Justices on the U.S. Supreme Court will be asked for an upcoming case to interpret the geographic descriptions logged by Lewis and Clark in their diaries from the early 19th century.

The matter of what Meriwether Lewis and William Clark saw on their expedition rose because of a legal battle between the state of Montana and a utility company over control of three rivers. The case, however, holds broader implications for the entire West.

State governments have long enjoyed ownership of rivers that were deemed navigable at the time of statehood. Montana shares this position, and has demanded $53 million in rent from power supplier PPL Montana , which operates hydroelectric dams on the rivers in question.

PPL argues that the state only has ownership of river segments that are navigable, leaving those portions where there are falls or other impediments (and where PPL is operating) in control of private entities. If the court sides with PPL, it won’t owe millions in back rent to Montana. Also, a victory for PPL could open up other states to lawsuits by utilities operating on rivers, which is why 26 governments are supporting Montana in the case.

Lewis and Clark’s diaries will be consulted to determine whether portions of the rivers back in 1805 were impassable or not. Lewis wrote about encountering the Great Falls of the upper Missouri River on June 13, 1805 . Montana became a state in 1889.

-Noel Brinkerhoff

PPL Montana, LLC v. Montana (SCOTUS Blog)