Story: Bush trams and other log transport
Page 6 – Moving kauri: dams and rafting
Water transport is very economical, but as kauri is the only native New Zealand timber that floats, this method was used only in the kauri logging industry of Northland and the Coromandel. The driving dams and rafting used there were quite unlike anything else in New Zealand.
The technology of log-driving dams originated in Europe and developed in North America. In the 1850s, driving dams to the west of Auckland were copies of the timber-built structures of Nova Scotia. But New Zealand’s dams evolved into a distinctive form. Improvements included a simple but effective lifting gate design that allowed the dams to be released by a rigger and then re-used. It is estimated that over 90 years, up to 1,000 log dams were built.
The dam collected a large amount of water on a fairly small stream. Over many months, as the dam slowly filled, logs were felled and laid in the stream bed. The gate of the dam was tripped, usually during a storm (to enhance the natural flood), and the power of the water drove the logs down to navigable water. There, a floating structure of wooden booms collected the logs. A dam might be re-used for several years before it was abandoned.
Witnesses described the power of a log drive as unforgettable.
Wood on water
A 1911 report in the New Zealand Herald described the power of the kauri dam. ‘Telegram from Dargaville: The fresh [sudden flood] that occurred after Tuesday’s rain has liberated great numbers of logs, and the various mills now have large stocks on which to work. In the Awakino Creek 5000 logs were poled out to the booms … in the Tangowahine 2000 are now available for the rafting steamers, and in the Mangonui there are over 2000 on the move.’ 1
The main structural form was the sill dam, built in a narrow gorge. In this type, the horizontal members were the key element, braced with diagonal legs. If only a wide site was available, a rafter dam would be built. Diagonal legs were its key element, linked by horizontal members.
Driving dams were a feat of engineering, built by talented craftsmen who learned by doing. Construction timber was felled locally and pit-sawn on site. Highly accurate hand sawing ensured the dam’s facing planks were watertight. Considered judgement was required on the numbers, size and position of the heavy structural members that held the water behind. The master builders of dams and booms became legends, and their expertise was much sought-after. A measure of their skill is that only three dams are recorded as bursting. The re-usable gate also showed considerable local innovation.
The journey onward to the mill involved chaining the logs into rafts and towing them by steamer on a river or the open sea. The largest operator, the Kauri Timber Company, rafted logs by sea to large sawmills at Auckland and at Mercury Bay in the Coromandel. The longest trip, 300 kilometres from Whangaroa to Auckland, took three days. At Auckland the logs were stored in the purpose-built Viaduct Basin.