Abstract Water exchange in lakes reflects on their trophic status and has implications on water quality. However, in contrary to common belief, beneficial effects of dilution or ‘flushing’, due to high hydraulic load, should not be expected in the case of polluted tributaries. Research conducted in Poland on several hundred lakes indicated, that 80% of lakes with intensive water renewal rates, (200%/y to 21400%/y) were characterized with bad water quality. Those lakes with low, (30% or less) water renewal rate per annum mostly had good or even very good water quality. Inferior water quality of lakes having low water renewal rate was usually associated with the presence of point sources of pollution. Contribution from the tributary sources would improve the lake water quality only if reduction of the loadings to the rivers is achieved through watershed pollution control.