The need for efficient and economical use of the world’s water requires the implimentation of modern methods to make decisions about water management. This inspired the principal philosophy of the management study of the Hellenic part of the Strymonas River catchment (6400 km2). An important issue in such studies is to simulate the hydrology of the river catchment. The state-of-the-art couple model MIKE SHE/MIKE 11 is a powerful, physically-based distributed hydrological and hydraulic simulation tool. To determine water balance, hydrology and hydraulic functions in the river catchment a water level monitoring network was established. Data collection was performed during 2004 and 2006. The philosophy and main assumptions that underlie the current work are described. Furthermore, the effectiveness of the irrigation networks in the Strymonas River basin is checked. Finally, the negative effect of the current irrigation network on the lake’s water level fluctuation is pointed out and simple but effective solutions are proposed.