The Wairau River is Marlborough's largest river. It has a drainage area of 2600 square kilometres and is 170km long. Maori aptly named it "many waters" owing to it being largely a braided river. Major tributaries include the Branch, the Waihopai and Rainbow rivers. The Wairau rises in the Spencer Mountains at an altitude of 2,202m and runs from the south to the north before turning to follow a part of the alpine fault line filled by glacial outwash. This leads more or less straight to the sea. A series of glacial moraine deposits west of Wairau Valley used to contain sizeable lakes before European times and signs of them can still be seen along the main road to Rotoiti. Most of the rock in the Wairau catchment is either hard, medium grained sandstone 'greywacke', or fine black mudstone 'argillite.
Most of the Wairau Plain was forested before humans arrived but with the advent of fire large areas on the eastern side were cleared and failed to regenerate. At the time Europeans arrived large areas of flax swamp and swamp forest covered the lower plains and the Wairau wandered freely amongst it. The north bank of the Wairau was almost entirely forested in mixed podocarp and broadleaf forest while the southern side was tall tussock grassland.
In its lower reaches as it nears the sea, the Wairau meanders to some extent, and Grovetown Lagoon is an oxbow lake formed by a former loop of the Wairau that was been cut off from the main channel during a flood.
The river enters the sea at the Wairau Bar where it flows into Cloudy Bay, and is navigable at high tide. Before entering the sea, the Wairau River feeds an extensive area of lagoons and salt marsh which is an important wildlife habitat, and originally an important source of food for Maori. A popular walking track crosses the salt marsh to the wreck of the Waverley, a decommissioned steamer intended as a breakwater, but washed up a channel in 1928 and abandoned in the lagoons during a flood.
The Wairau River is a major source of irrigation water for the vineyards on the Wairau Plain and provides opportunities for trout fishing, particularly in the lower reaches, but it has also been responsible for severe floods which have reached over 55000 cumecs. The most recent severe flood was in July 1983 when Renwick, Spring Creek, and particularly Tuamarina were flooded.
Due to flooding problems, a second artificial outlet for the Wairau was created, the Wairau Diversion. The mouth of this channel has become a popular fishing and picnic spot.
The lower reaches of the river are navigable, and in the early years of European settlement, vessels used the Wairau as part of an important connection between Blenheim and Wellington. The last important merchant vessel that navigated the Wairau, the Echo , retired in the 1960s.
The Wairau is crossed by four main bridges, The single lane Wash Bridge, on State Highway 63 between Renwick and Kawatiri, double lane bridge at Kaituna on State Highway 6 between Blenheim and Nelson, a double lane bridge on State Highway 1 at Tuamarina, and a single lane bridge at Spring Creek.
With growing electricity needs in the region, an electricity generation company Trustpower, has proposed a series of canals to take water from the Wairau to feed generators. There has been a considerable amount of public opposition to the proposal due to concerns over the effect on the environment, including potential loss of recreational fisheries.
References and Links
Wairau Mountain Lands: Marlborough Catchment Board and Regional Water Board Copyright 1980.
A River Ran Through It: Marlborough Express 15/7/2013