Description: We examined the cesium-137 ( 137 Cs) contamination of river food webs in a gradient of initial fallout deposition (net density estimates 2.5–3.5 months after the Fukushima Daiichi Nuclear Power Plant accident in March 2011), in Fukushima Prefecture, Japan. Litter, aquatic insects, and salmonid fish were collected in five headwater stream reaches (watershed-average fallout density, 368.1–1398.4 kBq/m 2 ) for the measurement of 137 Cs concentration and stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) in June 2014. The stable isotope ratios suggested that the detrital food chain was a dominant energy pathway in rivers originating from a basal resource (litter) to primary (aquatic insects) and secondary (fish) consumers. The 137 Cs concentration decreased with an increase in the trophic level, with the highest value for litter (10930 ± 5381 Bq/kg, mean ± SD), the lowest for fish (2825 ± 2451 Bq/kg), and the intermediate one for dominant (numerically and biomass-wise) detritivorous insect, Ephemera japonica McLachlan (4605 ± 1970 Bq/kg). 137 Cs concentrations of three trophic levels were linearly predicted by the initial fallout amount of 137 Cs. The evacuation of the gut contents of E. japonica during field experiments led to a reduction in their 137 Cs concentration by approximately 50% within 1–2 day(s) without loss of body weight. This suggested that a substantial portion of 137 Cs contamination of E. japonica was derived from highly contaminated fine solids deposited in depositional habitats at a disproportionately high density. Overall, the initial fallout amount of 137 Cs was helpful in roughly predicting the contamination levels of headwater river-riparian ecosystems with the detrital food chain as a dominant energy pathway. Long-term monitoring of the dynamics and fates of 137 Cs associated with fine organic and inorganic particulates appears important for better prediction of 137 Cs contamination of food webs in forested headwater streams.