(defendant, public water authority, took plaintiffs' riparian rights entitling plaintiffs to compensation; defendant used power of eminent domain to acquire land and divert by inter-basin transfer up to 30.5 million gallons of water daily from Deep River Basin to Haw and Yadkin River Basins; dam constructed and lake filled with diverted water to create public water supply; water flow in Deep River diminished as result of project which impaired plaintiffs' ability to produce electricity; plaintiffs have sufficiently defined interest in rate of water flow in Deep River and riparian rights are vested property rights in natural flow of water arising out of land ownership of land bounded or traversed by navigable water; reduction of flow is compensable taking; capitalization of income approach appropriate method to calculate plaintiffs' compensation owed).