If you think of California when you hear about water crises, you’re only seeing a part of the picture. In fact, writes Abrahm Lustgarden, who reports on water issues for ProPublica, “pretty much every state west of the Rockies has been facing a water shortage” in recent years. And there’s something surprising to blame for the lack of water, he tells Fresh Air’s Dave Davies: a 90 year-old math mistake.

The miscalculation occurred in 1922, says Lustgarden. That’s when seven Western states (Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming) entered into the Colorado River Compact, an agreement that provided for the “equitable division and apportionment of the use of the waters of the Colorado River System.”

The agreement is central to modern water policy — the river and its associated system now provides water for 30 million people and is what National Geographic calls “one of the most contested, recreated-upon, and carefully controlled rivers on Earth.”

Lustgarden tells Davies that though most people think current Western water woes are the result of drought or climate change, that’s only the tip of the iceberg. In fact, he says, the 1922 compact contained a fatal miscalculation: while drawing up their agreement, each state overestimated the river system’s capacity and promised themselves more water than the system could sustain.

“When officials divvied up rights to Colorado River water nearly a century ago, it happened to be a wetter period than usual,” he explains in his piece for ProPublica. And now, even though river reserves are at a historic low, states continue to claim the same amount of water they have in the past — an amount that hasn’t been adjusted in 93 years.

So is there hope for Western water? Some experts remain pessimistic, pointing to federal subsidies that “finance the water crisis” and make change difficult to achieve. Lustgarden disagrees. He tells Davies that there are three opportunities to make the most of “plenty of water” in the West: achieve water efficiency in cities, prioritize water-saving crops, and make changes in farming techniques. In the meantime, he writes for ProPublica, nature will ensure that states that don’t act soon will face a new — very dry — status quo.