Considering how much water they often hold, is it all surprising that a dam needs to be built well? What would happen if it burst and couldn't stand up to Mother Nature's ebb and flow of water over the long haul? No matter how big they build 'em and how safe they say they are what a broken or overtopped dam can do is scary stuff indeed. Hydrologists and oceanographers recognize that water doesn't fall from the sky in even amounts - it is scattered both geographically and over time. Once every hundred years you can count on one mother of a flood. Every 200 years you can count on one still bigger. And what about every 1000 years? Well you get the picture - life here on earth comes in waves. As much as we like to see nature as a stable force - which it is overall, there is still *always* going to be catastrophic happenings - and this is true for rivers and weather as much as anywhere.
Still, your average politician and dam engineers gets an outright thrill from trying to dominate the forces of the planet. And they don't win them all, despite their cocky propaganda!
So as you can see from the links below... strong meaures are needed when dealing with the once in a century or millenial flood - which they never build for. Because it's too costly to do so. At least in terms of short-term cash. Tell *that* to the farmers of the American midwest or the people on the St. Louis-de-HaHa! River in Quebec. That's right - dams aren't utterly safe. And some hydrologists believe that damming and diking actually make the flood problem *worse*. But I betcha you'll never hear Kemijoki Oy or Hydro-Quebec mention that when they're trying to sell their wares overseas! Or at home.
Here's one example - the mississippi/missouri river flood of '93. Check it out! The Great Flood of 1993 Post-flood Report, from the US Army's Corps of Engineers.