Abstract:The water and sediment conditions influence the channel evolution around the river boulder regions in mountainous sediment-laden rivers, which significantly changes the characteristics of water and sediment movement and riverbed deformation. Through field investigation of Baisha River and Longxi River, the tributaries of Minjiang River in Dujiangyan City, the bed sand composition, and morphological characteristics of the typical boulder reaches were analyzed. Field investigations found that due to the torrential rains and floods of mountain rivers and the sediment carried by them, as well as the changing terrain of the river, a large amount of upstream sand was silted in the section of the boulder river, resulting in the development of many beaches in the riverbed. Based on the field investigation of typical riverbed and physical model experiments, the effects of the boulder and sediment supply on hydrodynamic characteristics and beach development in river reach were analyzed. According to the experimental results, it can be concluded that the heavy rain and flash floods produced abundant sediment supply including boulders, resulting in the large average particle size of the Baisha River and the Longxi River, and forming a large number of boulder reaches. The hydraulic factors around the boulder abruptly, lead to considerable changes in the water level and riverbed forms. Under the sediment supply, the upstream sediment is mainly lateral banded source deposition, while the downstream sediment deposition is prominent on both sides, which is easy to form a drift stone beach. The development of boulder beaches can increase the local riverbed gradient and slow down the river channel cutting. While the non-uniformity of the upstream sediment supply changes the local water level and sediment deposition scale around the boulder. Therefore, the morphological characteristics of the boulder riverbed under the condition of water and sediment are revealed. It shows that the sediment and boulders from the upper reaches of the mountainous rivers jointly affect the development process of the beach, and the boulders have a prominent impact on the movement of water and sediment in the mountainous sediment-laden river.