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River gorge in New Zealand

The Cromwell Gorge is a steep gorge cut by the former Clutha River (Māori: Mata Au) in the Central Otago region of New Zealand's South Island. It winds between the Dunstan and Cairnmuir Mountains, linking the townships of Cromwell and Clyde. It is one of three substantial river gorges in Central Otago, the others being the Kawarau Gorge to the west of Cromwell, and the Roxburgh Gorge south of Alexandra. Long associated with gold mining, orchards and the production of stone fruit, the gorge (including part of old Cromwell) was flooded in the early 1990s to form Lake Dunstan behind the hydroelectric Clyde Dam. The former Otago Central Railway which traced the river through the gorge from Clyde was removed, while State Highway 8 was realigned above the newly formed lake. The Dunstan Trail, a major new cycle route, was opened on the lake's right bank in 2021.

The Cromwell Gorge formed in response to the uplift of the Dunstan and Cairnmuir Mountains and simultaneous antecedent incision by the Clutha River. At the northern (upstream) entrance to the gorge lies the township of Cromwell, while the township of Clyde lies to the south. To the east, the gorge is bound by the Dunstan Mountains and to the west, the Cairnmuir Mountains. The overall relief from the highest point of the gorge (Leaning Rock, 1647 m) and the floor of the gorge (Lake Dunstan, 194 m) is
The Cromwell Gorge is the entrance to the Upper Clutha Valley and was historically an important thoroughfare for early Māori moa hunters, and later pastoralists and gold miners in the late nineteenth century. It remains an important access route into Central Otago's interior via State Highway 8 (SH8).

The Cromwell Gorge is dominated by the characteristic rocky tors and craggy outcrops of the Haast Schist Group; grey quartzofeldspathic metagreywacke interlayered with micaceous meta argillite and greenschist formed during the Rangitata Orogeny. The schist in Central Otago has a well defined pervasive schistosity, with shallow dips defining the broad regional scale warps in schistosity caused during Miocene deformation (the Kaikoura orogeny). The warped geometries are antiformal over mountain ranges and synformal under the intervening basins. Associated reverse faulting (the Otago fault system) along the south eastern flanks of many Central Otago mountain ranges (i. e. Taieri Ridge, Lammermoor Range, Rock and Pillar Range, Rough Ridge, Raggedy Range, Dunstan Mountains, Pisa Range) gives rise to the basin and range topography of parallel ridges and basins with steep south eastern limbs and gently dipping north western flanks. The Cromwell Gorge exits the Dunstan and Cairnmuir Mountains immediately to the north of Clyde. Both mountain ranges are controlled to the south east by large reverse faults; the Dunstan Fault and Earnscleugh Fault, respectively. Investigations have found that geologically recent deformation associated with the Dunstan Fault has extended south west towards Clyde, rather than being translated along the southern margin of the Cairnmuir Mountains. The former Clutha River followed several of these faults as it coursed through the Cromwell Gorge. The arrival of humans in Central Otago resulted in particularly extensive changes in vegetation as Māori hunter gatherers used clearance fires improvidently. Early Māori firing of woody cover from about the thirteenth century have heavily modified the vegetation cover since European settlement. This is further worsened by Central Otago being the driest region in New Zealand. and thyme, all of which flourish. The introduction of thyme to Central Otago is most often attributed to Jean Desire Feraud, a French goldminer and orchardist who was part of the Otago gold rush. There is evidence of native plant regeneration (such as kōwhai) in areas where conservation has been put in place, Forests of kōwhai (Sophora microphylla) have recently been postulated to have also been present in the Cromwell Gorge, upon which moa grazed.

