Abstract: The deposit layer in Yalong river bed is usually over 30 m thick and can be thick up to 51.6 m. The deposit thickness varies substantially horizontal direction. The deposit layer can be divided into three parts along the depth. The upper and lower parts are normal river faces, while the middle part is composed of alluvial, pluvial, barrier lacustrine, fluvial-glacial and comparatively thick. The underlain bedrock has a 20 m thick unloaded and relaxed zone. This zone has weak weathering, high permeability, poor integrality and poor engineering-mechanics features. It has an engineering implication in dam construction since it can be a leakage zone beneath dam base. The thick deposit layer was resulted from a second series of deposition. The unloaded and relaxed zone is due to the fact that the river bed was formed 25000 years ago, which had adequate time for the bedrock to unload, relax, and be weathered.