Abstract: Boring hole data showed that there was a deep overlay and an associated unloading and relaxation zone in the valley bottom of a hydroelectric station in southwestern China. The deep overlay could be vertically divided into three layers. The upper and lower parts were normal river faces, while the middle part was a sequence of aggradation of multi-genesis deposits. The rock mass in the valley bottom was relaxed on structure, bad integrality, splay cranny and fill with clay, weathering badly in the faces of cranny. Study on the relaxation zone in the base of river valley showed that when the river cut adown, it caused stress concentration and led to shearing failure. After far-flung unloading rebound and weathering, the rock mass in the valley bottom formed certain thickness of relaxation zone.