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The dynamics and present state of the radioactive contamination with 137 Cs of littoral soil of Lake Kozhanovskoe and Lake Svyatoe on the Besed' River are presented. The parameters of the vertical migration of 137 Cs in soil -the rate of directional transfer with soil moisture, the diffusion coefficient, and the average velocity of vertical migration -are estimated by comparing the experimental and model distributions of 137 Cs content over soil depth. It is shown that at the present time the two 5-cm layer of soil can contain 20-90% 137 Cs depending on the type of soil and landscape. The average values of the diffusion coefficient, the rate of directional transfer, and the vertical migration velocity for 12-, 13-, and 20-year periods after the Chernobyl accident are 0.1-2.8 cm 2 /yr, 0.1-0.3 cm/yr, and 0.1-0.8 cm/yr, respectively.The slowly draining Lake Kozhanovskoe and the nondraining Lake Svyatoe on the Besed' River in Bryansk Oblast are water reservoirs most strongly contaminated with 137 Cs and 90 Sr as a result of the Chernobyl accident [1]. At the present time, these nuclides enter the lakes as a result of being washed off the contaminated water sheds by rains and floods. 137 Cs makes the main contribution to the contamination of the soil. The 137 Cs content is approximately 50 times greater than that of 90 Sr. Experimental data on the soil content and vertical distribution of 137 Cs in the soil in the littoral zones of these reservoirs were obtained from 1992 to 2006 as part of the radioecological studies of surface water objects in Bryansk oblast [1,2].The purpose of the present article is to estimate the parameters of the vertical migration of 137 Cs in the soil of the indicated littoral zones on the basis of data on its concentration in 1998, 1999, and 2006. Lake Kozhanovskoe is located in the watershed of the Iput' River. The average depth of the lake is 1.5 m, and the area of the lake and watershed are 6.5 km 2 and ∼3 km 2 , respectively. The littoral zone consists mainly of boggy-peat and soddy-podzolic soils. According to the 1993 data, a high fraction of insoluble and a low (2%) fraction of exchange 137 Cs predominate in the boggy-peat soil; the mobile 90 Sr fraction is 40-50%. Lake Svyatoe is located in the watershed of the Besed' River. The areas of the lake and watershed ara 0.1 km 2 and 0.34 km 2 , respectively; the average depth of the lake is 3 m. The dominant soil in the littoral zone is soddy-podzolic, in which according to 1993 data the mobile fractions of 137 Cs and 90 Sr are 5-20% and 40-80%, respectively.The 137 Cs content in the soil was determined by two methods. The first one is based on determining the 137 Cs content using a RKG-095N (Korad) spectrally sensitive portable gamma-radiometer with detection limit 18.5 kBq/m 2 with measurement time 300 sec [3,4]. The exposure dose rate at heights 5 cm and 1 m above the soil surface were measured with a

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