Whenever, in the opinion of the commission, it is necessary and desirable for it to erect a dam or spillway across a stream or at the outlet of a lake, or to alter or reconstruct an existing dam or spillway, so as to increase or decrease its permanent height, or to permanently affect the water level above the structure, it shall proceed with said project by first adopting a resolution of necessity to be placed upon its records, in which it shall describe in a general way the work contemplated.