Almannagjá Gorge was closed to public for a while, i.e. the southern most part of it - due to the ground caving in and a big hole/fissure appearing on the path. First a small hole appeared on the 31st of March 2011 and then underneath it a fissure, 10-14 meters deep and in parts 4 meters wide :( It was repaired temporarily by putting a plank on it, but when I visited in September 2011 this part of Almannagjá Gorge had been closed to public.
Thank God that nobody was hurt, as we had no idea there was a fissure beneath the path. And it seems like people have been walking there on a thin layer of sand for some time! They closed part of it, so now there is no walking from the "Hakið", see my tip, and down Almannagjá Gorge. They are trying to fix it somehow, but I think that nobody knows what to do with it. When I last visited there were some workers there with a tractor. One suggestion is to fill it with sand, but a gigantic amount of sand will be needed to fill up the fissure. Building a bridge over the fissure is another suggestion... another suggestion is to close it with steel sheets and cover it with sand.
The most likely reason for this is that something shifted there in the earthquakes in 2000 and 2008 and then the spring melt made it worse.
Almannagjá has now been opened and the fissure has been bridged.
When you're standing above the lake and overlooking the fissures in the valley, it looks like a really peaceful place, but when you think that this is land that is literally being pulled apart by the forces of nature, you realize there's more to it than meets the eye. Those strips of water that look like shallow little creeks are actually incredibly deep fissures in this delicate landscape. If you fell into the water and couldn't swim, you might never come back out, not to mention the fact that the water is incredibly cold. Be careful that you don't get too close to the edge.
We tried to go out to Þingvellir (a national park) one day. We had rented a car. Before leaving Reykjavik, we headed over to the tourist center to confirm the weather and they said that it was "fine".
Well, we hit the roads and about 50 miles out of town we hit howling winds, iced roads, and zero visibility and we were like the only car on the road in a completely isolated area. Seeing that the weather was reminiscent of Siberia, we ultimately turned back.
So beware, you should do your own research because sometimes the tourist centers can be unreliable.
. . . Especially in the winter time, especially if the roads are slushy, especially if you have a tiny rental Toyota! It could be hard gettting roadside assistance out here!