Complex sentence: Jökulsá á Fjöllum streams over the waterfalls Selfoss, Dettifoss, and Hafragilsfoss, the second of which is the most powerful waterfall in Europe, and through the canyon at Jökulsárgljúfur National Park, which was formed by the explosion of a volcano situated directly beneath the river.
Split into simpler sentences: Jökulsá á Fjöllum streams over the waterfalls Selfoss, Dettifoss, and Hafragilsfoss, the second of which is the most powerful waterfall in Europe. It continues through the canyon at Jökulsárgljúfur National Park, which was formed by the explosion of a volcano situated directly beneath the river.