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THE CRISIS / EXPLAINER

How water rights work

Water on the Colorado River goes to whoever claimed it first. “First in time, first in right”: when there is not enough, the newest rights go dry before the oldest lose a drop. The stack below is the Lower Basin's 7.50 maf of rights, the most senior at the bottom. Drag the shortage and watch which blocks drain.

The priority stack

01.25 maf adopted3.00 maf sideboard

No shortage: every block full

Arizona
0
California
0
Nevada
0
01234567mafCalifornia priorities 1–3: 3.85 maf of 3.85 maf remainingCalifornia priorities 1–33.85 mafArizona on-river rights and contracts: 1.20 maf of 1.20 maf remainingArizona on-river1.20 mafMetropolitan Water District, fourth priority: 550 kaf of 550 kaf remainingMetropolitan (California 4th)Nevada: 300 kaf of 300 kaf remainingCentral Arizona Project and other priority-4 contracts: 1.60 maf of 1.60 maf remainingCentral Arizona Project1.60 mafMexico1.50 maf by treatyafter 2026: not negotiatedLower Basin, 7.50 maf · senior at the bottomno shortage
  1. 1,229 ftHigh-water line
  2. 1,125 ftConsultation on increases
  3. 1,075 ftThe old shortage trigger
  4. 1,050 ftSecond 2007 tier
  5. 1,037.2 ftToday
  6. 1,025 ftThird 2007 tier; ICS consultation
  7. 1,010 ftConsultation on more cuts
  8. 1,000 ftNo banked water; ROD consultation
  9. 950 ftNo hydropower
  10. 895 ftDead pool

What the 2027–2028 rules actually do

Until the end of 2026, cuts were tied to a table of Lake Mead levels: below a projected 1,075 feet on January 1 the first tier applied, deeper tiers at lower levels. For 2026 that meant a Level 1 shortage: Arizona 512 thousand acre-feet, Nevada 21 thousand, Mexico 80 thousand, California nothing.

The new Operating Guidelines, signed in August 2026, break that link. For 2027 and 2028 there is a shortage condition whatever the lake does: the Lower Basin gives up 1.25 maf a year, Arizona 760 thousand acre-feet, California 440 thousand, Nevada 50 thousand, the split the three states proposed themselves, plus 700 thousand acre-feet of paid, voluntary conservation. If Lake Mead is projected below 1,010 feet, the Secretary consults on more; above 1,125 feet, on less. The ten-year framework behind the guidelines allows up to 3.00 maf of shortage, allocated on applicable law unless the rights holders agree otherwise, and adopts no distribution for anything beyond the first 1.25 maf. That is why the slider has two bands: the adopted cut, and a model of what the ladder would do if it went deeper.

Four things to take away

  1. The river was divided on paper before anyone knew how much water there was. The 1922 negotiators assumed 16.40 maf a year and promised 16.50 maf; the river has carried 14.60 maf on average since 1906 and 12.40 maf this century.
  2. Shortage is not shared. It runs up the ladder from junior to senior. A farm district with an 1877 or 1901 right in the Imperial or Palo Verde valleys is protected by the 1968 Act; a Phoenix suburb on Central Arizona Project water is first in line.
  3. The 2027–2028 rules cut California, which took no shortage in 2026: 440 thousand acre-feet a year from 2027. They are the first guidelines written under the new ten-year framework, cover two years, and set no precedent for the next two.
  4. Mexico and the tribes sit outside the state-by-state math. Mexico's 1.50 maf comes from a 1944 treaty; its cut was 80 thousand acre-feet in 2026 and its share of any post-2026 cut is not yet negotiated. The five reservations along the river hold present perfected rights with dates from 1865 to 1917, satisfied first regardless of state line; other tribal claims, such as the Navajo Nation's, remain unresolved (see its page in the register).

The paper river

Three promises stacked against three measurements. The Compact split 15.00 maf between the two basins and the treaty added Mexico's 1.50 maf; the Upper Basin states hold their half as percentages of what is left after the Compact's delivery obligation at Lee Ferry.

