Two very popular attractions in the Taupo area, Huka Falls and the Aratiatia Rapids, are both part of the longest river in New Zealand, the Waikato River. They are connected by the 7km Aratiatia Rapids Track. I found this description of the track to be very informative. That writer advises starting from the rapids end, which makes sense because you can time your start to coincide with the periodic opening of the dam release gates, which is a spectacular show! I failed to plan for that, and we started from Huka Falls.
The falls only drop 6m, which is not remarkable. What makes them special is that just a few hundred meters upstream the river channels narrows dramatically, accelerating the water and giving the falls extraordinary power.
The hiking track starts out in native bush, and than in about a kilometer the vegetation opens up and you will have sunshine (hopefully) the rest of the way to the rapids. Along the way you get views of the geothermal energy lines feeding into the Wairakei Power Station. Completed in 1963, it was only the second geothermal power station ever built and currently generates over 150mW, enough to satisfy 5% of the electricity needs of the North Island.
And at the end — or maybe the start — of your walk you can watch the dam release water into the Aratiatia Rapids. The Waikato River is the only outlet of Lake Taupo, the largest lake in New Zealand. Gates at the mouth of the river in the town of Taupo regulate the outflow. Local residents start to worry If the lake level rises too much. To control the water level and regulate the flow of water to the five hydroelectric generating dams downstream, water is released at the Aratiatia Dam several times daily: 10, 12, and 2 in the winter, and 10, 12, 2, and 4 during the summer.
Before the water release, just below the dam the river looks like this (note the high water mark, where the rock turns a lighter color):
And this is the view from a bit farther down looking back up towards the dam:
The two dam gates are raised about 2 meters to release water for approximately 10 minutes, and the gorge is transformed (both of the next two photos are taken from the dam looking downstream):
Here are before/after photos side-by-side taken from the same viewpoint:
This 5 1/2 minute video shows the transformation: