Instant Rapids

First you need to find a rocky watercourse downstream from the sluice gates of a hydro-electric power station reservoir.
Then, just

add

water...

And you're away:

The Aratiatia Rapids on the Waikato River north of Lake Taupo are one of my favourite things. They are dry most of the time, but several times a day the sluice gates used to control the reservoir level are opened and the rapids run again. The gates open for half an hour on each occasion, with the rapids fully developing in less than ten minutes. Once the gates close they are mostly dry again after a quarter of an hour. I've seen them many times and the spectacle of watching them fill, run and empty hasn't gotten old yet. Some blurb about the whys and hows of the rapids is here, with a cool 1960's geological and general description of them and nearby Huka Falls here.

Just downstream of the gates there is a pool about 50 metres across that acts as a sort of header for the rapids themselves. To give an idea of scale, inside the red box at centre left of this pic is a supermarket shopping trolley (which isn't visible at the resolution of this copy, but trust me its there on the hi-res version):

I don't know how it got there either, but there are also a few traffic cones scattered around the upper reaches as well as other stuff people have tested the rapids with. Anyway, a few minutes after the gates open it looks like this:

Another part of the same pool:

Making the rapids cooler is that they are totally free to view, you just have to know how to get there. There are three separate vantage points to watch from, each with a different perspective worth checking out (during our stay last week, we went to the rapids three times to go to each lookout. And because Charlotte enjoyed seeing the 'water falling out'). It is only a very short detour from SH1 if you are passing by and looking for something more interesting than the usual rest-stops to take a break. Recommended for residents and tourists alike :)

And because movies sometimes convey things better than stills, here are some appropriate youtube vids I found:

Filling up

Drawing down

And someone running the rapids in a kayak....