The scenes at Dettifoss, where Stewart disposes of Graham’s body, were filmed on the western side of the Jökulsá á Fjöllum river.
Dettifoss, located in the Vatnajökulsþjóðgarður (Vatnajökull National Park), is 45m high and 100m wide. With a volume of 212 tons per second it makes the ground shake in the vicinity. One of the greatest and most majestic of Iceland’s waterfalls it is though to be the most powerful waterfall in Europe. 
During the filming of the scene the cameraman Alex Scott was lowered over the edge to film the ‘dummy’ churning in the water. More can be read about the filming of the scene at Dettifoss here.
Images © BBC Scotland, © The Bagley Brief.
1. Steindórsson, S. Hálfdanarson, Ö. Hálfdanardóttir, E. (2007) Dettifoss – Vegahandbókin 2007. p412.