Waterfalls start with a riverbank bursting over something like a cliff and fall into a pool at the bottom.
Then the falling water loosens rock particles or boulders and wears away the softer rock.
The harder rock at the top of the new waterfall becomes a lip as all the water falling into the bottom pool starts to erode the softer rock underneath.
The hard rock breaks from the pressure of the water and falls into the pool at the bottom to be washed away by the river and the position of the rock falls move back.
The Erosion continues and the waterfall slowly makes its way upstream between a few thousand years to millions of years leaving a small or extremely large gorge in its trail.
Famous waterfalls include: Mokau Falls, Huka Falls, Browne Falls, Kitekite Falls and Okere Falls