Abstract: This paper presents results of measurements of 210Pb in sediments of the humanmade lake Koz³owa Góra. The measurements were done on the complete sediment cores taken in 1999 and covering the whole 60 years span of life of the lake. For the first time in Poland, the gamma ray spectrometry was employed, additionally to the alpha spectrometry method, what gave the opportunity to date the sediments and also to determine the sedimentation rates, and to correlate their changes with modification of the dam. Analyses of measured 210Pb radioactivity and dating based on a modified CRS model and led to a model of sedimentation that explains most of experimental results.