Complex sentence: The Manawatu Gorge is significant because, unlike most gorges, the Manawatu River is a water gap, that is it runs directly through the surrounding ranges from one side to the other.
Split into simpler sentences: The Manawatu Gorge is significant because, unlike most gorges, the Manawatu River is a water gap. That is, it runs directly through the surrounding ranges from one side to the other.