This north Westland river, 75 miles long, rises at the main alpine divide and flows generally westwards to the Tasman Sea. Its major tributary, the Ahaura, also rises at the main divide, but several other important streams join the Grey from the coastal Paparoa Range. In the 1860s a route from Nelson by way of a low saddle from the Maruia River near its head extended down the river to the gold-mining areas of the West Coast, but it has not become a modern route because of gorges in the middle reaches of the river. The Maori name both for the river and for the pa at its mouth was Mawhera, but in 1846 Heaphy named it the Grey, after the new Governor, Sir George Grey; the town of Greymouth now stands on the site of the pa. In the following year Brunner discovered coal on its banks a few miles from its mouth, and subsequent development of the Greymouth coalfield was largely possible because the river mouth provided a workable bar harbour. The wider terraced lower valley provides some farming land; the first was taken up on the West Coast at Waipuna by Samuel Mackley in 1862. Both in the main valley and in many tributaries, alluvial gold mining and dredging have left heaps of tailings as their memorials.

Greymouth (pop. 8,877) has a typical bar harbour through which much of the output of the Greymouth coalfield passes. It is often closed because of shallow or rough water on the bar, and frequent dredging is necessary to keep the berths free of the sand and gravel brought down by the river when in flood. The maximum recorded discharge is 250,000 cusecs and the minimum is about 5,000 cusecs, there being no well-defined seasonal flow. The Grey Valley serves as a communication route for both main road and rail traffic between Greymouth and Ikamatua.

by Frederick Ernest Bowen, B.SC.(DURHAM), New Zealand Geological Survey, Otahuhu and Richard Patrick Suggate, M.A.(OXON.), D.SC.(N.Z.), F.R.S.N.Z., New Zealand Geological Survey, Christchurch.