When does pollution kill a river?

A river will die when pollution reaches such a level that all the available oxygen is absorbed. This can happen if the oxygen is used up by presence in the water of an excessive number of waste organisms. Large amounts of nitrogen or phosphorus make a river’s oxygen-producing plants grow so rapidly that they become over-crowded and die of exhaustion. Then the Finally various forms of bacteria decay and the water becomes putrefied. All human industrial waste produces pollution. Excessive quantities of chemicals and minerals deposited in rivers will kill the water by destroying its oxygen content. Some rivers flowing through industrial areas have been so changed that it is possible to set fire to them. This happened to the Cuyahoga River in Ohio in 1969.