Clutha River

The average discharge of the Clutha is around 614 cubic metres per second (21,700 cu ft/s),[1] comparable to many much larger rivers. This heavy flow, combined with the relatively small size of the river in global terms, makes the Clutha notoriously fast-flowing. It is often listed as one of the world's most swiftly flowing rivers, alongside Australia's Macleay and Fitzroy Rivers, the Amazon and Atrato Rivers in South America, and the Teesta River in the Himalayas. The highest recorded flow on the Clutha was during heavy storms in June 2015, peaking at 1,621 cubic metres per second (57,200 cu ft/s).[5]

The waters of the Upper Clutha are clear turquoise, the result of glacial and snow-melt filtering when the water flows through Lake Wānaka. This is a rare characteristic for a high volume river. Below Cromwell, where the silt-laden Kawarau enters, the waters are less turquoise.

Such is the nature of the geology of the New Zealand region that the true river tells only half the story of the course of the Clutha. Beyond its mouth, a submarine canyon system extends for over 100 kilometres (62 mi) into the South Pacific Ocean, eventually becoming the Bounty Trough. The canyon system bears a remarkable resemblance to the pattern of river and tributaries visible on land, so much so that many of the rivers which empty into the sea along the Otago coast can virtually be considered tributaries of the Clutha's submarine system. These rivers include the Tokomairaro, Taieri, Waikouaiti, Waihemo / Shag, and even the Waitaki.

The quartz and schist geology of much of the Clutha River catchment leads to the river transporting white quartz material downstream and depositing this on coastal beaches. The general northerly current in the Pacific Ocean leads to this sand being transported and eventually deposited on beaches north to the Otago Peninsula and beyond. The Roxburgh Hydro Scheme has blocked the supply of new sand from the river, causing erosion of beaches further north such as St. Kilda Beach and St. Clair Beach, Dunedin, and along the Coastal Road at Kakanui.

The river is known in Māori as Mata-au, meaning current or eddy in an expanse of water.[6]

The official name has been Clutha River / Mata-au[7] since the Ngāi Tahu Claims Settlement Act 1998, a landmark Treaty of Waitangi settlement which added dual names to approximately 90 geographic features throughout the South Island to recognise the "equal and special significance" of both the English and Māori names for the river.[8][9]

The spelling of the Māori part of the name is likely to be updated to Matau,[7] while the river takes the English component of its name from Cluaidh, the Scots Gaelic name for the River Clyde in Scotland which runs through Glasgow. During New Zealand's early colonial history it was known as the Molyneux, that name given to it by Captain Cook.

The upper reaches of the river once abounded in flightless moa, predated upon by the Haast's eagle.

The river was an important route for Māori, providing direct access from the coast through Central Otago into lakes Wānaka, Hāwea and Whakatipu Waimāori.

The mouth of Mata-au was heavily populated, with many permanent and temporary Ngāi Tahu kāika (settlements) throughout the lower stretches of the river. Murikauhaka, a kāika near the mouth of the Mata-au, was at one stage home to an estimated two hundred people.[10]

During early European settlement, a whaling station was established close to the river's mouth at Port Molyneux, and during this period the sea was the source of almost all of the area's economy.

The river featured greatly in the Central Otago goldrush. The first major gold deposits in Otago were discovered around the Tuapeka River at Gabriel's Gully by Gabriel Read in 1861, and the following year large amounts of the precious metal were discovered close to the site of modern Cromwell.

By Christmas 1861, 14,000 prospectors were on the Tuapeka and Waipori fields. The gold rush was short-lived, with most of the alluvial gold played out by 1863, but prospectors continued to arrive, swelling to a maximum of 18,000 miners in February 1864.[11]

The Clutha passes under the third Balclutha Road Bridge. The first bridge at this site was destroyed during the flood of 1878.

Several major floods have occurred on the Clutha, most notably the “Hundred year floods” of 14–16 October 1878 and 13–15 October 1978. The 1878 flood is regarded as New Zealand's greatest known flood. During this, a bridge at Clydevale was washed downstream, where it collided with the Balclutha Road Bridge, destroying the latter. The course of the river near its mouth was greatly affected, with the mouth moving north from its former outlet to create two new outlets at its current mouth. The thriving town of Port Molyneux, which was located at the river's (former) mouth, dwindled as a result and today no longer exists.

The 1978 flood breached the banks of rivers from the Ōreti in the south to the Tokomairaro. Over 12,000 hectares (30,000 acres) of land was inundated, with the loss of over 21,000 livestock. Towns and areas affected stretched from Makarora in the north to Invercargill in the south. The town of Wyndham was completely evacuated, and the towns of Balclutha, Milton and Mataura were seriously affected with many residents moved. The small settlement of Kelso on the banks of the Pomahaka River was completely abandoned and was not rebuilt once the waters subsided.[12] At its peak, at 6.00 a.m. on the 15th, the Clutha's flow was measured at just over 4,500 cubic metres per second (160,000 cu ft/s).[13]

A major flood in 1999 seriously damaged river communities, especially Alexandra. The flooding in Alexandra was attributed to a rise in the riverbed, resulting from silt loading in the Roxburgh reservoir behind the Roxburgh Dam downriver from the town. The 1999 flood had significantly higher water levels in Alexandra than the 1878 flood,[14] despite being only 80% of the volume of the latter.