The Clarence River, 125 miles long, drains an area of 1,270 square miles and rises on the eastern slopes of the Spenser Mountains. The river drains Lake Tennyson and flows south for about 25 miles, after which an easterly and then northerly direction is followed over a distance of 15 miles to the junction with the Acheron River. From here the river flows in an easterly direction for 12 miles and subsequently follows a fault angle depression in a north-easterly direction for more than 50 miles, and separates the Inland Kaikoura Range to the north-west from the Seaward Kaikoura Range to the south-east. The river then cuts through the Seaward Kaikoura Range in a winding gorge 7 miles long, entering the sea 30 miles north of Kaikoura.
The vegetation of the upper Clarence region consists of tussock on the lower slopes of the ranges, with grassed river flats between; these are in part used as cattle-runs for Molesworth Station. A seasonal, dry, north-westerly climate, together with overstocking and introduction of noxious animals, has caused considerable erosion in the middle Clarence, where briar and manuka form an additional pest. This region is used as mixed cattle and sheep-runs, the only homestead being Bluff Station, which can be reached only by pack track and by air. Quail Flat Station is the other station and is used only seasonally.
The river gradient is relatively steep and frequent gorges give this river system a hydro-electric potential.
by Geert Jan Lensen, New Zealand Geological Survey, Lower Hutt.