Complex sentence: Selfoss is a waterfall in the river Jökulsá á Fjöllum in the north of Iceland which drops over some waterfalls about 30 km before flowing into Öxarfjörður, a bay of the Arctic Sea.
Split into simpler sentences: Selfoss is a waterfall on the river Jökulsá á Fjöllum in the north of Iceland. The river drops over a number of waterfalls over about 30 km before flowing into Öxarfjörður, a bay of the Arctic Sea.