Sionyfoss is one of the smaller waterfalls on the Grófarlækur river. It is interesting because a natural bridge crosses it. It is possible to walk across this natural bridge, but personally, I am not a fan of doing so, as you can never be sure when the bridge might collapse.
You can find Sionyfoss upstream from the more well-known Grófarlækjarfoss waterfall; from there, you will climb a steep hill, which takes about 20 minutes.
