At high tide, the main reason why seawater will flood into inland rivers is tidal gravity. The moon and the sun attract the earth, leading to periodic changes in ocean water bodies. When tidal gravity acts on the ocean, water will be stretched and pushed to form tidal waves. These tidal waves will enter the river with the movement of ocean water, which will lead to the tide going deep into the river. In addition, topography and geographical location will also affect the degree of tidal infiltration into rivers. Some estuaries have wide estuaries and shoals, and these topographical features are helpful for tidal waves to further enter the river.