Abstract: In order to measure the degree of the deposition-caused innavigability of shoals quantitatively,"navigation hindering siltation thickness(NHST)" is proposed.Analyses show that for the majority of the Yangtze River′s shallows characterized by "deposition in the flood season and scouring in the dry season",there is a critical flow range: 14,000~18,000 m3/s for the upstream of Jingjiang River and 12,000~16,000 m3/s for the downstream of Jingjiang River.NHST is inversely proportional to the difference of continuous days between erosion critical flow and deposition critical flow.The influence of the river morphology is in the performance of the larger river phase coefficient;the larger widening rate and the maladjustment of radius of curvature between the flow dynamic axis and the river channel lead to the formation of shallows.