The disposition of radionuclides in and along the 380-mile reach of the Columbia River between the Hanford, Wash., Atomic Energy Commission reactors (the major radionuclide source) and the head of the estuary is a first step in understanding the environmental cycling and ultimate fate of the discharged radioactive materials. Preliminary results for the study period, July 1962 to September 1963, are presented for radionuclide, sediment, and water data from stations on the Columbia River and from stations near the mouths of the Snake River, the Willamette River, and other Columbia River tributaries.