Three water reservoirs and their reserve catchment area, which preserves a fragment of the original tropical forest, occupy the center of the island. Hills have been leveled, swamps drained and filled, and many of the fifty-odd small islets and reefs have been enlarged or joined to form new larger islands suitable for industrial uses. Some of the larger streams are dammed at their mouths to form fresh-water reservoirs, and the major stream courses through built-up areas are lined with concrete to promote rapid drainage.

Rain falls throughout the year, but is heaviest during the early northeast monsoon from November through January. The average annual rainfall is 237 centimeters, and much of the rain falls in sudden showers. The greatest natural hazard is local flash flooding, the threat of which has increased greatly as buildings and paved roads have replaced natural vegetation.

In spite of the high rainfall, it is necessary to import water from Malaysia. The water, from reservoirs in upland Johor, comes through an aqueduct under the causeway linking Singapore with the Malaysian city of Johor Baharu. Singapore has responded to this dependence on a foreign country for water by expanding its reservoir capacity and constantly urging household and industrial users to conserve water.

Between 1977 and 1987, the Ministry of the Environment carried out a major program to clean up rivers and streams by extending the sewer system, controlling discharges from small industries and workshops, and moving pig and duck farms to resettlement areas with facilities to handle animal wastes. The success of the program was demonstrated by the return of fish and aquatic life to the lower Singapore and Kallang rivers.