The whole or any portion of any natural watercourse, the whole or any portion of which lies within any district established under this chapter, or the whole or any portion of any ditch or drainage system already constructed or partially constructed prior to the passage of this chapter, may be improved and completed as a system under the provisions of this chapter: PROVIDED, That vested rights in any such watercourse acquired by appropriation of the water thereof for irrigation, mining or manufacturing purposes under existing law, shall not be disturbed.