The Manawatū is unique among New Zealand rivers in that it crosses a mountain range. The river has formed a "water gap" across the mountains because it is older than the Ruahine and Tararua Ranges. Most rivers arise from an already-existing range of mountains or hills, but beginning about 3 million years ago the central North Island mountain ranges began to uplift across the Manawatū's current course. Because it drained a large catchment, the river had sufficient flow to keep pace with and erode the rising mountains, eventually forming the Manawatū Gorge; other rivers were unable to and were diverted into the Manawatū instead.