Some 750 lighters along the Singapore River and Kallang Basin in 1977. Waste, oil spills and wastewater from these boats and lighters added to the pollution of the rivers. In 1977, Prime Minister Lee Kuan Yew called for a clean up of Singapore's rivers, which included the Singapore River and the Kallang River. The clean up cost the government $300 million and involved a relocation of about 4,000 squatters, along with hawkers and vegetable sellers, whose daily waste flowed into the river. Public housing was found for the squatters, while street hawkers were persuaded to move to hawker centres. The government then dredged foul-smelling mud from the banks and the bottom of the river, clearing the debris and other rubbish.