Sediment discharge from mountainous areas causes severe sediment and flood disasters. Therefore, it is important to predict the sediment discharge from mountainous areas in order to prevent increased damage due to climate change. To improve the accuracy of sediment transport model for mountainous areas, we included the effects of the expansion of the river width and hiding effect due to boulder protrusion in the numerical model “SiMHiS” which integrates sediment production, sediment supply, and sediment transport. As a result, we succeeded to reproduce the amount of sediment discharged from the upper reaches of the Pekerebetsu River and the expansion of the river width during the 2016 heavy rainfall in Hokkaido, Japan. The original model involved extreme riverbed change. However, because the new model is able to consider the decrease in shear stress due to the widening of the river, extreme erosion is suppressed. Furthermore, because it considers the hiding effect of fine sediment appropriately, extreme deposition is suppressed.