Construction of bridges and other infrastructure over western rivers would be more difficult if the U.S. Supreme Court decides in favor of hydroelectric powerplant owner and operator PPL Montana in the company's case against the state of Montana, the state's attorney, Gregory Garre, told the justices on Dec. 7, during oral arguments in the case.

On the other hand, if the court rules for the state, about 1,000 existing dams on U.S. rivers, including some dams that have been operating for more than a century, could be subject to back rent, PPL allies contended in briefs filed earlier.

In the case, PPL Montana vs. Montana, the state argued that the land under nine PPL Montana dams is navigable and therefore belongs to the state.

The state's claim was upheld by the Montana Supreme Court, and Montana state says the Billings-based utility firm, a subsidiary of PPL Corp., Allentown, Pa., owes it $41 million, plus $8 million in interest for back rent, as well as about $7 million a year for rent going forward.

PPL Montana argued that the land was not navigable at the time of statehood in 1889, when navigable rivers would have become the property of the state.

In addition, PPL Montana contended that, at the time the company's predecessor built the dams, the state actually helped get the dams constructed by giving the company eminent domain. Seven of the dams have been operating for more than 100 years.

Justice Antonin Scalia called it "extraordinary" that, 100 years after these hydroelectric dams were built on Montana's rivers, the state has laid claim to the land under those dams and demanded millions in back rent.

Most of the discussion during the oral arguments focused on whether the Madison, Missouri and Clark Fork rivers, where the dams are located, are navigable and whether portages to avoid the rapids and falls on those rivers make them technically navigable or un-navigable for purposes of commerce.

The court is expected to issue a ruling in the case by the end of June, when its current term concludes.