Steve Greene from White Horse said the project was "a fantastic achievement"
Experts from Oxfordshire are bringing new life to a once-polluted river at the London Olympics site.
Upstream from the 2012 Aquatics Centre specialists from Abingdon's White Horse Contracting Ltd, are populating the River Lea with new fish.
The river runs through the former wastelands that are now being transformed for the games.
The work also involves the creation of a wetlands area in the heart of the Olympic Park in Stratford.
The company has refined its skills developing lakes and wetlands for the likes of the Royal Society for the Protection of Birds.
Acting managing director Steve Greene said: "It's a fantastic achievement for the company. We've been operating in the Oxfordshire area since 1957 and I think this is probably the pinnacle contract that we've won. It's a huge achievement for all the team."
The badly-polluted river had already been scoured and cleaned before the company started work on site.
Not that the area was ecologically "dead": 2,000 newts and hundreds of toads were relocated away from the future park before the first diggers moved in.
But it remains a challenging job.
The area was home to polluting industries such as petrol factories, and even a vast Victorian rubbish tip.
The Oxfordshire workers have to scrape away the earth as they work and take it to be cleaned at a "soil hospital" on the site. Plenty of vintage rubbish has emerged along the way.
The company have over 100 people working on the site and are importing something in the order of 3,500 trees and 36,500 herbaceous shrubs.
The team at White Horse is used to challenging assignments.
The new horse-racing circuit it completed at Ascot in the summer of 2010 was described by champion jockey Frankie Detorri as "probably the best racetrack in the world."
It involved laying 17 miles of drainage.
And a recent job in Abu Dhabi presented challenges that put the Olympics in the shade.
In one of the hottest countries in the world, the company had to use technical ingenuity to create several international-standard football pitches - with real grass - for the FIFA World Cup Local Organising Committee.
They had to load four articulated lorries with all the equipment they'd need, including a laser-guided trenching machine, and drive it to Abu Dhabi through Greece, Turkey, Syria, Jordan and Saudi Arabia.
Olympic gold medallist Jonathan Edwards, who serves on the board of London 2012, said he marvelled at the transformation of the Queen Elizabeth Olympic Park.
He said: "I can remember when this was just wasteland, and it doesn't matter how many times I come here, I always get goosebumps to see how this has changed.
"It's going to be beautifully landscaped, and this Oxfordshire company is part of it.
"For the athletes, being in a lovely environment will be part of the inspiration they need to do their best."
He said landing an Olympic contract was a challenge for any company.
"This is very competitive indeed. You've got to train hard, and when it comes to the day when you're pitching, you've got to get it right. It's a bit like winning an Olympic medal."