For decades, water law in all four Pacific Northwest states (Washington, Oregon, Idaho, and Montana) has been based on the prior appropriation doctrine. 1 That doctrine has been a fixture in the western United States for over a century, 2 and the fundamental rules of water law based on prior appropriation are well established. These basic rules provide water users with a high degree of certainty and security, creating private property rights to use3 a resource that is owned by the public, 4 but they also limit water use in some significant ways. For example, the traditional rules restrict where, when, how, and how much water may be used, and specify how water rights may be established and lost. 5 The Northwest's water resources and those who rely on them have come under increasing stress in recent years. Causes of this stress include overappropriation, drought, population growth, the decline of salmon and other fish populations, aquifer depletion, water quality impairment, assertion of tribal water rights, increasing competition for water supplies, public demands for environmental protection, and other factors. 6 These

1 The prior appropriation doctrine was established in the Idaho Constitution, dating to 1889. Idaho Const. art. XV, § 3. In Montana, the doctrine is rooted in statutes of the 1860s and 1870s. Mettler v. Ames Realty Co., 201 P. 702, 706-07 (Mont. 1921). Oregon codified prior appropriation as the law of the state in 1909. OR. REV. STAT. §§ 537.110-537.330 (1997). Washington followed suit in 1917, becoming the last state in the West to adopt a comprehensive water code. Wick Dufford, Washington Water Law: A Primer, 11 ILLAHEE 29, 31 (1995); WASH. REV. CODE §§ 90.03. 005-90.03. 611 (1992 & Supp. 1998).