Seljalandfoss is one of Iceland's many well know waterfalls. It is easily accessible and photographed by thousands of photographers, amateurs and professionals alike. The cliff face the water falls from is overhanging, and there is a path behind the waterfall; there is every opportunity to take a picture from almost any angle.
One option, with the sun setting, would be to freeze the water and choose an angle which would emphasize the rainbow reflected by the spray forming as the water hit the pool beneath the waterfall. Another to use a slow shutter speed, or ND filters to smooth out the water. But none of this would convey my perception of the waterfall - the force and power of the water gushing over the cliff edge. The force that you can feel as you get close to rushing water. So I dropped the tripod and started to experiment.
One of the things that occurred to me was that by choosing a slow shutter speed and panning the camera to follow the flow of the water, just like a speeding sports car, I was able to achieve what I was looking for - an image, which to me, captured the waterfall the way I saw it. Then it was just a question of details like finding a spot where the light fell from the right direction and getting close enough to keep the details from the surroundings at a minimum.