Revitalising the Han, Nakdong, Geum and Yeongsan rivers was the centrepiece of the outgoing president's "Green New Deal", a bold plan to create jobs after the 2008 global downturn.
Started in 2009, the USD 20.1 billion project involved the construction of 16 weirs and dams along the four rivers that were straightened and dredged to improve water quality and prevent flooding.
But a report by the Board of Audit and Inspection suggested the three-year effort had fallen far short.
"Due to faulty designs, 11 out of 16 dams lack sturdiness, water quality is feared to deteriorate... and excessive maintenance costs will be required," the report said. Silting would require another round of dredging at an estimated cost of 289 billion won, it said.
Driven by tight timetables, work was pushed through without proper inspection and the river bed protection of 15 dams has partially subsided or been washed away. "Preparations were not thorough, while the construction period was too short," Yu In-Jae, director of the BAI's Construction and Environment Bureau, told AFP.
As water became stagnant behind the dams, the amount of organic pollutants surged above pre-construction levels, the report said, urging comprehensive measures to bolster faulty structures, boost water quality and ensure proper maintenance.
The project had targeted the creation of 340,000 jobs and an economic boost of 40 trillion won, including the trickle-down effects of tourism.
A network of bicycle lanes was also constructed along the river banks, stretching more than 1,600 kilometres.
But the project had its critics from the outset, with opposition parties and civic groups questioning its cost and environmental credentials.