In the spring of 2009, Muggli went looking for irrigation records in the old file cabinet on his grandfather's porch. He found a bundle of letters, more than 100 pages in all, thin and delicate, on carbon paper. They were all addressed to members of the Yellowstone River Compact drafting committee, of which his grandfather was a part.

"It was just a treasure trove of stuff," he says. The letters gave Muggli a sense that the drafters left some things unwritten because they trusted the states to interpret the Compact fairly. But what Muggli sees as trust could have been a simple lack of foresight -- that as water grew scarcer and technology more advanced, irrigators would scramble to make the most of their rights.

The precedent set by the Supreme Court's ruling may not reach far, particularly not to downstream states whose water allotments are quantified in interstate compacts. (The Colorado River Compact, for instance, appropriates 7.5 million acre-feet a year each to groups of upper and lower basin states.) But in states like Montana, where compacts are less specific, the decision could become problematic.

"Nobody complains in good years," says Montana Assistant Attorney General Jennifer Anders. "But what's going to happen five years down the road if Wyoming keeps consuming more without regard for downstream users?" The Supreme Court's interpretation still guarantees Montana its pre-1950 rights, at least on paper, but Anders will be watching closely when the next dry year comes. "We have a lot at stake," she says.  "It's the ability of our farmers to make a living off the land as they've done for generations."