The Waihopai River is the largest tributary of the Wairau River, with its confluence a few kilometres west of Renwick. The river has its source 103Km from the sea, to the south westof Blenheim, north east of the Waihopai Saddle, bounded to the north west and south east by unnamed ranges climbing to over 1000m in its upper catchment, eventually opening out to a flat plain in the lower valley. Parts of the river, particularly in the lower reaches show some braided characteristics, but parts flow through a steep sided rocky gorge.
In a 2014 report commissioned by the Marlborough District Council on the natural character of Marlborough rivers, the Waihopai achieved a moderate rating, with good water quality and low level of introduced aquatic organisms, but substantial modification to the flow, channel and surrounding landscape.
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.
The river has six bridges between State Highway 63 and the end of the public road.
Craighlockhart is a popular summer swimming area just upstream of the Craiglockhart No. 2 Bridge.
Trubutaries include the Avon and Spray rivers.
Waihopai Hydroelectric Dam
The river is notable for having the first hydro-electric scheme built in Marlborough, which began operation in 1927. During construction of the dam, shingle build up behind the dam was not taken into account, and the original small lake that was formed has now largely filled in with shingle carried downstream by the river. The hydroelectric scheme was operated by the Marlborough Electric Power Board until the 1990s, when electricity reforms introduced by the National government resulted in the sale of generation assets. Initially providing 1MW of capacity, the power station was upgraded in 1996 by Trustpower, the current owner, to a capacity of 2.4MW.