It was not necessary to expose the river bed to mine it. There had been plans to use a submarine, the Platypus, to recover gold from the bed of the Clutha River, but the idea was not practical. More useful were floating barges known as dredges, which scooped gravel off river bottoms, separated the gold on board, and dumped the waste rock. In 1863, spoon dredges began working the Clutha. These were crude and only involved dragging a spoon dredge (a leather bag tied around an iron loop attached to a pole) across the river bed. Some gold was won, but spoon dredges could not dig deep or process much gravel.
In 1868 the bucket dredge was developed. A series of buckets on a long chain continuously dug up gravel. Originally these were powered by wheels turned by the river’s current. In 1881 the first steam-powered dredges were used on the Clutha River, and electric-powered dredges arrived in 1890. These could also be used in ponds away from the main river channel, allowing old river channels to be worked.
In 1888, the Chinese businessman Charles Sew Hoy of Dunedin ordered a steam-powered bucket dredge to be built in a Dunedin foundry. It had a string of buckets on a ladder that could be lowered to the river bed and onto river flats on the shore. It was immediately successful on the lower Shotover River. This is considered to be the prototype for the New Zealand style of dredge. Gradually this design and variations of it proved to be adept at removing gold from river beds or artificial ponds on river flats. By the 1890s Dunedin was at the forefront of gold-dredge design and it was not long before New Zealand-style dredges were being successfully used overseas.
Over the next 20 years hundreds of dredges recovered tens of thousands of ounces of gold from the beds of Otago’s rivers and old river channels. A dredging boom that reached its zenith about 1900 was characterised by the use of many small dredges. In 1900 there were 228 dredges working in Otago and Southland. By contrast, a second boom in the 1930s and 1940s saw a few large, powerful dredges.
Dredging created unique local landscapes such as the Earnscleugh tailings near Alexandra. Essentially they are piles of stones – in Otago’s dry climate, vegetation has not covered them.
West Coast dredging
Dredges worked the West Coast in lesser numbers – an estimated 150 dredges mined there. It was, however, where they remained in use the longest. From 1956 the Kanieri, a large bucket dredge that employed 38 men, worked the Taramakau valley before moving to the Grey River, where it dug up gravels near Blackball and Ngahere. The Grey River dredge, New Zealand’s last big dredge operation, won its last gold in 2004.