The first settlers in the region were the Māori as they travelled through Central Otago en route to the West Coast on pounamu expeditions, as well as in search of seasonal food resources. Although the Lindis Pass was the usual route for early Māori to access their summer camps at Lake Hāwea and Lake Wānaka, there is sufficient evidence the Cromwell Gorge was used as an important thoroughfare. Early Māori archaeological sites are concentrated through the gorge, particularly on the true left of the former Clutha River, and include small Moa hunter camps with associated Moa bones. Four sites have been identified at Rockfall I and II, Italian Creek, Muttontown Gully and Clyde West. that the numerous overhangs within the craggy rock formations of the Central Otago river gorges may have been favoured moa nesting locations. Such locations abound in the Cromwell Gorge and eggshell fragments found in many shelters confirm their usage. accompanied by two Māori guides: Reko, a Ngāi Tahu chief from Tuturau, and Kaikōura, a man who had runaway from the Kaikōura ranges. Chalmers made his trip into Central Otago's interior when no roads or tracks existed, hoping to retrace the steps that his guide, Reko, had made from the Waitaki some twenty years earlier. Horatio Hartley and Christopher Reilly worked the sides of the Cromwell Gorge for three months until they were forced to take the huge hoard of gold they had secretly amassed to the Chief Gold Receiver in Dunedin in August 1862. Hartley and Reilly's discovery caused great excitement as they deposited some of gold, sparking a gold rush to what would become known as Hartley's Beach. Hartley and Reilly, in divulging the location of their rich finds (approximately one mile downstream of the Clutha River's confluence with the Kawarau River), were rewarded with £2000 from the Otago Provincial Government. On 23 September 1862 the Dunstan goldfield was proclaimed, the selection of this name in preference to Hartley's being largely in deference to Reilly, who was 'jealous of the pre eminence' accorded Hartley as the discoverer. but like the earlier gold rush, the times of the dredges drew to a close only several years later.

In 1865, the Otago Provincial Council invited Chinese miners working on the Victorian goldfields in Australia, to rework the Otago goldfields. European miners had departed in their droves for more lucrative goldfields in Marlborough and the West Coast, opening the door for Chinese mining immigrants to come directly from China, particularly Guangdong province. although it was intended to link the Lakes District (Kingston/Queenstown) via the Kawerau Gorge and not the Cromwell Gorge at the time. Reference to a railway, presumably through the Cromwell Gorge, linking to the Taieri Railway was also made in 1877, but nothing would come of this for another thirty years. The railway finally made it as far as Clyde in 1907, with the station's official opening on 2 April 1907 by the prime minister, William Hall Jones. The Cromwell Railway and Progress League, having been formed in 1906 to influence the extension of the railway through the gorge to Cromwell, set about lobbying the Government with increasing frequency. However, work on the railway line stopped at Clyde until 1914. The lobby group stressed the high cost of transport between Cromwell and Clyde, the impact on the region's farmers and also pointed to the inadequacies of the road:

"Irrigation first", was the Government's reply, This would not stop the representations, however. An announcement to restart the railway extension was made on March 1914, and works were established in the following months with new surveys taking place. The railway started in earnest in September and by the end of 1916 some 8.5 mile (14 km) of rail had been laid. The line to Cromwell was not completed until July 1921 significantly overrunning the predicted two to three years it would take. When the dam itself was completed, the line to Clyde had little other traffic and the section from Middlemarch to Clyde was closed on 30 April 1990. This made the Cromwell Gorge unique in the region and subsequently it produced some of New Zealand's best fruit. The Annan's were one of the first families to establish an orchard at the southern end of the gorge on what would become Fruitgrowers Road. In 1901 William Annan cleared his piece of land, built a cob shelter, and planted his fruit trees. Imported from Australia, the first trees Annan planted were a diverse mix to test what would be most suitable for the challenging Central Otago climate and included citrus, almond, walnut, cherry, apricot, apple, pear, peach, nectarine, quince and grape. The last orchardist to leave the Cromwell Gorge ahead of inundation by Lake Dunstan, was Kevin Jackson, who had run an orchard in the gorge from 19691989. Only a few trees from what was Jackson's orchard still remain at the edge the lake. but it was not until the 1940s that it began to be a more serious option for power in Otago. An examination of the Clutha River's potential in 1944 culminated in the selection and construction of the Roxburgh Dam with first power being produced in 1956. The impact on existing activities from any scheme was recognised from the outset:

