The present work considers the section of the Mesta River on Bulgarian territory using the integral method for evaluation of climate and anthropogenic impact on the river flow. The level of this impact is determined by the index K(i) (flow module), the coefficient C(i) for the deviation of the average annual water volume Q(i) from the flow norm Q(o) and the index h(i) for the deviation of the average annual rainfall volume H(i) from the average multi-annual rainfall volume H(o). The dynamics of the average annual flow Q(i) at two typical hydrometric stations - Yakoruda and Khadzhidimovo, as well as the dynamics of the average annual rainfall for the Yakoruda station was examined for the period 1955-2003. The data for the considered period 1955-2003 exhibit a decreasing trend of the average annual water volumes dynamics for both stations due to the impact of climate changes in the Mesta River catchment.