Yeah, I remember watching a Discovery channel special on Mayan civilization and it was proposed that the population overgrew the water supply and what the land could support. Irrigation was a serious problem. The inability to supply a large city with enough water (and surrounding farmland with enough irrigation) seemed to be the key to many civilization downfalls. If I recall correctly, there was a lost city in Indonesia somewhere that had the same problem.
Lesson learned: Overpopulation = death. Of course, I'm reminded of the water shortage in Beijing, and the horrible drought problem in Cali back in the early 90's. All warning signs that our civilization is about to collapse if we don't figure out how to get enough water to go around.
In fact, it's been talked about that the next major World War will probably be waged over water rights, especially in the Middle East and in Tibet. (Well, from what I hear anyway, since two of China's major river origins come from the Tibetan plateau, so if China loses control of Tibet, then essentially Tibet will control China's water access, hence why the government is fighting so hard to keep the area inside China's borders.) Sorry, didn't mean to drag this into politics.