Clarence River, river in eastern South Island, New Zealand. Rising on the eastern slopes of the Spenser Mountains, it flows south, then northeast between the Inland and Seaward Kaikōura ranges. Cutting eastward by a gorge 7 miles (11 km) long through the Seaward Kaikōura Range, the river flows south and east, entering the Pacific Ocean 20 miles north of the town of Kaikōura. The main stream, fed by Lake Tennyson and its principal tributary, the Acheron River, is 130 miles long and drains a 1,270-square-mile basin. The Clarence, bordered by mixed sheep and cattle runs, has a steep gradient that, in conjunction with several gorges, presents hydroelectric potential.