How have cities like Paris, Singapore and Seoul cleaned up their rivers?

Cities have cleaned up their rivers by stopping the dirt at its source (treating sewage and fixing overflows), by moving polluting activities away, and then by waiting for the river to recover. The Seine in Paris and the Singapore River are two well-documented cases, and neither was quick.

Paris is the clearest example. By 1970 about 60% of Paris's urban sewage was allowed to flow into the Seine untreated, and the resulting oxygen depletion reduced the number of fish species to three. Measures taken in the early 2000s reduced organic carbon, phosphorus and ammonium, and in 2009 it was announced that Atlantic salmon had returned. A swimming ban in place since 1923 was lifted in July 2025, after a €1.4 billion 'Swimming Plan' launched in 2018 that included a basin to store rainwater and release it slowly into the sewer system.

Singapore took a different route. Its river was polluted by garbage, sewage and industrial by-products, and the clean-up that began in 1977 cost $300 million and involved relocating about 4,000 squatters, along with hawkers and vegetable sellers. The re-emergence of monitor lizards and otters has been attributed to its success.

Success is rarely total. Paris still has a 19th-century 'single system' that combines runoff and sewage, so heavy rain can push untreated sewage into the Seine, and E. coli levels have often been found to be far higher than is safe to swim in, though this could depend on the season.