the proposed hydropower projects is insufficiently known, and environmental impact assessments are often inadequate.
Here, we provide a multidisciplinary synthesis of the conservation value of the Vjosa in southern Albania which is one of the last large European rivers in excellent hydromorphological condition throughout its entire course. The river has recently come under threat from two commissioned hydropower dams at its lower section (see Fig. 2a). While it is evident that the construction of these hydropower plants would have a severe impact on the conservation value of the Vjosa, the decision for constructing these dams has been made without a comprehensive assessment of possible environmental and socio-economic effects and without considering possible alternatives.
Thus, a joint action of research teams from Albania, Austria and Germany was instigated in 2017 to survey the river sections that would be directly affected. While the detailed results have been made available in a recent monograph (Schiemer et al. 2018a, b), here we provide a synthesis of the results and discuss them in a broader context. In particular, we address the geomorphological dynamics of the river and its floodplains, examine habitat diversity and habitat turnover rates, present a model on vegetation succession and discuss characteristic aquatic and terrestrial fauna. Our results support the urgency for conservation.

Study site and methods
A detailed description of the Vjosa and its accompanying landscapes is available in Schiemer et al. (2018a, b). In brief, the river extends over 272 km from its sources in the Pindos mountains (at 1343 m.a.s.l.) east of Ioannina in Greece through the south of Albania to the Adriatic Sea. Its catchment covers 6704 km 2 of which 4365 km 2 are on Albanian territory. The first 80 km are situated in Greece, where the river is named Aoos. The overall geomorphology of the Vjosa basin is characterized by a NW-SE orientation of the folded structures and tectonic planes (Fig. 2a). Along its course a variety of channel types occurs, with gorges in the upper parts, braiding and anabranching sections in the middle and lower courses and meandering stretches close to the river mouth. The climate of the lower catchment is Mediterranean, grading further upstream into sub-Mediterranean, temperate and finally-at its source in the mountains-alpine climates. The hydrological regime is classified as pluvio-nival. On average, the flow of the river exhibits a distinct seasonal pattern with high discharge from December to April, intermediate flows Fig. 1 Hydromorphological status of rivers with a catchment size over 500 km 2 in the Balkan Peninsula (acc. to ''Ecomasterplan Balkan 2018). Class 1 near-natural, class 2 slightly modified, class 3 moderately to severely modified. Balkan countries SL (Slovenia), HR (Croatia), RS (Serbia), BA (Bosnia-Herzegovina), ME (Montenegro), AL (Albania), KV (Kosovo), GR (Greece), BG (Bulgaria), TR (Turkey). Neighboring countries AT (Austria), HU (Hungary), RO (Romania), IT (Italy). Within the Balkan 25% of the rivers are hydromorphologically in ''near to natural '' conditions (compared to 10% in Germany) in May and October and low flow from June to September. Daily discharge is highly unpredictable (Fig. 2b). The annual mean flow at Poçemi is 141.5 m 3 s -1 , the recorded maximum was 3140 m 3 s -1 (1963).
Our assessment focused on the lower section of the Vjosa at Poçemi and Kalivaçi, which represents a barbraided and island braided river type with extensive floodplains (Fig. 3a). We identified the hydro-morphological and habitat dynamics of the river-floodplain system at a range of spatio-temporal scales. The landscape development over the past 50 years was reconstructed from a series of satellite images from 1968 onwards. In a detailed review of about 30 available images, those from 1968, 2006, 2012and 2016(Google Earth 2018 were analyzed with regard to the long-term translocations of the active channel within the floodplain. Satellite images were selected according to their temporal distribution, water level and spatial resolution for interpretation. To gain a better understanding of the driving forces of the landscape dynamics, terrestrial surveys along three representative transects of the river-floodplain morphology were conducted using a Leica TC805  1958-1990. After 1990 the gauging station was closed. The broken horizontal lines at 300 m 3 s -1 and 1,000 m 3 s -1 indicate the initiation of bed load transport in the channel and the discharge overtopping into the floodplains, respectively total station in April 2017 and July 2018 (Figs. 3a,4). Integrating this analysis of relief changes of the riverine landscape with available flow data allows for an approximation of the extensive sediment transport and morpho-structural dynamics. Morphological classification (following Nanson and Knighton 1996) of the floodplain at Poçemi was achieved with a onedimensional hydrodynamic-numerical model: bankfull discharge capacity (m 3 s -1 ), bottom shear stress (Nm -2 ) and unit stream power (Wm -2 ) were analyzed for characteristic discharges (mean flow -maximum recorded floods). For measurements of sediment transport processes see Hauer et al. (2019).  Table 2). Vegetation surveys were carried out in 69 plots (relevés) along the three transects in April and September 2017 and compared to the analysis of satellite images. Plot size varied between 25 and 100 m 2 , depending on vegetation type. Obtained data were classified using TWINSPAN (Hill 1979) to describe succession patterns of floodplain vegetation (Fig. 6b). For details see Drescher 2018. Habitats covered by the EU Habitats Directive (Annex 1, European Commission 2013) of the river corridor of the Poçemi-Kalivaçi section were The study of the diversity of benthic invertebrate fauna was designed as a faunistic survey, conducted as a stratified sampling program along three transects. Larval samples were taken with a 250 lm 25 9 25 cm hand net, observing standardized sampling routines. Additionally, adult aquatic insects were collected with sweep nets and light trapping.
