to a magnitude of 6. The island is mostly composed of black basalt rock. Since it is along the sea coast, it has a sandy belt on its western coast. The southern region of Old Mumbai is mostly at sea level. However, the parts which were erstwhile shallows are below sea level. Many parts of the city are hilly. There is laterite soil and rocks at a point on Mumbai Island. Other natural formations Lakes There are three major lakes on the island: Powai Lake, Tulsi Lake and Vihar Lake. The latter two supply part of Mumbai's water requirements. Numerous other smaller ponds and lakes are present. Rivers The Mithi River (Mahim), Poisar River, Oshiwara River and Dahisar River originate in the national park and empty into the Arabian Sea. The Mithi River originates at Vihar Lake. Vasai and Thane creeks are estuarine distributaries of the Ulhas River. Creeks A number of saline or brackish creeks extend inland from the coastline. Mahim Creek separates the city from the suburbs in the west. Further north on the western coast, the Oshiwara river empties into Malad (or Marvé) Creek and the Dahisar River into Gorai Creek. The eastern waterfront also has many small creeks. Wetlands The small southern part of the eastern waterfront of Salsette forms Mumbai Harbour. North of this region lie vast amounts of protected wetlands at Sewree, home to migratory birds. The northern and northwestern part of the island and parts of Mahim River have government-protected marshlands. These swampy regions form massive, dense mangrove forests. See also List of topics on the Portuguese Empire in Goa, Bombay-Bassein, and the East Indies Navi Mumbai (New Mumbai) References Geography of Mumbai Former Portuguese colonies Islands of Mumbai Geography of Thane district