The Waihopai River is the largest tributary of the Wairau, with its confluence a few kilometres west of Renwick. The river has its source 103Km from the sea, to the south westof Blenheim, below the Waihopai Saddle. The river is notable for having the first hydro-electric scheme built in Marlborough, which began operation in 1927. Initially providing 1MW of capacity, the power station was upgraded in 1996 by Trustpower, the current owner, to a capacity of 2.4MW.
Median flow (Source: NIWA) near the confluence with the Wairau is a little over 10 Cumecs, although this has significant seasonal variation.
In the lower valley, water is taken from the river for vineyard irrigation.
Craighlockhart is a popular summer swimming area just upstream of the Craiglockhart No. 2 Bridge.