Juvenile and adult fish were caught by point abundance electrofishing (EF) (Copp and Peňáz 1988) at wadable sites in April 2017. For EF, a backpack-generator (ELT60-IIH from H. Grassl) was used. Additional gill nets were placed at selected sites with a mesh size of 20 mm and 50 mm. For more detailed information see Meulenbroek et al. (2018).
Surveys on the riparian invertebrate fauna were conducted from 24.04.2017 to 28.04.2017. In particular, 150 pitfall traps were applied at 30 sites arranged along Transect 2 ( Fig. 3a) with 20 additional sampling sites studied by time-standardized handcollections. To record amphibian and reptile species, transect campaigns along the Vjosa River were performed in 2016 and 2017.

Landscape dynamics
There is a clear distinction between the active channel (bright band) and the elevated floodplain (Fig. 3a). The width of the active channel (AC) varies between 400 and 1000 m. Larger islands and gravel bars within the AC are strongly structured by erosion channels and show the cumulative effects of changing water levels and flow pulses. Within the AC large bars are deposited and more perennial islands are formed as is characteristic for island braided river sections. The river segments under consideration are partially constrained by bedrock structures leading to a high variability in the lateral extent of the riverine landscape. Gorges constitute bottlenecks in sediment transport resulting not only in lateral but also in longitudinal sorting processes. Besides the high gravel load, a large amount of finer sediments is transported by the river -a significant feature that creates large silt deposit areas but also causes high compaction and reduced porosity of the river bed. The floodplain is characterized by extraordinarily high dynamics and relocation of the AC, partially due to channel avulsion (Fig. 3b). Its average relocation rate for the period between 1968-2016 was in the order of 1-1.4% per year of the AC area, which means that the riverfloodplain system is completely restructured within less than 100 years.
The impact of the flow dynamics is apparent from the two series of cross-sectional measurements over  . The percentage fraction of the various habitat types (aquatic: dark grey, upwards directed; terrestrial: bright grey, downwards directed columns) refer to the study area at Poçemi delineated in Fig. 3a during the period 2012-2016. The columns are arranged along the x-axis with respect to their theoretical turnover rate. b Vegetation succession model: The heading denominates the progressive succession from the initial stages B0 and C0 to woodland (BC4). The dashed lines with arrows indicate a continued development. The succession is frequently interrupted and reset by flood events, depending on the distance from the main channel and elevation. The B-and C-series represent the difference in the succession series starting on coarse and fine grained sediment respectively. Dominant species are printed in bold. For a detailed discussion see text the mean water level of the main river arm. The analysis of the discharge capacity of the AC indicates an overtopping of flow into the overbank areas at around 1000 m 3 s -1 (Fig. 1b), which represents the height of an approximately annual flood event in the investigated river sections. The modelling approach enabled a quantification of driving morpho-dynamic parameters like bottom shear stress or stream power (22.8 W m -2 -39.8 W m -2 ) for 1000 m 3 s -1 discharge. These numbers are at the lower boundary of gravel-dominated laterally active rivers (Nanson and Knighton 1996), such rivers being transitional between meandering and braided types (Deslodges and Church 1989). In the study site, extended stretches can be classified as island-braided with tendencies to anabranching. Channel sinuosity as well as the ratio of island length/channel width supported the classification of a laterally active gravel-bed river.
