A special master appointed by the U.S. Supreme Court in a long-running dispute over state water rights says Wyoming is obligated to provide enough water to fill a reservoir in Montana.

But Special Master Barton Thompson Jr. is rejecting Montana’s bid to be reimbursed for some of the more than $4.6 million it has spent in the 2007 lawsuit against its southern neighbor.

Thompson said in a ruling this week that Montana is not entitled to recover costs incurred since he issued preliminary recommendations in February 2010. Montana Justice Department spokesman Eric Sell did not have an estimate for those costs.

Montana claims it was shorted on water flowing down the Tongue River from Wyoming during 15 years. Thompson concluded the shortage was just two years and Montana deserves $35,877 in compensation.