Water falls to the earth when it rains or snows. Then it evaporates from rivers and lakes and rises into the air to form clouds. When those clouds get full enough, it rains again and the cycle begins anew. This is called the hydrologic cycle or the water cycle. Watch a movie about the water cycle Earth movers Rivers carry more than water.
Rivers are like big conveyor belts.
A river is constantly scooping up and dropping sediment, literally changing the face of the earth. The faster a river travels, the more sediment it carries. The Mississippi River has carried billions of tons of sediment from its headwaters in Minnesota to the Gulf of Mexico, forming the ground on which the city of New Orleans sits. The slowest moving rivers have the greatest amount of sediment and silt on the bottom, allowing the greatest number of plants to grow. Some fish, like trout, need fast clear water.