Explanation:

 The Tista River (also known as the Teesta River) is referred to as the lifeline of the state of Sikkim in India. The Teesta River is a 309 km long river flowing through the Indian states of West Bengal and Sikkim through Bangladesh before emptying to the Bay of Bengal. It carves out from the verdant Himalayas and Singalila in temperate and tropical river Valleys and forms the border between Sikkim and West Bengal