It arises in the Crawford Range of the Southern Alps and flows generally southward through the Arthur's Pass National Park to join the Poulter River.

[1] The river was named for J. W. M. Cox, a landholder in the 1860s at the junction of Cox River and Bull Creek.

This route was later blocked by a series of terminal moraines deposited by the Cox glacier during the Pleistocene period.

The river then created a new outlet through to the main Poulter valley via a rocky gorge, known as McArthur Gorge, which now contains the East Branch of the Poulter.

This article about a river in Canterbury, New Zealand is a stub.