Mont. Code Ann. § 85-1-111

Navigable waters and all streams of sufficient capacity to transport the products of the country are public ways for the purposes of navigation and such transportation. This section shall not be construed so as to affect or impair, in any manner, any rights acquired prior to July 1, 1901, by any person, association of persons, or corporation. The right of any person, association of persons, or corporation to take and use any water, as now provided by law, from any stream or streams for the purpose of irrigation or any beneficial or industrial pursuit shall not be abridged.