In the forest, Cs gradually migrates from the tree canopy to the forest floor and from the litter layer to the soil layer. Three transport pathways for Cs to fish living in mountainous streams are possible: direct litterfall into rivers, lateral transfer from the litter layer, and lateral transfer from the soil layer into rivers. The developed model simulates the Cs transfer between forest compartments (e.g., leaves, litter layer, river water) to understand the time evolution of Cs concentration in each compartment.