Nothing in title 25 of this article shall be held to curtail the powers of the department as to river or drainage work within a river regulating district prior to the time that such district is charged with the powers and duties above set forth as to river or drainage work or both, nor in any event shall the powers of the department under sections 15-1945 and 15-1957 be curtailed or changed. Should the department at any time form or have formed a river or drainage improvement district in a basin now or subsequently formed into a river regulating district, which district is later given the extended powers herein provided, such river or drainage improvement district shall become a sub-district administered by the board. In such cases, all outstanding indebtedness and financial liabilities of the district shall remain undisturbed and not in anywise altered by the transfer and shall be met by the board.