Which is the longest river in the world? Why is the water hyacinth known as the 'million-dollar weed'? Which country receives around five metres of rain every year? A river is a body of flowing water. Sometines it rushes and rages. At other times it flows slowly along. Rivers always travel downhill, pulled by Earth's gravity. They can start from high up in hills or mountains, from lakes, or from lots of little streams joining together to form a river. Most rivers end up flowing into the sea.