A New Approach for Determining the Environmental Flow Regime in a Greek River Christina PAPADAKI 1* , Konstantinos SOULIS 2 , Rafael MUÑOZ-MAS 3 , Lazaros NTOANIDIS 4 , Stamatis ZOGARIS 1 , Nicholas DERCAS 2 , & Elias DIMITRIOU 1 (1) Hellenic Centre for Marine Research, Institute of Marine Biological Resources and Inland Waters, *corresponding author e-mail: chrispap@hcmr.gr, (2) Department of Natural Resources Management and Agricultural Engineering, Division of Water Resources Management, Agricultural University of Athens, Greece, (3) Institut d’Investigaciό per a la Gestiό Integrada de Zones Costaneres (IGIC), Universitat Politècnica de València, C/ Paranimf 1, 46730 Grau de Gandia,Valencia, Spain. (4)D. Argyropoulos & associates, Tinou 2, 15562 Cholargos, Greece Abstract: The ecological integrity of freshwater lotic ecosystems depends on the flow regime which has a pronounced natural variability in the natural Mediterranean streams. The increasing human demands for water resources has led to the adoption of the environmental flow concept which focuses on an integrated management of water for long-term sustainability by coupling anthropogenic and environment demands. This has been poorly researched and applied in Eastern Mediterranean rivers. In Greece, management actions usually focus on prescribing minimum acceptable flows estimated by hydrological methods according to summer mean monthly flows. This study applies a combination of different approaches for the estimation of an environmental flow regime in a Greek montane river, which includes fish habitat hydraulic requirements and a temporal variability of minimum flow. The proposed method integrates the results of a hydrologic and a fish habitat simulation procedure in a temporal scale that takes into consideration the natural hydrologic seasonality. The results highlight the advantages of using different policy-relevant methods to estimate environmental flow regime and not just a static minimum flow requirement. Finally, this preliminary application promotes the need for further science-based eco-hydrological approaches that respect the local biodiversity with respect to distinctiveness and holism but also to the current EU Water related policies for water management. Key words: flow regime, Greek river, habitat simulation method, hydrological model, IHA 1 INTRODUCTION Even though a review of the methodologies for Environmental Flow Assessment (EFA) revealed the existence of more than 207 individual methodologies in 2003 in 44 countries (Tharme, 2003), there are still millions of kilometers of rivers that remain unprotected from anthropogenic activities linked to intense exploitation of water resources. It has often been observed that the growing water demand for irrigation, hydropower, industry and domestic uses may result to the over-allocation of water to off-stream uses as well as other alterations of their natural flow regime (Poff et al., 2010), thus degrading streams even in formerly pristine upland areas. When it comes to Greek rivers, very limited work has been carried out regarding the development of adequate EFA by taking into account local conditions. According to the current Greek legislative framework the mitigation of the environmental impacts due to hydrological alterations requires the implementation of a Minimum Flow (MF) equal to a portion of the average summer months' discharge. In cases of rivers with adjacent diversions and the presence of ichthyofauna the MF is defined as such in order to ensure a minimum river depth of 0.2 m at the thalweg. Besides the simplified approach of the above mentioned rule of thumb the most common process for the assessment of MF, capturing the dynamic behavior of the river flow, is the application of hydrological methodologies based on the statistical analyses of natural (unaltered) historical flow- series (Mathews and Richter, 2007). In this way the natural seasonality and variability of flows, including floods and low flows, is identified and included in the assessment process (Acreman, 2004).