The tram pauses next to a winding street running through a Mexican village. The tour guide demonstrates a movie "rain" effect involving rain bars which direct a spray of water upwards which then falls looking like rain. The tour guide then demonstrates a thunderstorm effect using powerful strobe lighting and thunder sound effects. The rain won't turn off - sometimes too much rain causes flooding - wait! What's that at the top of the hill? A wall of water rushes down the hill and threatens to engulf the tram. A wooden cart swings perilously close to the tram and a signpost is thrown over by the force of the flood water. Then, just as the tram drives off, the water drains off the set and is pumped back to the dump tanks ready for the next tram to pass.
The effect was added to the tour in 1968 and has appeared in countless films including Big Fat Liar and Fletch Lives. There's a story that when a group of dignitaries (including John Wayne) visited the Flash Flood in the early days, the water levels were still being perfected, and the group got soaked.
Guides commonly use two "gags" in this area: Early film-makers realised that pure Californian water doesn't show up well on film, so they added powdered milk to the water to make it more visible. Gene Kelly wasn't really "Singing in the Rain" - he was singing in the milk!! (the story is true - only the punchline is cheesy!). The other gag is about the fact that the water is not wasted - it's all recycled... right up through the drinking fountains around the upper lot. (The water is reused and is pumped back into the tanks ready for the next tram to visit.)
The Flash Flood consists of 10,000 gallons of water from two tanks. One flood comes down the hill and one comes through the buildings at the left of the area. Two additional bursts of water splash up right next to the tram, but these are generated by water being fired under pressure through two additional ducts right next to the tram.
In the early days of the attraction there was a tree that was knocked over by the flood. That was replaced by the current signpost and cart.