Dinajpur, before entering Bangladesh. As most of the rivers are fed by snow, they are perennial in nature and often flood during the rainy season. The entire region is made up of sand, gravel, and pebbles laid down by these rivers. The Mahananda rises from the Dow Hills forest, near the town of Darjeeling, and runs in a zig-zag course through the district of Malda and joins the Padma in Bangladesh. In the central region, the main river is the Mahananda. The Tangon, Punarbhabha, and Atrai rivers arise in the plains; the former two join and flow into the Mahanadi, while the Atrai flows into the Padma. Wetlands The Sundarbans delta is the largest mangrove forest in the world, situated in the South 24 Parganas district. It lies at the mouth of the Ganges and is spread across areas of Bangladesh and West Bengal. The Bangladeshi and Indian portions of the jungle are both UNESCO world heritage sites, separately listed as the Sundarbans and Sundarbans National Park respectively, though they are parts of the same forest. The Sundarbans are intersected by a complex network of tidal waterways, mudflats, and small islands of salt-tolerant mangrove forests, and present an excellent example of ongoing ecological processes. The average elevation of the area is 10 m. This area has been created by the deposition of silt from its numerous rivers—the Hooghly, Matla, Jamira, Gosaba, Saptamukhi, and Haribhanga—and their tributaries. The formation of the delta is an ongoing process, and new bars and islands are being created along the rivers and at the river mouth. A large percentage of the area is underwater during incoming tides. The area is known for its wide range of fauna. The most famous among these is the royal Bengal tiger, but numerous species of birds, spotted deer, crocodiles, and snakes also inhabit it. It is estimated that there are now 400 Bengal tigers and about 30,000 spotted deer in the area. Climate West Bengal's climate varies from tropical savannah in the southern portions to humid subtropical in the north. The main seasons are summer, rainy season, a short autumn, and winter. While the summer in the delta region is noted for excessive humidity, the western highlands experience a dry summer like northern India, with daily high temperatures ranging from to . At nights, a cool southerly breeze carries moisture from the Bay of Bengal. In early summer, brief