valleys The pattern of valleys and plains within Singapore reflects, on the one hand, geology and relief, and on the other, rainfall and erosion. Larger river systems have been responsible for a greater amount of long- term erosion and have therefore created more extensive valley networks.

The largest area of flat land associated with river systems is found in the west of the island, where Sungei Tengah and its tributaries flow northwards into the Strait of Johor and Sungei Jurong flows southwards into the Strait of Singapore. To the east of the central hills, the northward- flowing Sungei Seletar and the southward- flowing Sungei Kallang carve out a natural route which is largely followed by the Central Expressway.

Smaller river systems have carved more localized valleys around the fringes of the island. In many places, these valleys have submerged with the building of estuarine barrages in order to create water storage reservoirs, such as the Tengah, Poyan, Serimbun and Murai reservoirs along the west coast. Similarly, in the centre of the island, former steep- sided river valleys, carved out by the headwaters of numerous rivers and their tributaries, have been dammed to create the Seletar, Upper Peirce and MacRitchie reservoirs.