All of these tidbits have been found through rumours, amateur research and wikipedia. Do not take them as truth.
The Chonggyechon river has been around for centuries and it has gone through many makeovers - in the 1400's the King at the defied Feng Shui warnings and it was begun to be used as the cities sewage system and then from then on it had many changes to accomodate Seoul's growing population.
When the Japanese invaded Korea in the early 1900's they had many plans for the Chonggyechon - including buliding roads over it, which all failed for lack of funding. After the World War II many Korean refugees moved to Seoul and lived on the side of the river. Which was really an open sewage system.
In the 1950's the river was thought of as a symbol of Korea's poverty and they soon relocated the refugees and simply and cheaply filled the river making it into a road and creating an industialised area around it. In the late 70's, a raised highway was also built over the road and this area was now thought of as a symbol as the modernisation of South Korea.
When the now president of Korea, Lee Myugn-bak was the major of Seoul he proposed to turn the highway back in to the Chonggyechon river. Beginning in 2001, it cost trillions of won in restoration as the remains of the river were still sewagey and the river was now completely dry. Tonnes of water were pumped into the 6km river and the many cars that used the road everyday were told to find another route or use public transport.
Most Koreans supported the project and many believe this lead Lee to winning the presidency. Personally I think it is great and now that it is getting warmer you see many people spending time walking down the river and crossing the adorable stepping stones.