West of Egilsstadir
One of the rivers draining Vatnajökull to the north. In the summer it should be filled to the brim with meltwater from the glacier, but they keep it to make electricity in the winter instead.
I don't know the name of the place, but we passed here some time (half an hour?) after leaving Egilsstadir on our way towards Myvatn.
I think I have managed to find the place with the help of Google Earth, and added it to the map. I am not 100 % sure, there is an place a few km further up the valley that I cannot conclusively dismiss.