Over 40% of Hakaluki Haor, one of the largest marsh wetlands of Bangladesh as well as of Asia covering a total surface area of 181.15 km2 falls within Barlekha. A very large number of nature tourists come to visit the haor each year. Hakaluki Haor represents a complex wetland system with more than 80 interconnecting beels in a shallow basin formed between the Patharia and Madhab Hills to the east and the Bhatera Hills to the west. The major sources of water are the Juri, Sonai Bardhal and Kushiyara rivers, which traverse the wetland and drain through a single outlet, the Kushiyara River. While the haor itself is a seasonal water body formed during the monsoon, the beels are low-lying depressions of the haor system retaining water even during the dry months of the season. Thus, the haor system is a complex of both lacustrine wetlands (with open water) and palustrine wetlands (marshy – with vegetation), depending on the hydraulic behaviour in different seasons. In rainy season it takes the shape of a sea. Hakaluki Haor has a wide variety of waterfowl as well as wintering migratory birds. Every winter, tens of thousands of guest birds of about 150 species from Siberia and other cold regions flock to the haors. They include Bright and Rose King-duck, Pati-duck, Bali Hash, Lenja, Chity, Sorali, Boikal, Nilshir Piyan, Pantamukhi, Pankouri, Buti-duck, China, Rangamuri, Black-duck, Peributhi, Chokachoki, Giria, Khonjona, Patari, Dolpipi, Water-hen, North-Giria, Dahuk, Patibatan, Common-chill, Cotton-Chill, Gergini, Cottontail, Pintail, Toughed Duck.