Based on direct sediment transport measurements in 2018, the initiation of bed load transport starts at 300-350 m 3 s -1 . Flows at or above these critical values occurred on average at 29 ± 21 days per year from 1958-1990 (Fig. 1b). Total load of transported sediments based on flow data for 2018, and on direct sediment transport monitoring techniques both of bed load and suspended sediments, yielded values of 5 Mil. tons a -1 for the Poçemi section, which constitutes a significant contribution to delta development and coastal protection (Hauer et al. 2019). The portion of suspended sediment is high: an interesting finding of the transect measurements ( Fig. 4) is that the water levels of the main arm, the disconnected arms and erosion pools differ by up to one meter. This confirms that the lateral groundwater exchange is low, due to the high clogging of the riverine sediments.
Dynamic rivers create a wide range of habitat types and transition zones which change continuously at different spatio-temporal scales (see Table 1). Within the AC, aquatic habitats range from fast current to stagnant conditions. Their ecological characteristicsat a microhabitat scale-vary continuously with water table height. Even small water level fluctuations and ''flow pulses'' lead to continuous changes in the flowage line and habitat conditions, especially for the aquatic biota and the riparian fauna.
Terrestrial habitat types on elevated bars within the active channel and the elevated floodplains exhibit an even higher complexity. The main factors for differentiation are: (a) sediment sorting processes and sediment composition (grain size diameter) as a main factor for water capacity of the sediments and thus the germination of plants, (b) distance from the main river branches, (c) elevation above mean water table, (d) frequency and dimension of flow pulses resulting in different time spans for vegetation development, (e) the groundwater table, (f) timing and mode of seed dispersal, and (g) human impacts by logging, burning, or grazing. Within the active channel we can identify successions on gravel (B series) and on sand/ silt deposits (C series) with higher water retention. On higher islands within the AC and in the elevated floodplain areas, the continuous dynamics of sedimentation and erosion processes lead generally to vertically structured depositions with alternating layers of coarser and finer sediments. As a consequence of several years of sedimentation a clear floristic distinction between a gravel (B-) and a fine sediment (C-) series is no longer possible (therefore BC3 and BC4 in Table 1, see Fig. 6b).
For the analysis of the quantitative composition of habitat elements and their spatio-temporal dynamics ( Table 2), some of the habitat types were combined because they were either continuously interchanging with water level fluctuations (e.g. the riverine habitats A1-A4) or were not clearly discernable in the satellite images. The river with its downstream connected or disconnected side-arms (A1-A4) represents approximately 15-20% of the AC at low and mean water levels. The areal extent of erosion pools (type A5) is very low, yet they are very significant as habitat and refuge for many riverine landscape species. The unvegetated gravel and sand bars (B0 and C0) cover the largest area, about 50% of the AC. Small patches of softwood floodplain forests (BC4) can be found on larger accreted islands. Within the elevated floodplain such softwood floodplain forest patches are also rare and cover only small areas. The same holds true for wetlands (A6 and A7). At the elevated floodplains where grassland is dominant (D), turnover is expectedly lower and the observed structural changes are mainly due to human activities by grazing and burning.
The habitat turnover rates are particularly high within the AC (see Table 2, Fig. 5). The position of the multichannel river and all the major habitat elements, including smaller islands pioneer vegetation and shrubs, changes in the order of 10-20% per year at the habitat scale. Isolated erosion pools within the active channel are specifically short-lived due to erosion and sedimentation, with a life-cycle of less than one year.
Our data highlight two significant features of habitat dynamics (see also Fig. 6a): firstly, the proportion of habitat elements remains constant over time, but-secondly-the continuous shift in the location of the various habitat types indicates a fast turnover and renewal cycle: the whole river-floodplain   The values refer to the study area delineated in Fig. 2. The average area of the AC and of the active floodplain (AF) are 560 ha each system is in a state of a dynamic hydro-morphological equilibrium. Accordingly, we found that: (a) Microhabitat availability is continuously shifting with water level fluctuations and small scale sediment transport processes above a flow of 300-350 m 3 s -1 . (b) The average renewal rate of aquatic and riparian habitats in the AC occurs at a time scale of approx. every 5-10 years due to the cumulative effect of regular, annual floods (HQ 1) (see Table 2 and Fig. 5). (c) Restructuring of the complete floodplain occurs in an order of 50-100 years as a compound result of major flood events.
We suggest that this pattern found for the Vjosa is characteristic for large, undisturbed Balkan gravel bed rivers.
