Waterfalls A waterfall happens where a river or stream flows over a vertical drop. A waterfall on the River Ceiriog, a tributary of the River Dee in Wales. The Horseshoe Falls, an artificial waterfall on the Dee near Llangollen. The highest waterfall in Wales can be found on one of the rivers near the bottom of the map Can you find it and click on it? (click on the map when your mouse pointer changes to a hand)