Abstract. The Yesa reservoir, the largest Pyrenean reservoir, was constructed in 1959 to supply water to new irrigated areas in the Ebro Depression, NE Spain. It is filled from October to May-June and then releases large quantities of water in the summer via the Bardenas Canal. The results confirm that the frequency of floods downstream of the dam decreased. The reduction mainly depends on two factors: i) the water storage level, and ii) the season of the year. Floods are very well controlled when the reservoir level is lower than 50%. Between 50 and 70%, only the highest floods are controlled. Finally, the reservoir retains mainly autumn and spring floods; most winter floods are released downstream to ensure the safety of the dam.

Keywords: reservoir, flood control, flood seasonality, flood frequency, river regime, Pyrenees