External water exchange of overflow lakes is presented as the integral indicator, defining the type of lake basin. Intensity of water replacement in the lake (Kw) determines many processes taking place in the lake and its drainage area. Simultaneously this process influences the runoff of the outflowing river. Among the processes in the lake, depending on Kw, transitory-accumulative possibilities, predominance of allochton and limnic processes, and the participation of Kw in forming trophic status are considered. Some examples of the dependence of the parameters of lake level distribution curves upon the Kw values are given. The problem of calculation of Kw for unexplored objects is also regarded. The text contains specific examples of the results of calculations and analysis.