The aim of the present research is to evaluate the contribution of ¹³⁷Cs global fallout to the contamination of bottom sediments in the mixing zone of the Ob fresh water and the Kara sea salt water. Sources of cesium-137 pollution were explored in the Kara sea continental catchment based on modeling and published data. The main sources are global radioactive fallout (24.8 - 25.2 10¹⁵Bq), Semipalatinsk nuclear test site (1.3 - 37.0 10¹⁵Bq), PA 'Mayak' (liquid radioactive wastes in the Techa river 1.9 - 13.0 10¹⁵Bq and Karachay lake 200 - 444 10¹⁵Bq, Kyshtym disaster 2.7 1015Bq), Krasnoyarsk mining and chemical industrial complex (0.01 - 0.11 10¹⁵Bq). Cesium-137 inflow to the Kara sea from the Ob river basin (including decay) is 0.04 - 0.44 10¹⁵Bq. Global fallout 137Cs runoff forms 48 - 73% of total 137Cs runoff. Cesium-137 runoff from the landscapes contaminated due to activity in the Semipalatinsk nuclear test site and PA 'Mayak' is 2 - 42% and 10 - 26% correspondingly.