In any case where a reservoir has been or shall hereafter under the provisions of this division be constructed, or surveyed, laid out, and proposed to be constructed, for the storage of water for a system, which water is to be used at one or more points under appropriations of water and rights held and owned by the person owning the reservoir site and constructing the reservoir, the reservoir, appropriations, and rights shall, in the discretion of the board constitute a single enterprise and unit, and work of constructing the reservoir, or work on any one of the appropriations shall, in the discretion of the board, be sufficient to maintain and preserve all applications for appropriations and rights thereunder.