The Nuclear Power Station accident of Fukushima in March 2011 emitted enormous amount of radioactive substances, and these deposited onto the forest areas of the northeastern region of Japan. To clarify the mechanisms of dispersion and export of cesium-137 within and from a forested ecosystem, we have conducted intensive observations in a forested headwater catchment in the northern Fukushima. Hypothesized two major pathways of cesium-137 transportations are by water movement in dissolved and particulate forms, and dispersion through the food web in the forest-stream ecological continuum. Monitoring showed that major forms of exported cesium-137 through the stream were suspended matters. High flow event accelerated the export of cesium-137 through streams from forested catchments. Estimation of cesium-137 export from the forested catchments requires precise evaluation of the high flow acceleration during storm events. Because the biggest pool of cesium-137 in the forested ecosystem is accumulation of litters and their detritus, the cesium-137 dispersion is quicker through the detritus food chain than the grazing food chain.