The generalized model of vegetation succession in context with habitat turnover (Fig. 6b) shows that succession on gravel deposits starts with herbs and grasses (Chondrilla juncea, Dittrichia graveolens, Cynodon dactylon, Tragus racemosus-B0), followed by seedlings of Tamarix parviflora, Salix spp. (S. amplexicaulis, S. eleagnos, S. alba) and Populus nigra (B1) which develop into a low shrub (B2). The initial stage of succession on silt and sand (C-series) is dominated by Agrostis stolonifera and Xanthium italicum (C0), followed by a pioneer stage with seedlings of Vitex agnus-castus, Platanus orientalis, Populus alba and Alnus glutinosa (C1) which require higher soil moisture for development to a shrubby pioneer stage (C2). Besides these species a wide variety of herbs prevail as pioneers.
At higher levels a shrub phase on mixed gravel and sand develops (BC3, see above) with woody species (Salix spp., Populus nigra, Platanus orientalis, Populus alba, Alnus glutinosa). On larger islands within the AC and at the elevated floodplain the succession is leading potentially to an early riparian woodland as the final stage of a floodplain turnover cycle (BC4). Platanus and Alnus dominate the tree layer and Rubus sanctus, Brachypodium sylvaticum are main elements of the understorey. Floristic differences to Alpine and Northern Balkan gravel bed rivers are outlined in Appendix A.
All herb species mentioned above as well as the dominant floodplain forest species represent different types of anemochory (wind dispersal). The only exception is Vitex agnus-castus which is barochorous and also able to reproduce vegetatively. Dispersal mode combined with the timing of receding water levels play an important role for germination and vegetation development on newly created sand and gravel bars.
Frequent aggradation of sand supports the spreading of the alien cogongrass Imperata cylindrica. At the elevated floodplain and at higher locations (''islands'' within the AC), regular burning and pasturing leads to a dominance of this species with tussocks of Saccharum ravennae and scattered shrubs of Vitex agnuscastus and Platanus orientalis as a final degradation stage (D). Without regular burning or when burning is stopped, the vegetation would develop into a Platanus orientalis-Alnus glutinosa woodland.
In the short-lived stagnant erosion pools (A5) filamentous algae, Chara cf. vulgaris and Nasturtion-communities are found. In former active channels abandoned from the main channel within the floodplain (A6), or in wetlands along hillslope streams entering the floodplain assemblages of Nasturtion, Glycerio-Sparganion and Phragmition alliances can develop, followed by Salix triandra and S. alba leading potentially to an Alnus glutinosa-Populus alba forest.
Fluvial dynamics as the basis for the high and specific biodiversity The widely undisturbed river dynamics in the Vjosa catchment enables the development of the entire spectrum of floodplain ecosystems from unvegetated gravel bars to floodplain forests. They are of outstanding importance and in an excellent conservation status. All riverine habitats typical for the Vjosa are listed in Annex 1 of the European Union Habitats Directive (92/43/EEC, amended document from June 10th, 2013). The riverine habitats 3220, 3250, 3230, 3240, 92D0, 6210 and 92C0 are covering 86% of the total area of the Poçemi-Kalivaçi river corridor. This underpins the importance of the Vjosa river corridor at a European scale (Table 3).
The fish fauna is an important indicator for the ecological integrity of large river systems as a broad spectrum of habitat conditions are required to fulfil the habitat needs of different ecological guilds in the course of their life cycle (Schiemer and Waidbacher 1992). The Balkan rivers are characterized by Europe's highest concentration of endemic fish species (Freyhof and Brooks 2011). Shumka et al. (2018a) list 31 fish species inhabiting the Vjosa river system including the critically endangered species Anguilla anguilla (European eel) and endangered species Aphanius ibericus (Spanish toothcarp) and Gobio skadarensis (Skadar gudgeon) according to IUCN criteria. Furthermore, the Vjosa's fish fauna is characterized by several species endemic to the Balkans e.g. Barbus prespensis (Prespa barbel), Luciobarbus albanicus (Albanian barbel), Pachychilon pictum (Albanian roach) and Oxynoemacheilus pindus (Pindus stone loach).
The Vjosa is a large migration corridor for both anadromous and catadromous species as well as other saltwater species entering the system. Meulenbroek (in litt.) documented Anguilla anguilla from 100 mm up to 510 mm length up to the boarder to Greece.
These findings indicate that Mediterranean rivers are significant for the conservation of the European eel stock. The river potentially provides habitat and spawning sites for anadromous sturgeons (Acipenseridae) such as the critically endangered Acipenser sturio and Acipenser naccarii, which are found along the Albanian coast (Freyhof and Brooks 2011). The assessment of the fish fauna (Meulenbroek et al. 2018) showed distinct differences in the distribution of species and their ontogenetic stages within the riverine habitat range (A1-A5) indicating the need for a broad spectrum of habitat conditions in terms of water depth, flow velocities and substrate types, that provides the basis for species and ontogenetic niches of the characteristic assemblages.
