The Waimakariri River, formerly briefly known as the Courtenay River, is one of the largest of the North Canterbury rivers, in the South Island of New Zealand. It flows for 151 kilometres (94 mi) in a generally southeastward direction from the Southern Alps across the Canterbury Plains to the Pacific Ocean. In Māori, Waimakariri has several meanings, one of which is "river of cold rushing water". The river is known colloquially in Canterbury as "The Waimak".
The river rises on the eastern flanks of the Southern Alps, eight kilometres southwest of Arthur's Pass. For much of its upper reaches, the river is braided, with wide shingle beds. As the river approaches the Canterbury Plains, it passes through a belt of mountains, and is forced into a narrow canyon (the Waimakariri Gorge), before reverting to its braided form for its passage across the plains. It enters the Pacific north of Christchurch, near the town of Kaiapoi.
Geological evidence indicates that the river mouth has been very mobile, at times flowing through the current location of Christchurch and even flowing into Lake Ellesmere / Te Waihora south of Banks Peninsula for a time.
Instead of being unoccupied crown land as are most New Zealand river beds, the bed of the Waimakariri River is vested in the Canterbury Regional Council (Environment Canterbury).
Chinook salmon (Oncorhynchus tshawytscha) were introduced from California in the 1900s and persist today.
In 1923 the river was mainly investigated for a hydroelectric dam to supply electricity to Christchurch. It received support from the community but the dam was never built since the Government offered inexpensive electricity from the Lake Coleridge scheme.
The Central Plains Water Trust is proposing to take 40 cubic metres per second (1,400 cu ft/s) of water from two points on the Waimakariri River as part of the Central Plains Water enhancement scheme.
In 2007 the Waimakariri was ranked as one of the ten most polluted of the larger rivers in New Zealand. Some of the pollution was caused by liquid wastes from industries such as a meat processing plant and wool scourers in the vicinity of the river. The wastes were discharged directly into it but as of 2012 it was piped to the municipal sewage treatment plant. There had been some non-compliance issues with the resource consents for water discharge.