河口，为什么都叫三角洲？

Rivers carry lots of sand and mud. 大河带着很多泥沙。 Where the river meets the sea, it slows down. 河水到了入海口，就慢了下来。 The mud drops down and piles up. 泥沙沉下来，一点点堆高。 Over years, new land grows out. 年复一年，长出了新的陆地。 That land often looks like a triangle. 这块陆地常常像个三角形。 That is why we call it a delta. 所以人们叫它三角洲。