The concept of the rate of transport of exchange and non-exchange forms of radionuclides over river catchment is introduced. On this basis, a differential transport equation is proposed for finding the radionuclide content in soil and radionuclide flow into a river with nonuniform contamination of the catchment area. The time dependence of the radionuclide wash-off time constant is determined from the measurements. It is found that transport is slow, so that radionuclide decay plays a large role. A large fraction of the radionuclides, aside from the long-lived transuranium nuclides, decays within the catchment area and does not enter the river. The flow into the river is largely determined by the radionuclide content near river edge. Examples of calculations of radionuclide flow into a river and bukhta are presented.To study the transport of radionuclides in a river and its basin, the wash-off from the catchment must be taken into account. This process affects their concentration in the water and soil of the catchment. Balance equation can be used to analyze transport in the catchment. Data from many measurements have been generalized for this purpose [1][2][3][4][5][6][7].The following problems are examined in the present article: 1) the determination of the radionuclide wash-off time constant at a given location in the basin; 2) analysis of the radionuclide content in rivers in the zone of the Chernobyl nuclear power plant, Siberia, Italy, France, and Finland to find the wash-off time constant and its time dependence;3) formulation of the transport equation for a radionuclide in a catchment with a constant wash-off time; and 4) determination of the wash-off characteristics for radionuclides in a river and bukhta using the transport equations, including with nonuniform contamination of the catchment.As a result of the radioactive contamination of catchment due to the operation of a nuclear power plant, enterprises reprocessing nuclear fuel, storage facilities for spent nuclear fuel, and bases for re-fueling reactors and salvaging submarines, radionuclides are transported by surface wash-off to a river (bukhta, lake). During this process, some radionuclides decay and only a fraction reaches a river. The shorter the half-life and the slower transport, the smaller this fraction is.It is assumed in this article that a radionuclide migrates toward a river at a certain rate. This parameter makes it possible to formulate the balance equation for the contamination density of the catchment. The solution of this equation determines the radionuclide flux into the river and the contamination density for known dependence of its flow onto unit surface area of the catchment.Determination of the Wash-Off Time Constant and Transport Rate of a Radionuclide in a Catchment. The transport rate determined from the expressions for the activity per unit area per unit time of the exchange and non-exchange forms