President Lee Myung-Bak likes to think big, but his cherished $19 billion project to dredge, dam and beautify South Korea's four major rivers has stoked fears it is too ambitious and insufficiently green.

Revitalising the Han, Nakdong, Geum and Yeongsan rivers is the centrepiece of Lee's "Green New Deal", a bold plan to create jobs after the 2008 global downturn and to nurture a new national growth engine.

Since it began in 2009, work has focused on building 16 weirs and dams with small hydroelectric power plants and natural passageways for fish.

It is now 80 percent complete and to its fans there is much to like.

Rivers have been dredged, with the silt used to reinforce banks and build playgrounds and parks for sports, camping and picnics. Roads, sewage plants and cultural and sightseeing infrastructure have also been constructed.

"Lee's project gratifies the requirements of the times -- green growth," says Chungnam University professor Jung Kwan-Sue, a water resources specialist. There is also the promise of a jobs bonanza.

The project will create 340,000 jobs and boost economic output by 40 trillion won ($34.6 billion) over its four-year lifespan, including the trickle-down effects of tourism, according to the Land and transportation Ministry.

In addition a network of bicycle lanes stretching more than 1,600 kilometres (1000 miles) set to open next month will encourage South Koreans to use the river courses for recreation.

More still, when its completed in December next year, the ministry says it will save 1.3 billion tons of water a year for drinking and farming, while also reducing floods.

Official data show floods and drought cost the country 4.2 trillion won ($3.65 billion) annually.

"This is a very essential project as flash floods will be reduced significantly in rainy seasons and a shortage of water will be resolved in dry seasons," Jung added.

But caution hangs over the scheme's potential economic impact.

"It is true that the project contributes to development of our national economy," said Samsung Economic Research Institute analyst Park Hwan-Il.

"But unlike other projects to build roads and industrial infrastructure, it's hard to quantify the economic benefits brought by the river project."

Some see the green-growth strategy as too heavily focused on economic rather than environmental gain.

In a recent report, two professors -- Yun Sun-Jin of Seoul National University and Cho Myungrae of Dankook University -- said 64 percent of the budget for the Green New Deal would be spent on projects related to civil engineering.

The study found opponents of the "four rivers" scheme also believe the work created is mostly short-term construction contracts and not those really needed to employ a highly-educated younger generation.

"Though the government announced that around 340,000 jobs will be created by the project, opposing groups argue that only 2,000 jobs will be created over the long term," their report said.

Civic groups and opposition politicians have rounded on the project, decrying it as wasteful and damaging to the existing environment.

"Water quality has already deteriorated in some areas because of new dams and excessive dredging, endangering the survival of fish," environmental activist Ahan Cheol told AFP.

After taking office in 2008, Lee pushed to dig a cross-country canal but scrapped it after widespread protests. Instead he promoted the four rivers project, saying it offers a dual promise of environmental and economic gain.

Defending the project Lee has said the work will "help secure abundant water resources, create flood control systems, improve water quality, restore ecosystems and create opportunities for rural development".