Walled up behind the Clyde hydro dam, the Clutha River has formed New Zealand’s newest lake, Lake Dunstan. Stretching more than 40 kilometres from the southern end of Cromwell Gorge to the Lowburn Flats to the northwest, the lake covers 26.4 square kilometres of former river bed and terraces.
Although construction of the Clyde dam began in 1977, it was not until April 1992 that filling started—a process completed in four stages by September 1993. However, creating the lake has not been a matter of simply letting the river back up behind the dam; it has entailed a lot of complex engineering.
A fault line runs through the dam site, and the surrounding mountains are not regarded as particularly stable. Considerable concern has been voiced over the years about the effect a large rockslide would have on the dam. Could the structure withstand such an event? To cope with the unstable geology of the region, the dam was constructed in two pieces with a vertical wedge in the middle, the theory being that the two sides can move up to two metres apart without breaking.
Fourteen kilometres of tunnels were drilled into the bowels of the mountains to drain destabilising water down into the lake, and at the northern end of the gorge 15 giant steps were carved into the so-called Cairnmuir Slide. The terraces, each about four metres wide, were constructed by welding together hundreds of rock-filled steel cages. The top of each terrace is tar-sealed, to catch and divert surface rain water away from the slide.
As a further stabilisation measure, a network of plastic pipes and culverts has been installed to collect water from above the terraces. This $15 million portion of the project was completed in May 1994. Elsewhere in the gorge, huge buttresses of compacted rock and gravel have been built to add strength to the base of other landslides.
The raising of the lake has dramatically changed the appearance of the Cromwell Gorge as well as that of the town of Cromwell and the Lowburn Flats.
The narrow orchard-covered river terraces in the Cromwell Gorge have been completely submerged, with the lake now filling the entire valley between the Cairnmuir Mountains to the west and the Dunstan Mountains to the east.
Cromwell itself has been changed beyond recognition. Prior to the filling of the lake, the town was perched high up on terraces above the Clutha and Kawarau Rivers. Today it sits on a peninsula jutting out into the lake and surrounded by water on three sides.
The lake has caused great upheaval for Cromwell’s residents, particularly the loss and subsequent relocation of the commercial heart of the town. However, because of its strategic location midway between the upper and lower reaches of the lake, Cromwell looks set to benefit the most from its presence.
With its artificial beaches, harbours and numerous boat ramps, Lake Dunstan is already becoming popular with anglers and water sports enthusiasts. One hundred and sixty residential sections have been developed around the shoreline between Cromwell and Lowburn, with another 60 planned. The filling of the lake has also provided the opportunity for Cromwell’s architectural heritage to be repackaged, with the creation of an historic precinct on the south side of the town overlooking the Bannockburn Arm. A total of 12 stone buildings, some rebuilt after being dismantled from their original now-submerged sites, make up “Old Cromwell Town.” Several more buildings will be constructed over the coming years to expand the precinct.
To a lesser extent, the town of Clyde is also benefiting from the lake, with the southern end of Cromwell Gorge rapidly developing a reputation as one of the best rowing venues in New Zealand. To maximise this potential, a new rowing complex is planned at Weatherall Creek near the dam.
While the development of Lake Dunstan has caused massive physical changes and socioeconomic dislocation for some, Central Otago people are coming to grips with their new lake and looking forward to building further on the benefits it has brought to the region.