[1] The canyon was formed by a deep incision of the Waimea River arising from the extreme rainfall on the island's central peak, Mount Waiʻaleʻale, among the wettest places on earth.

The canyon has a unique geologic history as it was formed not only by the steady process of erosion but also by the catastrophic collapse of the volcano that created Kauaʻi.

With lava flows dated to about 5 million years ago, Kauaʻi is the oldest of the large Hawaiian islands.

In the time since, rainwater from the slopes of Mount Waiʻaleʻale have eroded Waimea Canyon along one edge of the collapse.

On the east side of the canyon, the cliff walls are built from thick lava flows that pooled in the depression.