(CN) - The Supreme Court ruled Monday that Wyoming, while off the hook for most of Montana's irrigation-related claims, is liable to its northern neighbor for two years of reduced river water levels.
Montana claimed in 2008 that Wyoming breached a 1951 agreement between the states called the Yellowstone River Compact by using more than its fair share of water from the Tongue and Powder Rivers.
Montana specifically alleged that Wyoming wrongfully allowed "pre-1950 water appropriators to increase their net water consumption by improving the efficiency of their irrigation systems," according to court records. Montana claimed the new systems used sprinklers that reduced the amount of wastewater returned to the rivers, depriving its "pre-1950 appropriators of water to which they are entitled."
In May 2011, the U.S. Supreme Court ruled 7-1 that Wyoming did not violate the Yellowstone River Compact by allowing for new, more efficient irrigation systems on farmland.
The high court rejected an exception Montana claimed in response to the first interim report filed by a special master appointed to the case.
"We conclude that the plain terms of the Compact protect ordinary 'appropriative rights to the beneficial uses of [water]...existing in each signatory state as of January 1, 1950,'" Justice Clarence Thomas wrote for the Supreme Court in 2011. "And the best evidence we have shows that the doctrine of appropriation in Wyoming and Montana allows appropriators to improve the efficiency of their irrigation systems, even to the detriment of downstream appropriators. Montana's allegation that Wyoming has breached Article V(A) of the Compact by allowing its pre-1950 water users to increase their irrigation efficiency thus fails to state a claim."
Thomas added that "the doctrine of appropriation in Wyoming and Montana allows appropriators to improve their irrigation systems, even to the detriment of downstream appropriators."
On Monday, the high court issued a one-page order on the special master's second interim report. It ruled in Wyoming's favor on a notice requirement for damages for the years 1982, 1985, 1992, 1994 and 1998.
The Supreme Court also held that Wyoming is not liable to Montana under the Yellowstone River Compact for the years 1981, 1987 to 1989, and 2000 through 2003.
However, it ruled that Wyoming is liable for reducing the volume of water in the Tongue River on the border of the two states by 1,300 acre-feet in 2004 and 56 acre-feet in 2006.
The high court remanded the case to its appointed special master for a determination of damages, without further explaining its finding of liability.
Read the Top 8
Sign up for the Top 8, a roundup of the day's top stories delivered directly to your inbox Monday through Friday.