Many taxonomic groups of aquatic and terrestrial invertebrates are not, or only sparsely treated in the IUCN Red List, the Bern Convention list or the Table 3 The characteristics, conservation value and the areal representation of the four main habitats of the Vjosa floodplain in the study site. Given are the habitat type (first column), a short characterization of the habitat type (second column), the corresponding Natura 2000-habitat type (  Albanian Red List of endangered species. To evaluate the conservation status of such taxa we have to refer to their decline in geographic occurrence and population size in Central Europe over the past decades. Macroinvertebrates are a main component for the biological quality assessment in the European Water Framework Directive (Hering et al. 2010). Our survey, which gives only a snapshot of the existing diversity at the catchment scale, yielded 143 species thus far, of which approx. 50%-according to ''Fauna Europaea'' (https://fauna-eu.org)-are new to the fauna of Albania. It includes species endemic to the western Balkans (e.g. Ephemeroptera: Rhithrogena neretvana, Ecdyonurus puma, Ephemerella maculocaudata, and Trichoptera: Rhyacophila diakoftensis, Rhyacophila balcanica, Rhyacophila loxias, Micrasema sericeum, Thremma anomalum, among others), and vital populations of pan-European species like the Plecoptera Marthamea vitripennis and Xanthoperla apicalis, the Ephemeroptera Neoephemera maxima and Prosopistoma pennigerum and the aquatic beetle Potamophilus acuminatus (Coleoptera: Elmidae). These species were formerly widely distributed in large, dynamic rivers across Central and Eastern Europe and are now on their decline (see Graf et al. 2018, Bauernfeind 2018 and are considered to be endangered at a European scale. A detailed list with an evaluation of the conservation status of individual species is provided in Appendix B).
The significance of the intact fluvial dynamics is illustrated here with the example of the ground beetle fauna (Carabidae, see Paill et al. 2018). The species richness in the Vjosa river corridor is enormous considering the limited collection time and the small river section which has been studied so far. 112 species have been documented. This exceeds the species richness of carabids in most other near-natural river systems in Europe (e.g. Plachter 1986). In the Tagliamento in Northern Italy, for example, 185 carabid species have been found based on an investigation over the whole river length from headwaters to the estuary over a period of more than 20 years (Kahlen 2009). From the 112 species (2,327 specimens) collected on the Vjosa, 70 (1566 specimens) were exclusively found within the active channel (AC). The relative abundance of stenotopic riparian species reached values of 93% to 97% (in terms of individual numbers) in the regularly flooded levels of the AC, but only 4% in the higher floodplains. In the AC the faunistic patterns vary at a small scale between different habitat types especially in the highly dynamic initial and early pioneer stages where 36% of individuals belonged to species almost exclusively found on gravel substrates (B0-B2) and 57% exclusively on sand (C0-C2). In the older succession stage (BC3 and D) the proportion of species found exclusively here is low (1-2% of all individuals). Also in terms of ecological traits (e.g. flight ability, body size) a clear pattern of habitat preferences is evident: in the continuously changing early pioneer habitats on sand (C0-C2), species with high mobility and re-colonisation capability dominate (e.g. Cicindela monticola albanica or Bembidion quadricolle) making up 40 to 50% of the abundance. This is linked to the patchy structure, small size and time-restricted availability of moist conditions of the sand bar habitats. The large number of species found and the high percentage of stenotopic floodplain-species which have become very rare throughout Europe (e.g. Bembidion quadricolle, B. scapulare, B. striatum, Stenolophus discophorus, and Poecilus striatopunctatus) underlines the high conservation value of the Vjosa. It represents a genetic pool of international significance (for details see Appendix C).
Other invertebrate species are also well represented. Among orthopterans Saga pedo is listed as a strictly protected species in Appendix II of the Bern Convention. Sphingonotus coerulans, Acrotylus insubricus, Xya pfaendleri and Xya variegata, are listed on the European Red List (see Rabl and Kunz 2018).

