Worth a read:<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Risks_to_the_Glen_Canyon_Dam" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Risks_to_the_Glen_Canyon_Dam</a><p>This gives a hint as to why huge dams cannot be drained for maintenance: there is so much water behind them that it would take too long to drain them via the spillways. In fact, the spillways would probably be irreparably damaged by experiencing full flow for that long.