The Rakaia River is in the Canterbury Plains in New Zealand’s South Island.The Rakaia River is one of the largest braided rivers in New Zealand. The Rakaia River has a mean flow of 203 cubic metres per second (7,200 cu ft/s) and a mean annual seven-day low flow of 87 m3/s (3,100 cu ft/s). In the 1850s, European settlers named it the Cholmondeley River, but this name lapsed into disuse.
It rises in the Southern Alps, travelling 150 kilometres (93 mi) in a generally easterly or southeasterly direction before entering the Pacific Ocean 50 kilometres (31 mi) south of Christchurch. As it reaches the river-coast interface; a hapua forms.
For much of its journey, the river is a braided river, running through a wide shingle bed. Close to Mount Hutt, however, it is briefly confined to a narrow canyon known as the Rakaia Gorge.
The Rakaia River is bridged in two places. The busiest crossing is at the small town of Rakaia, 20 kilometres (12 mi) from the river mouth, where State Highway 1 and the South Island Main Trunk Railway cross the river using separate bridges. These two bridges are New Zealand’s longest road and rail bridges respectively, approximately 1.75 kilometres (1.09 mi) long. A second bridge, much shorter and less used, spans the Rakaia Gorge.
The Central Plains Water Trust is proposing to take up to 40 m3/s (1,400 cu ft/s) of water from the Rakaia River as part of the Central Plains Water enhancement scheme.
The Rakaia River is a celebrated Chinook salmon fishery. It has been identified as an Important Bird Area by BirdLife International because it supports breeding colonies of the endangered black-billed gull. The river is also known for its large wrybill population which represents 73 percent of the total population. Other important bird species using the riverbed are black-fronted tern and banded dotterel.