This study examined 137Cs detachment from needle litter of Japanese cedar (Cryptomeria japonica) through leaching and decomposition in a headwater stream located 45 km from Fukushima Daiichi Nuclear Power Plant. We placed litter bags in pools and riffles and removed them after 4, 15, 75, 150, and 240 days. The 137Cs activity concentration in litter collected after 4 and 15 days was 48–72 % of the initial 137Cs activity concentration in litter. The 137Cs activity concentration in litter after 240 days in the pools decreased by 26 %, while that in litter of riffles decreased by 9 % compared with the initial 137Cs activity concentration. The dry weight of litter became 71 and 86 % of initial weight after 75 days, and from 32 to 51 % after 150 days. The decay coefficient of litter (k = 0.0060) in the riffles was 1.6-fold higher than that in the pools (k = 0.0037). The greater decrease in litter weight with longer submergence time was possibly associated with a combination of leaching and decomposition, in particular in litter that was submerged for 100 days in the water column. The difference in the rate of decomposition and 137Cs detachment from litter between the riffles and pools was due to the substrate and flow conditions at the litter bag sites. Examining the different patterns of 137Cs detachment from litter among channel morphologies is necessary for comprehending the mechanisms of 137Cs accumulation in aquatic biota. The findings of this study suggest that elucidating leaching and decomposition processes in litter of stream channels is important for understanding 137Cs dynamics in forested headwater ecosystems.