Rivers become polluted whenever anything harmful is poured into them. Some rivers are so badly polluted that they contain no living creatures at all. River animals are sometimes killed by industry.But they can also be killed by poisonous materials like fertilizers , detergents, sewage and even hot water. These materials pollute the water by seducing the amount of oxygen which is dissolved in it. River animals need oxygen to live. The smaller amount of oxygen dissolved in the river water , the smaller is the number of animals which can live in it.