Many living things rely on healthy rivers for their wellbeing. Free-flowing rivers carry several necessities: fresh water, which can be purified and used as drinking water by communities; sediments, which help create coastal wetland features such as marshes that provide habitats for many animal and plants; and oxygen, which fish and other aquatic life need to breathe. Pollution poses the largest threat to rivers and the broader network of tributaries and basins that make up river systems.

The majority of a river's living inhabitants — bacteria — need oxygen to break down and purify organic matter in the water. They get this oxygen naturally from the water. If untreated sewage and agricultural runoff enter the water, nitrogen and phosphorous levels can suddenly climb, and algae and microorganism populations boom. This boom uses up oxygen more quickly than it can be naturally restored, so oxygen levels plunge, causing fish and other animals to die and the waterway to fill up with undecomposed organic sludge.

Industrial waste also affects water quality. Organic chemicals — including pesticides, herbicides, and pharmaceutical drugs — can enter freshwater food chains and ultimately reach drinking water supplies. Furthermore, many inorganic pollutants, such as acids, zinc, and lead, are known to be toxic and disease-causing. Heat, another form of industrial pollution, reduces the amount of oxygen that water can carry. One of the chief sources of heat pollution in waterways is water that has been used to cool power stations.

Over the last half of the twentieth century, the water quality of one of Western Europe's most commercially important rivers, the Rhine, rapidly deteriorated. By 1958, once-plentiful salmon — known for their sensitivity to pollution — were gone. Decades of dredging and straightening had altered the river's flow, which in turn changed the clarity and temperature of its waters. Overfishing depleted many of the native fish species, and industrial chemicals and raw sewage were discharged into the river.

In 1970, the nations bordering the Rhine allied to treat the effects of pollution, but a devastating chemical spill in 1986 upset much of the progress made. The coalition redoubled its efforts, resolving to tackle the causes of pollution. Cleanup efforts, including those in tributaries like the Emscher River that feed into the main channel, followed a standard prescription for restoring water quality: First, end the dumping of raw sewage and other pollutants into the water. Then, do away with artificial structures that restrict water's flow and its interaction with sources of sediment. Finally, restore natural vegetation to shorelines. Since this video segment was produced in 1990, the Rhine has become a model for other nations seeking to "renaturalize" their dying streams.

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