Discussion and summary
The Vjosa and its surrounding habitats are of high conservation value as both the main channel and the riparian landscape comprise a mosaic of habitat types with specific turnover cycles along an undisturbed longitudinal river continuum. A ''natural flood regime'' (Poff et al. 1997) associated with a ''natural sediment regime'' (Wohl et al. 2015) generate a high spatio-temporal heterogeneity, a continuous habitat rejuvenation and biotic successions. The wide range of habitat types and ecotones in combination with dynamic water level fluctuations provide the conditions for the specific associations of well adapted rheophilic and riparian species, and high levels of alpha, beta and gamma diversity (Ward et al. 1999). Extensive studies of other large undammed gravelbedded rivers in the temperate zone-e.g. the Tagliamento in Italy (e.g. van der Nat et al. 2002) or the Flathead river in Montana, USA (e.g. Stanford et al. 2005)-revealed a similar critical role of fluvial dynamics for maintaining habitat and biotic biodiversity. In the majority of European countries such natural characteristics of large, dynamic rivers are no longer present (see Fig. 1).
The high abundances of a typical and critically endangered riverine fish community throughout the whole river course highlight the importance of habitat diversity and the undisturbed longitudinal river continuum (Meulenbroek et al. 2018;Schiemer 2000). The aquatic invertebrate communities harbor viable populations of highly threatened species that have largely or completely disappeared from other European rivers. The syntopic occurrence of such potamophilous species underlines the high conservation value of the Vjosa. The key features of rivers still colonised by such characteristic guilds in their different development stages are high fluvial dynamics which provide connectivity within a spatio-temporal habitat mosaic and buffer capacity against physical disturbances. The same holds true for the riparian fauna and vegetation. The high numbers and prevalences of stenotopic floodplain carabids for example, are unparalleled, while densities of rare habitat specialists among the accompanying flora were entirely unexpected.
Moreover, all species found are classified as typical elements of the Natura 2000 habitat types 3220 (alpine fast flowing rivers) and 3230 (alpine rivers, gravel bars with ligneous vegetation). Thus, the high conservation value of the Vjosa is evident. With the impending habitat degradation and fragmentation by dam constructions we expect a massive decline in abundances and the local extinction of several species in the area. Lessons learned from other European rivers underpin the negative ecological effects of river-damming (Poff et al. 2007). Dams homogenize the flow dynamics of rivers and reduce their geomorphic complexity (Graf 2006).
Decision-makers and stakeholders should be aware of the profound consequences and major environmental threats of dam construction on intact rivers (Tockner et al. 2008). Today, the ecological and morphological status of highly modified rivers in many European countries is being improved by costly restoration measures. Besides environmental issues, negative socio-economic side-effects should be considered when deciding on the construction of hydropower plants (see Buijse et al. 2005). In the case of the Vjosa, a major challenge is the high sediment load of the river that (i) will reduce efficiency of dam operation while (ii) increasing river bed incision downstream of the dam and coastal erosion. These effects will affect the quality-of-life and economic prospect of the local population (Hauer et al. 2019).
Finally, there are operational and legal concerns regarding the planned hydropower dams: during the past 20 years, water body management is developing standardized operational procedures, which should be adhered to. In EU countries, for example, river management must follow the European Water Framework Directive (2000/60/EC), Natura 2000 Directive, EU Birds and Habitats Directives (92/43/EEC) and EU Flood Risk Directive (2007/60/EU). These regulations stipulate that projects have to evaluate short-, medium-, and long-term impacts on nature and affected residents and must consider alternative lowimpact concepts. Important water policy instruments such as the Water Framework Directive of the EU request a comprehensive catchment-wide planning process with clearly defined procedural steps. We call for a platform to be established at the science-policy interface, where scientists interact with policymakers, authorities and other stakeholders. Such a platform should be used to explore scenarios for the sustainable development of the Vjosa river corridor, acknowledging the interdependence of the integrity of riverine ecosystems with economic, social and cultural aspects of human well-being. The present multidisciplinary baseline studies on the ecological and morphological status of the Vjosa form a prerequisite for any assessments of the ecological effects caused by dam construction. Such studies should be adopted as a standard for river management in the Balkan area, where widespread data deficiency currently prevents the elaboration of appropriate environmental impact assessments.
Finally, in light of the extraordinary biodiversity and uniqueness of the Vjosa river-floodplain ecosystem, we call for the Vjosa to be protected by evaluating different options including the establishment of a protected area (e.g. national park) to ensure that the outstanding conservation value is being secured. The Bern Convention of the European Council has already opened a case file at the 38th Standing Committee meeting in 2018 on the Vjosa. The Convention recommended to the Albanian Government to suspend all dam projects on the Vjosa river and prepare an integrated River Basin Management plan, as well as a strategic environmental impact assessment including social aspects in collaboration with Greece.