During your first visit to Svartelheim, you will come across a locked gate. To get it open, you will need to use a nearby waterwheel to move the gears. There is not enough water pressure to make it move, unfortunately, so you have to figure out how to fix it. Here is what to do.
You will need to use this strategy for a few different puzzles in Svartelheim so it is good to learn how the mechanic works. When you freeze one of the water spouts, you will notice that it causes other, nearby waterspouts to shoot out with more pressure.
If you climb up to a higher area of the region nearby, you will be able to get a good shot of the water spout that is higher up. Throw the leviathan axe into this and it will freeze the water, increasing the water pressure below the water wheel and making it start spinning.
The video guide below will show you exactly where to go so you can freeze the water and how to get the water wheel to start spinning to unlock the path forward.
