Hangzhou Bay is wide outside and narrow inside. It is100km wide at sea, but only 3km wide to the west near Yan Guan Town in Haining County. The tide has just entered Hangzhou Bay, and the water surface is wide. The farther to the west, the more restricted by the topography on both sides of the river, so we have to pile up, and the tide heads pile up higher and higher, like a vertical water wall, pushing westward. At the same time, due to the action of tidal current, a large amount of sediment spilled into the sea from the Yangtze River is continuously brought to Hangzhou Bay, forming a huge "sandbar" like a threshold at the mouth of Qiantang River. When the sea water pushes the river into the mouth of Qiantang River, the sandbar blocks the tide head, forming a spectacular scene in which the back waves chase the front waves and wave after wave.