The Wairau River, 105 miles long, drains an area of 1,630 square miles and rises in the Spenser Mountains; its main headward tributary, the Rainbow River, rises in the St. Arnaud Range. The river at first flows in a northerly direction for 30 miles and separates the St. Arnaud Range to the west from the Raglan Range to the east. Subsequently it flows in a north-easterly direction through a fault-angle depression – the Wairau Valley – filled with glacial outwash material; these terraces carry prosperous sheep farms. The Wairau River enters the sea at Cloudy Bay, where a long boulder spit, Wairau Bar (Te Pokohiwi), extends south-eastwards towards White Bluffs. Behind this spit lie the Wairau Lagoons, connected by channels dug in pre-European times. The lower reaches of the Wairau River have a low gradient and are consequently prone to flooding; 165,000 cusecs have been measured. The main tributaries are the Branch and Waihopia Rivers, which drain from the ranges to the south-east of the Wairau River. Near its mouth on the south side of the river lies the town of Blenheim, which serves the Wairau Valley.
The literal meaning of Wairau is “many waters”, but the real meaning is obscure.
by Geert Jan Lensen, New Zealand Geological Survey, Lower Hutt.