This is a list of rivers in East Timor. The list is arranged by catchment or drainage basin from west to east, with respective tributaries indented under each larger stream's name. Overview East Timor has over 100 rivers originating in the highlands and discharging into the coastal zone. As the island of Timor is small and the country's topography is steep, the rivers are short and drain quickly, with fast flowing discharges. A drainage divide trends approximately from southwest to northeast along East Timor's central mountain range; rain falling north of the divide flows northwards, and that falling to its south drains southwards. The country has also been broadly divided into twelve 'hydrologic units', groupings of climatologically and physiographically similar and adjacent river catchments or drainage basins. Each hydrologic unit is made up of a number of rivers. In total, there are 29 main river systems in East Timor – 12 in the north and 17 in the south. Very few of East Timor's rivers flow perennially, because the country is narrow from north to south, and has steep terrain and infrequent rain. Most of the rivers have short courses, are dry most of the year, and are not navigable. In the dry season, the ephemeral rivers often form pools of stagnant water. The south has more perennial rivers than the north because of its larger catchment areas, longer rainy season, greater rainfall and prevailing winds. According to a Timor-Leste Water Sector Assessment and Roadmap published by The World Bank in 2018: During periods of greater precipitation, the rivers in the south cannot accommodate the whole of their flow, and therefore overspill into coastal swamps and lagoons known as coilões. On the north coast, most rivers are choked with nearshore gravel, and there are few deltas. The reason is that the offshore gradient, into either Ombai Strait or Wetar Strait, is very steep, with the consequence that sediment that flows offshore is deposited in deep water. On the south coast, where the offshore gradient into the Timor Sea is much gentler, deltas are common. Many of East Timor's rivers are polluted by sewage released from homes and light industrial premises, due to a lack of basic sanitation. Additionally, the majority of the population uses river water for washing clothes, bathing and other domestic uses, and thus promotes the spread of diseases. East Timor's longest river is the North Laclo, at