DENVER (AP) — The intensifying crisis facing the Colorado River amounts to what is fundamentally a math problem.

The 40 million people who depend on the river to fill up a glass of water at the dinner table or wash their clothes or grow food across millions of acres use significantly more each year than actually flows through the banks of the Colorado.

In fact, first sliced up 100 years ago in a document known as the Colorado River Compact, the calculation of who gets what amount of that water may never have been balanced.

“The framers of the compact — and water leaders since then — have always either known or had access to the information that the allocations they were making were more than what the river could supply,” said Anne Castle, a senior fellow at the Getches-Wilkinson Center at the University of Colorado Law School.

EDITOR’S NOTE: This is part of a collaborative series on the Colorado River as the 100th anniversary of the historic Colorado River Compact approaches. The Associated Press, The Colorado Sun, The Albuquerque Journal, The Salt Lake Tribune, The Arizona Daily Star and The Nevada Independent are working together to explore the pressures on the river in 2022.