1. READ/play the text
2. CLICK the picture (and video)
3. TRY this:
Q1: What is a reservoir used for?
Q2: What are the main things we need to remove from water to make it safe and pleasant to drink?

[Answer]

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...for drinking

Rivers provide most of our drinking water. We store some water behind dams in reservoirs in case of dry weather. We can then release it into rivers and collect it near our cities.

However, all rivers are unsafe to drink from. The water has to be cleaned of harmful chemicals, bacteria and mud using filter beds and balancing chemicals.