When does pollution kill a river?

Dying river - will die when pollution reaches such a level that all the available oxygen is absorbed. This can be happen if the oxygen is used up by the 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 overcrowded and die of exhaustion. Then the fish also die because they are deprived of plant food. 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.

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