Freshwater feature: Ohinemuri River, Coromandel
Ohinemuri River, Coromandel
Motorists travelling from Waihi to Paeroa are treated to views of sheer bluffs, tumbling rapids with boulders the size of cars, and serene pools as the Ohinemuri River winds through the spectacular Karangahake Gorge. During floods the raging river is an adrenalin thrill for kayakers. On sunny weekends families stroll along the banks, soaking up the river’s beauty and the many relics of gold processing plants. But the Ohinemuri has a checkered history as one of the most abused rivers in New Zealand.
Before European settlement the river drained bush and scrub in the Waihi basin and peat swamps and kahikatea forest in the lower catchment above the confluence with the Waihou River which flows to the Firth of Thames. The Ngati Koi had settlements on the natural levees along the lower river, and eel and whitebait fisheries contributed to their food. European impacts began with forest clearance and wetland drainage for farming from about 1840. Extensive exploitation of the upper catchment began in 1875 when the area was opened up for gold mining. In 1895 the river was declared a sludge channel by the Government, and processed ore from stamping batteries was dumped into the river until 1952 when gold mining first ended at Waihi. The dumping made the river very turbid and silted up the lower reaches, and the sediment and heavy metals would have severely degraded fisheries and other aquatic life.
Today the river is much improved, but still under stress. The modern mining operations have minimal impact on the river, but leachate legacies from old mines and associated mullock piles and tailings raise heavy-metal levels above guidelines at a few sites, and nutrients from agricultural runoff and treated sewage discharged from Waihi support abundant algal growth. Macroinvertebrate Community Index values have been consistently low since monitoring began in 1989 at the National River Water Quality Network monitoring site at Karangahake. As we enter the 21st century, the worst toxic pollution of the resource-exploitation era has been solved, but the challenges of dealing with nutrients discharged from our settlements and insidious diffuse pollution from agriculture remain.
The full name is Te Waitangi-o-Hinemuri: literally "the weeping water of Hinemuri, the youngest daughter". In Maori legend the river and floodplain were formed by the tears of Hinemuri. She was the youngest daughter of the Hauraki chief who turned away her many suitors because her older sisters remained unwed until finally the suitors fell away and she was left alone and disconsolate. Her copious tears formed the river and floodplain.