A watershed is the word for a segment of land that drains into a common body of water. Watersheds don't just drain the water, though -- they pass along any contaminants and chemicals that have been dumped into them. As a result of this pollution, the world's water resources are rapidly becoming compromised.
Of course, no good can come from pollution of any kind. Water pollution affects marine and freshwater wildlife, often disrupting the delicate ecological balance by altering habitats and the local food chain, eliminating important species while introducing invasive species, the latter of which can have disastrous effects on biodiversity.
Pollutants simply overwhelm nature's ability to take care of them. Sewage and other organic materials in water are normally broken down by organisms called beneficial bacteria. When enormous amounts of pollutants mix with water, the free oxygen supply is depleted, killing off the beneficial organisms. Some pollutants are even non-biodegradable.
Two types of pollutants come to Earth in the form of acid rain. Sulfur dioxide and nitrogen oxide are released into the air by the burning of fossil fuels (car exhaust, industrial output, etc.). A chemical reaction turns them into sulfuric and nitric acid. Rain, snow and sleet wash them out of the air and ruin life in ponds and lakes. Other examples of water pollution include phosphates, which encourage excessive plant growth and disturb the ecological balance. Meanwhile, heated water from the electric power industry creates thermal pollution, which depletes free oxygen.
Human development activities have greatly modified watershed topography as well, altering natural drainage systems. Construction of roads and buildings has resulted in creating "fast lanes" where rainwater drains quickly into streams and lakes instead of being absorbed by soil and plants. The threat of erosion also exists, where running water eventually destabilizes the land itself.
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