Of these, the North Mathias River is the longest, flowing predominantly southwards from its source northeast of Mount Williams.

The combined waters flow a further 17 kilometres (11 mi) southeast across a braided, shingle strath, which meets with the valley of the Rakaia River 15 kilometres (9 mi) west of Lake Coleridge.

The river was named by Sir Julius von Haast after his companion Alured George Mathias, during his Rakaia trip.

[1] "Place name detail: Mathias River".

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