Water isn't the only thing that falls down the mountains. We saw a few places where avalanche barriers were built to keep a town from being in the path of tons of snow, ice and rocks descending a mountainside. The picture on top is twisted steel from a fish factory and town flattened by an avalanche near Seydisfjordur.
There is also a constant chance of a Jokulhlaup, a torrent of water coming off a mountain when a glacier is melted by an erupting volcano. They transport boulders weighing tons along with sand that can cover many square miles of land.