Abstract: In this paper， the distribution characteristics of sediment concentration and sediment grain size are introduced in brief， and the formulas of the sediment incipient velocity and the sediment carrying capacity in the estuary are established. The characteristics of the large variation amplitude of the riverbed， the rapid deposition after rapid erosion， the flood-induced eroding and the tide-induced silting in the upper reach， and the flood-induced silting and the tide-induced eroding in the lower reach are analyzed. The riverbed variation has a great impact on flood and tides. When tidal bore arrives， the flows of the tidal bore violently erode riverbed and result in sharply increased sediment concentration， which mostly reaches a peak value after twenty minutes， thus forming a high sediment concentration region. In normal and dry periods， sediment discharge in flood tide is larger than that in ebb tide， resulting in silting in the upper reach. The tidal bore is one of the mechanisms for serious erosion and deposition of riverbed in the Qiantang estuary.