Unlike the rest of the subcontinent, Assam retained many elements of its tribal economy well into the 19th century. With the British invasion the picture began to change gradually. Opening up of the Brahmaputra Valley in 1826 brought about two major changes in the mode of surplus extraction. One, colonial capital flowed into tea plantations, along with indentured labourers from mainland India. Though immensely profitable for the planters, this had a limited impact on the larger peasant economy of Assam.