Greece and Bulgaria, are crossed by three very important transboundary rivers, namely Strourna, Nestos/ Mesta, and Maritsa. However, until recently no effective plan concerning their Integrated Water Management has been made. This is rather justified because an Integrated Water Management must be based on accurate and reliable data coming from a large number of observations and measurements and covering large time periods, in order to have an appropriate statistical treatment of the data and extraction of much trustworthy results. In this paper, histograms of mean discharges at the Greek part of the hydrological catchment and correlation coefficients of values relating Greek and Bulgarian stations at 21 river sites in Nestos/Mesta were analyzed in a monthly basis. The main aim of this study is a first-time presentation of useful meteorological data from both countries and to contribute towards an integrated study of the runoff for the whole hydrological basin, in agreement with directive E.U. 60/2000. This was accomplished within the framework of Interreg 11 transboundary collaboration between Greece and Bulgaria. The hydrological budget equations for the earth's surface were used. The water balance of Nestos/Mesta and Dospatska rivers to the Greek frontiers, as a total, for the period 1935-95, with observed precipitations, produced an annual average runoff coefficient of 0.482. With corrected precipitations, the coefficient acquires a value of 0.438 revealing the good correlation between the Bulgarian and Greek meteorological stations.