|Many bridges were broken due to a large channel change during the Hokkaido heavy rain in 2016. We investigated the damage situation of the four bridges (the Kyusen Bridge, the Chiroro Bridge, the Kobayashi Bridge and the Kiyomi Bridge) immediately after the flood. Then we analyzed the collapse mechanism of the Kyusen Bridge using a two-dimensional riverbed deformation model (iRIC – Nays2D). According to the observation result, the bed height near the Kyusen Bridge rose about 2 m. This implies that sediment supply was large during the flood. The results of numerical simulations indicated that the river bank was eroded largely due to both of a large flow discharge and a sufficient supply of sediment.