Greenman, Dams come with built-in time limits, as mud and muck fill in those lakes behind them. Not to mention the break-down of the aging technology and the dam structure as well. Our over estimation of water and power encouraged rapid over growth of the population in areas, creating long term problems we will not be able to solve. One barely can imagine the cost of having to move tens of millions of people from areas now experiencing rapid growth and hundreds of millions from older over grown population from older decaying areas of past overgrowth.