It's better than a grant.
It's a pre-existing right of possession, a valid claim, requiring protecton of government.
http://caselaw.lp.findlaw.com/scripts/getc...8&invol=645
The Mining Act of 1866 was not itself a grant of water rights pursuant to federal law. Instead, as this Court observed, the Act was "`a voluntary recognition of a preexisting right of possession, constituting a valid claim to its continued use.'" United States v. Rio Grande Dam & Irrig. Co., supra, at 705. Congress intended "to recognize as valid the customary law with respect to the use of water which had grown up among the occupants of the public land under the peculiar necessities of their condition." 10 Basey v. Gallagher, 20 Wall. 670, 684 (1875). See Broder v. Water Co., supra, at 276; Jennison v. Kirk, 98 U.S. 453, 459 -461 (1879). 11 [438 U.S. 645, 657]