The Colorado River Compact of 1922 divided water that didn't exist. Eight men at Bishop's Lodge near Santa Fe allocated 7.5 million acre-feet annually to each basin, upper and lower, using flow data from 1899 to 1921. The estimate, 16.4 million acre-feet per year at Lee Ferry, was derived from gauges hundreds of miles downstream and extrapolated through assumptions one peer-reviewed analysis has called "grossly optimistic."
That measurement window was among the wettest in twelve centuries. A USGS hydrologist named E.C. La Rue had a lower number, 15 million acre-feet, and warned the commission before signing that the data period was anomalous. He wrote directly to Herbert Hoover. He received a thank-you note.
The compact, the 1944 Mexico treaty, and subsequent agreements committed 16.5 million acre-feet per year. Nobody modeled evaporative losses from reservoirs that hadn't been built yet. By 2022, the river's twenty-year running average had dropped below 13 million acre-feet. Western water law distinguishes between "paper water," the legal entitlement, and "wet water," what actually flows. A century of dams, cities, and irrigated agriculture was financed against the first number. The river kept running on the second.
Assumed annual flow (1922): 16.4 MAF at Lee Ferry, based on 1899–1921 gauge data
La Rue's competing estimate: 15.0 MAF, derived from upstream gauges and submitted before signing
Total legally committed: 16.5 MAF/year — 7.5 upper basin + 7.5 lower basin + 1.5 Mexico (1944 treaty)
Long-term gauged average (1906–present): ~12.4 MAF
20-year running average, 2022: Below 13 MAF, roughly 20% under the compact's assumption
Tree-ring record: The current 22-year mean has fallen below the lowest in reconstructions back to A.D. 762, the driest stretch in over twelve centuries
What the compact enabled: Hoover Dam (authorized 1928), the All-American Canal, hydroelectric development, infrastructure now serving roughly 30 million people
Unmodeled loss: Lake Mead evaporation alone runs approximately 1.2 MAF/year — from a reservoir that didn't exist when the math was done
"Paper water": Legal entitlement on documents that does not correspond to a physical volume available for delivery