Inter annual water temperature variations in Nestos River-reservoir system, northern Greece

The drainage network of Nestos River is consisting of a transboundary system entering the Greek territory from Bulgaria with two main channels Nestos and Dospatis, draining a mountainous watershed area. In the Greek territory two reservoirs were constructed, Thesauros and Platanovrisi, with their hydroelectricity power plants. An automated monitoring system is installed in the Greek part of the watershed including four permanent monitoring stations recording water temperature among the environmental parameters. One station is installed in each of the two main branches entering Greece (Nestos and Dospatis); a third floating over the deepest area of Thesauros Reservoir; and a fourth in the main river channel in the Nestos delta plain. Continuous monitoring, over almost a decade after the construction of the dams, enables the determination of long term water temperature variations. Downstream of the dams the river water derived from the cool hypolimnion is characterized by low temperatures, even if it flows in the delta region, during the summer. However, the recorded temperature variations show a small positive trend in the recent summers. Thy hypolimnion of the Tesaurus Reservoir exhibits a constant increase form 4 - 6.5 degree C in 1998-2000, to 8-11 degree C in 2007. The very cold river water temperatures during summer in the delta area (during the first years after dam construction) seem to improving recently (19-25 degree C), with the temperatures rarely drop below 20 degree C. This change could be attributed to the dramatic decrease of the number of days that cold water masses (