Marlborough region

There are hydroelectric power schemes on two of the Wairau’s tributaries. The Branch power scheme  discharges into the Wairau about 7km downstream from the Branch confluence, causing significant variations in the flow of the Wairau (up to 20% either side of daily mean flow). The second power station is on the Waihopai River, a catchment known for its short intense rainfall and often turbid waters.

Most water takes from the Wairau are between the Branch confluence and SH6 and in the lower reaches of the Waihopai sub-catchment. There are major losses to groundwater in the reach between the Waihopai confluence down to Wratts Road, on average about 7-8 m3/s. As a consequence flows can be very low at Barnetts Bank, SH1,  in dry seasons. Less than 2 m3/s has been recorded at the lower end of this reach. Below SH1 the river divides into two: the tidal, meandering main channel that discharges at the Wairau Bar; and the more direct Wairau Diversion, built in 1963 to reduce flood levels. The main sections of the Wairau River are discussed in more detail below.

Upper Wairau River

The mountainous Upper Wairau catchment upstream of the Wash Bridge is important to the flow of the river due to its geology, high rainfall and winter snow cover.

North Bank catchments

The North Bank of the Wairau is dominated by high rainfall in the Richmond Range and a number of short, steep rivers that contribute significantly to flood events. The major North Bank catchments are the Goulter River, Top Valley Stream, Timms Creek, Pine Valley Stream, Bartletts Creek, Onamalutu River, Are Are Creek and the Waikakaho River. Of these Are Are Creek is the odd one out with its low gradient waterway compared to the mountainous backdrop of the other North Bank streams.

Branch River

The Branch River is the largest tributary of the Wairau River.  It flows generally northwards from high peaks of about 2200 metres to its confluence with the Wairau River.

The Branch River has a catchment area of about 560 km2, and is about 37 km long. It has two similar sized sub-catchments: the main Branch River and the Leatham River.

South Bank catchments

A number of smaller tributaries enter the Wairau River from the southen side between the Branch and Waihopai Rivers.  These waterways are generally ephemeral in their middle reaches when they leave the south bank hill country until they gain some spring flows near where they join the Wairau.

Waihopai River

The Waihopai River is a major tributary of the Wairau with a catchment area of about 800 km2. Its upper reaches extend back to the Acheron Saddle surrounded by peaks of more than 2000 metres.  From there it flows generally north-east for about 60 km, eventually joining the Wairau River just west of Renwick.

Rainfall in the Waihopai catchment varies from about 800mm a year in the lower catchment to more than 1800mm in the headwaters.  The range of flows also fluctuates from less than 1 m3/s to more than 1000 m3/s.