Idaho Code § 18-7013
Whenever any individual, individuals or corporation, municipal, public or private, owning, operating or maintaining any reservoir, pond, tank or any structure or place wherein or whereon water is stored, either in whole or in part for domestic use, has the area or ground on or wherein such reservoir, pond, tank, structure or place is located, wholly inclosed by any fence or artificial barrier, and has posted or caused to be posted, warning signs in conspicuous places on such fence or artificial barrier forbidding entry, it shall be unlawful for any person to enter, or to throw or place, or to cause to be thrown or placed, any substance or thing whatsoever, within the area so inclosed by such fence or artificial barrier, without the consent of such individual, individuals or corporation, and any violation hereof shall constitute a misdemeanor and shall be punishable accordingly.
Idaho Code § 18-7013