I've been watching information the past few years of how Lake Mead's water storage is slowly dropping to critically low levels. This is interesting to me because I visited the Hoover Dam in June of 1983 when the lake was full enough to send water over the spillways (the first and only time this has happened, other than for testing after construction). I dug out my old movies and slides of the historic event and posted them here.
I remember there was some concern at the time that continued upstream flooding would cause the lake level to rise a few more inches, triggering an automatic dump of the spillway gates to protect the dam, inflicting extensive downstream flooding. That obviously didn't happen, but it was close.
Update after Hurricane Harvey: As an illustration of scale, the NASA Earth Observatory estimates that Lake Mead holds about 9.3 trillion gallons of water at maximum capacity (as seen in this video). Hurricane Harvey, which devastated parts of Texas and Louisiana in late August 2017, is variously estimated to have left 27 to 33 trillion gallons of water in its path. It's staggering to think that Hurricane Harvey could have filled a totally empty Lake Mead to overfilling perhaps three times.