Kaiaraara Kauri Dam
In the storm of Tuesday 10th June 2014 New Zealand's biggest kauri dam, 110 years old, was destroyed. The storm was believed to be the biggest to hit the island in living memory. Over 300mm of rain fell overnight, within the space of a few hours.
The Great Barrier Land Harbour and Mining Company logged kauri in the Kaiaraara, Wairahi and Kiwiriki area from 1862 until 1867 although it is told that the first kauri were taken from Port FitzRoy for use as ship’s spars as early as 1794.
In 1890 the first sawmill on the island was established at Bush’s Beach in Kaiaraara Bay. When the company went bankrupt the mill and machinery were removed.
In 1909 the Kauri Timber Company built a mill at Whangaparapara originally to process kauri from the mainland. In 1914 the mill closed, a result of labour shortage and lack of demand for timber but in 1926 logging resumed and three dams were built on the Kaiaraara Stream. The milling was concentrated on stands of kauri that had previously been considered inaccessible and the kauri forest was decimated. It is estimated that up to 90 million feet of timber was milled.
To transport the logs the three dams erected from kauri logs built across the headwaters of the Kaiaraara Stream, directly below Hirakimata. The largest dam is the lowest. These dams allowed loggers to transport kauri logs from areas too remote and steep for them to be removed by other means.
Water was allowed to build up behind the dams and then released allowing the kauri, the only buoyant native timber, to flood downstream to the mouth of the Kaiaraara River. The process caused erosion of the valley and contributed to the development of the broad silt deposit at the river mouth.
A Kauri Dam is constructed of wood with a "trapdoor" near the bottom large enough for the logs to pass through. The logs were typically a couple of metres diameter and 4-5 metres long so the door was considerable size and the dam was tens of metres high. The lower Kaiaraara dam is braced into solid rock to take the tremendous load of the kauri. The trapdoor was constructed so that when the dam was full it could be tripped and the water released. The logs floating above the dam were sucked down through the hole and swept down to the sea.
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. 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. A dam might be re-used for several
years before it was abandoned.