Blockquote|text=Any further Clutha Valley development poses a very complex problem as there are considerable areas of better lands, orchards, allied industries, communication routes, closer settlement, and whole townships or effects on townships and local bodies to be taken into account.|author=Upper Clutha Hydro Electric Interdepartmental Committee, December 1971 were all ignored with the high dam becoming part of the National Government's Think Big policy. however it was not commissioned until 1992 at a total cost of $955M NZD (September 1987). Despite considerable amounts of concrete being pumped into tunnels across the fault to act as shear pins, presented a compilation of 617 sites of archaeological significance, 85 of which were submerged in the Cromwell Gorge due to the filling of Lake Dunstan. A total of 250 sites were lost in the greater Lake Dunstan area due to the filling of the lake. These landslides underwent extensive geo engineering in the early 1990s to mitigate their movement, and all now have extremely low movement rates as a result. Engineering works included extensive drainage to draw down the groundwater level, toe buttresses and in the case of the Cairnmuir Landslide, a drainage blanket. Thirteen large diameter tunnels extend into the toes of several landslides in order to drain groundwater which otherwise lubricates the basal shear zone. Combined with smaller drainage tunnels (49 in total), the total extent of the tunnels is In addition to the mitigation measures, some 6500 instruments have been installed to measure and monitor slide movement. With the establishment of a safe bench to work from, excavators and an abseiling team cleared the remaining loose material over the following days. The National Party's Think Big economic policy was heavily criticised and debated during the early stages of the dam construction, and the dam was protested extensively by all those affected by it. Workers unions during the construction of the dam held many strikes. As the landslide stabilisation of the Cairnmuir Slide neared completion, a protest slogan was painted on the lower portion of the drainage apron which read Hands Off Beaumont referring to the potential of a further dam being built near Beaumont, downstream on the Clutha River. The remnants of this graffiti are still visible. Wider impacts with the filling of Lake Dustan have continued to impact communities with the spread of lagarosiphon (lake weed), and the continued sedimentation and silting up of the Kawerau Arm of Lake Dunstan.

The Cromwell Gorge is accessed primarily by two routes. On the true left bank of Lake Dunstan runs State Highway 8, a single carriageway linking Clyde to Cromwell via Deadman's Point Bridge. On the true right bank of Lake Dunstan is the Dunstan Cycle Trail, a purpose built cycle path which extends the existing Otago Central Rail Trail. Lake Dunstan itself is navigable from the Clyde Dam through the gorge to Cromwell and beyond.

Lake Dunstan floods the site of the old State Highway 8 (SH8) and former rail line from Clyde to Cromwell. Rail services to Cromwell had ceased by 1980 and therefore a new line to Cromwell was not required ahead of Lake Dunstan being filled. The realignment of SH8 was necessary and was constructed above the shore line of the true left of the gorge (above the former SH8 on the floor of the gorge).

The Clyde power project roading construction took a total of ten years and involved of roading and several bridges and culverts. Twenty major contracts were involved and at least of rock and gravel moved. The majority of the sites were associated with heritage gold mining activities (stone walled buildings, stone walls, dams, water races, stone stacks), although six sites were pre historic rock shelters.

In May 2021 the Lake Dustan Trail was opened through the Cromwell Gorge. Although it does not form part of the existing Otago Central Rail Trail, it is a purpose built extension on the opposite true right side of the gorge, running along the foot of the Cairnmuir Mountains. The Lake Dustan Trail links Clyde and Cromwell, offering cyclists and walkers a Grade 23 (easyintermediate) route across bridges, switchbacks, cantilevered boardwalks pinned to sheer bluffs suspended above Lake Dunstan, or the gentle rolling vineyards of Bannockburn. The trail through the gorge from Clyde to Cromwell's Historical Precinct is , although the trail currently extends a further north of Cromwell along the western shore of Lake Dunstan.

On 21 October 2023 the Historic Clyde Incorporated Charitable Society unveiled a steel sculpture of a Kārearea (New Zealand falcon), erected overlooking the Clyde dam at the entrance to the gorge. The sculpture by artist Dan Kelly is fashioned from recycled steel, and will stand as guardian to the gorge and the township of Clyde below.