The Ganges Delta has the shape of a triangle and is considered to be an "arcuate" (arc-shaped) delta. It covers more than 105,000 km² and lies mostly in Bangladesh and India, with rivers from Bhutan, Tibet, and Nepal draining into it from the north. The Gorai-Madhumati River, one of the distributaries of the Ganges, divides the Ganges Delta into two parts: the geologically young, active, eastern delta, and the older, less active, western delta. The average maximum temperature across the basin is 30.3°C in summer and 21.1°C in winter. The average minimum temperature across the basin is 21.5°C in summer and 6.4°C in winter. The Ganges Delta lies mostly in the tropical wet climate zone, and receives between 1,500 to 2,000 mm of rainfall each year in the western part, and 2,000 to 3,000 mm in the eastern part. Hot, dry summers and cool, dry winters make the climate suitable for agriculture. Three terrestrial ecoregions cover the delta. The Lower Gangetic plains moist deciduous forests ecoregion covers most of the delta region, although the forests have mostly been cleared for agriculture and only small enclaves remain. Thick stands of tall grass, known as canebrakes, grow in wetter areas. The Sundarbans freshwater swamp forests ecoregion lies closer to the Bay of Bengal. This ecoregion is flooded with slightly brackish water during the dry season, and freshwater during the monsoon season. These forests, too, have been almost completely converted to intensive agriculture, with only 130 km² of the 14,600 km² protected. Where the delta meets the Bay of Bengal, Sundarbans mangroves form the world's largest mangrove ecoregion, covering an area of 20,400 km² in a chain of 54 islands.