There are tall waterfalls, wide waterfalls, waterfalls with multiple drops, and this one: the highest volume waterfall in Iceland. Urriðafoss spans the wide Þjórsá river and cascades in multiple flows. It narrowly escaped being harnessed for power early in the century, but it is again under threat by a planned power plant that would destroy the waterfall to provide 930 gigwatts of power to nearby Reykjavik.
The river is the longest in Iceland (at 230km), and flows through the Þjörsárhraun lava field, possibly the largest lava flow on earth since the Ice Age. The falls are shallow, though, only 4m (12 feet) or so, and so the falls is much less impressive than the other ‘biggest’ falls.
The location is lovely, and easy to get to from the main Ring Road, turning off on Route 302 to the parking lot.