Montana water rights are based on the appropriation system, meaning that the earliest rights are the most valuable. The deadline to timely file claims for water rights in use as of June 30, 1973, i.e. historical water rights, was April 30, 1982. Many of these historical water rights date back to the late 19th century. As you can imagine, finding information to substantiate claims that were filed 35 years ago based on events that happened over a century ago does not lend itself to quick Google searches.