Mount Isa was built on ore, but it survives on water, and for years water was the field's great anxiety. The answer was the Leichhardt Dam, begun in 1956 and completed on 6 November 1958 at a cost of 1.7 million pounds. It was, at the time, the largest water scheme in Australia financed by private enterprise - paid for by Mount Isa Mines to supply both the city and its sprawling mining lease. The American Utah Construction Company started the work; the Australian firm Thiess Brothers finished it. In 1961 a competition to name the new reservoir was won by a local, Danny Driscoll, and the dam officially became Lake Moondarra.

Long before any engineer studied this stretch of river, people gathered here for a different reason. This was the site of one of Australia's largest stone axe quarries, on the traditional country of the Kalkadoon people. The stone worked here was prized, and the finished axes travelled astonishing distances - traded hand to hand across as much as a thousand kilometres of the continent. It is a reminder that the lake's banks were a centre of industry and exchange for countless generations before the mine arrived, that this was a meeting place and a workplace long before it was a reservoir.

Lake Moondarra is where Mount Isa comes to breathe. There are picnic areas and pontoons, a ski jump and water-sports facilities; sailors and birdwatchers and anglers share the shoreline. One stretch is called Transport Bay for a frankly outback reason - the company's trucks hauled in tonnes of sand to build an actual beach on the lake's edge, a slice of seaside conjured up a thousand kilometres from any ocean. Since 1999 the Lake Moondarra Fishing Classic has drawn crowds, and for good cause: the lake is stocked with around 10,000 fingerlings a year, and barramundi and sooty grunter swim among the 22 freshwater species that fill it. Anglers here have pulled in barramundi of genuinely startling size.

Not every story here is about people. In the late twentieth century the lake faced an invasion - Salvinia molesta, a floating fern that can blanket a waterway, choking out light and life beneath. The remedy was almost absurdly small. A species of weevil, a beetle that feeds on the weed, was released here from 1980 onward in one of the world's landmark experiments in biological pest control. The tiny insects did what bulldozers could not, destroying tens of thousands of tonnes of Salvinia and clearing the water again. It is a quiet outback parable: the smallest creature, given the right job, saving a whole lake.