There is granted to the United States and its assigns, including its successors in control of the operation and maintenance of the Riverton reclamation project, Wyoming, a flowage easement and an easement for a dam site, together with all rights and privileges incident to the use and enjoyment of said easements, over tribal and allotted lands of the Wind River or Shoshone Indian Reservation within that part of said reservation required for the construction of the Bull Lake Dam and Reservoir on Bull Lake Creek, a tributary of the Wind River, in connection with the Riverton reclamation project, Wyoming, and for the impounding of approximately one hundred and fifty-five thousand acre-feet of water, including a ten-foot freeboard: Provided, That in consideration of the said rights insofar as they affect tribal lands there shall be deposited into the Treasury of the United States pursuant to the provisions of section 155 of title 25, for credit to the Shoshone and Arapaho Indians of the Wind River Reservation the sum of $6,500, from moneys appropriated for the construction of the said Bull Lake Dam and Reservoir, and the said sum when so credited shall draw interest at the rate of 4 per centum per annum.