Then there is pollution.

Because such a large fraction of humanity treats water as if it were free, we have slipped into the habit of using it as a conveyance medium for our domestic and industrial wastes. As a result most of the rivers that move through, and lakes that in the vicinity of, the world's major population centres are too filthy for words.

Developing new sources has historically been the option preferred to wastewater treatment. Thankfully, this situation has now begun to change.

A good example is the Thames River, which less than 100 years ago flowed through London like a big open sewer, has been cleaned up to the point that many fish species have now returned. This is one of the few items of really good news on the global water scene, and it shows what can be done when serious public pressure for change is focussed on a democratically elected government.