Stream transport is an important process in the dispersion of radioactive material that has been released by man into his environment. Some portion of the radionuclides that enter the stream system may remain in solution. However, a significant amount will become associated with aquatic organisms and sediments in the stream. The relative distribution will depend upon the particular radionuclide and the environmental factors of the stream system. Contemporary sediment samples from preselected locations in the Guadalupe River drainage basin were analyzed for their mineral composition, ion exchange capacity, and radionuclide sorption. The clay minerals were the most significant agents in the radionuclide sorption processes. X-ray diffraction patterns of the clay-sized particles show the presence of a heterogeneous mixed-layered clay mineral complex of poor crystallinity. The degraded nature of these crystallites is suggested by the low intensities and the broad and diffuse diffraction maxima. Ion exchange capacity measurements gave values of 10 to 47 meq/100 grams and projected values of 20 to 73 meq/100 grams of clay material. Radionuclide sorption studies using cesium-137 and strontium-89 were made on the naturally occurring sediments and sediments from which the organic fraction had been removed. Sorption values were compared with mineralogy and ion exchange capacity to obtain points of correlation. Rates of radionuclide sorption and release were highest during the first few moments of contact, but totals slowly continued to increase throughout the seven-day duration of the experiments. Sediments containing the naturally occurring organic fraction had the highest sorption values both for strontium-89 and cesium-137. The organic fraction contributes to the over-all sorption capacity of the sediments, even though some of the organic molecules may block exchange sites on the clay minerals against exchange with the radionuclide. These laboratory experiments indicated that radionuclide sorption in the dynamic fluvial environment was temporary. When the contaminated sediments were placed in conditions simulating a marine environment, 43 percent of the cesium-137 and 42 percent of the strontium-89 were released because of competition for the exchange sites from the more abundant ions in sea water. Even though fluvial releases are considered an acceptable means of radionuclide disposal, it should be emphasized that all stream systems are dynamic, and this disposal is at best temporary. The brief respite gained today may be overshadowed by the ultimate contamination of tomorrow. / text