Because of the snow melting it can become quite high water levels in the rivers. This often happens in May and June when the large mass of snow in the mountains is melting.
Some years this can even cause floods. Not normally something dangerous, but just be aware of the fast flooding water if you go near the rivers.
When you are out on the ice in the winter you shall always look for holes in the ice. Even if it's a lot of snow and cold weather it can always be hidden holes in the ice, especially at lakes where the water level vary.
The risk for holes is as biggest in the beginning and end of the winter.
Be aware of the fact that it can become quite cold in the winter. It's not unusual that the themperature drops below -30 C (sometime even -40 C) in the winter, especially in December and January.
So remember to bring some warm clothes.
During the winter the roads must be cleared from snow, and you will not and then meet a snow mover machine on the road. They are used to cars and people walking so no problem, but try to keep eye contact with the driver to see if gives you some signs.
But remember to keep away from the snow coming out from the machine. It's often the machine throw up more than snow, and getting a big stone in the head might hurt some.
When driving around in the north, always be aware of the reindeers.
In the area around Storuman, the most risk for reindeer encounter is in April-May and in September-October. Also keep in mind that reindeers often isn't afraid of cars, so don't expect that they will leave the road when you are driving by.
And if you have spotted one reindeer, you can be sure that there are more of them in the area. In areas with a lot of reindeers, the locals often put up black plastic bags on the side of the road as a warning.
If you are out driving snowmobile, skiing or walking in the forest, be aware of the small rivers that can be hidden under the deep snow. On many places the water path is so narrow so you will not see that it's flowing water below.
If you are uncertain about how the ground looks like, follow the already used paths.