Upper Basin 7.50 mafLower Basin 7.50 mafMexico 1.50 mafpromised: 16.50 mafassumed in 1922: 16.40 mafaverage 1906–2024: 14.60 mafaverage 2000–2024: 12.40 maf

How this model simplifies

Three classes stand in for hundreds of individual rights, each with its own date; inside every class the decree orders them one by one. The first band follows the adopted state split and places each state's cut on its junior block; California decides its own internal split and the card says so. The second band drains the stack strictly by priority, pro rata inside a class, which is one of the two readings the environmental study modeled; the other combined state shares with pro rata cuts. Neither is adopted. Every volume on this page is from the documents listed below; where a document is ambiguous the number is left out. For what the water is used for once it is delivered, see where the water goes.

Sources

Every number on this page is read from one of these documents, at the place named. Nothing is live; the Today mark is a dated reading, updated by hand.

  1. Colorado River Compact · Signed at Santa Fe; Bureau of Reclamation copy, 1922-11-24.
    Article III(a): 7,500,000 acre-feet per annum to each basin; Article III(d): 75,000,000 acre-feet per ten years at Lee Ferry.
  2. Upper Colorado River Basin Compact · Bureau of Reclamation copy, 1948-10-11.
    Article III(a): 50,000 acre-feet to Arizona, then Colorado 51.75%, New Mexico 11.25%, Utah 23.00%, Wyoming 14.00%.
  3. Treaty between the United States and Mexico on the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande · Bureau of Reclamation copy, 1944-02-03.
    Article 10(a): a guaranteed annual quantity of 1,500,000 acre-feet.
  4. Arizona v. California, Consolidated Decree, 547 U.S. 150 · Supreme Court of the United States; Bureau of Reclamation copy, 2006-03-27.
    Article II(B)(1): 2,800,000 acre-feet Arizona, 4,400,000 California, 300,000 Nevada; Appendix, pp. 167–182: the present perfected rights by state.
  5. Agreement requesting apportionment of California's share of the waters of the Colorado River (the Seven-Party Agreement) · Bureau of Reclamation copy, 1931-08-18.
    Article I, Sections 1–3: priorities 1–3 not to exceed 3,850,000 acre-feet; Section 4: fourth priority 550,000 acre-feet to Metropolitan.
  6. Colorado River Basin Project Act, Public Law 90-537 · Bureau of Reclamation copy, 1968-09-30.
    Section 301(b), p. 3: Central Arizona Project diversions limited to assure 4,400,000 acre-feet to California's present perfected rights, existing contracts and federal reservations, and to users of the same character in Arizona and Nevada.
  7. Arizona Department of Water Resources, Arizona's water supplies and Colorado River priorities (Water Resources Research Center forum slides) · ADWR, hosted by the University of Arizona WRRC, 2015-04-10.
    Slide "Arizona Priorities – 2.8 MAF Total": On-River 1.2 MAF, CAP 1.6 MAF.
  8. Joint ADWR and CAP shortage preparedness briefing · Arizona Department of Water Resources and Central Arizona Project, 2021-04-29.
    p. 16: Arizona's priorities 1–6 defined; p. 20: the CAP pools in delivery order; p. 11: Lake Mead dead storage at 895 ft.
  9. Record of Decision, Colorado River Interim Guidelines for Lower Basin Shortages and the Coordinated Operations for Lake Powell and Lake Mead · Department of the Interior, Bureau of Reclamation, 2007-12-13.
    Section 2.D: shortage tiers at projected January 1 Lake Mead elevations below 1,075, 1,050 and 1,025 feet.
  10. Reclamation announces 2026 operating conditions for Lake Powell and Lake Mead · Bureau of Reclamation news release, 2025-08-15.
    Level 1 Shortage Condition for 2026; Arizona 512,000 acre-feet, Nevada 21,000, Mexico 80,000; projected elevation 1,055.88 feet.
  11. 2027–2028 Operating Guidelines: Colorado River Guidelines for Coordinated Operations of Lake Powell and Lake Mead, Operating Years 2027 and 2028 · Department of the Interior, Bureau of Reclamation, 2026-08-21.
    Section 5.3, pp. 8–9: 6.25 maf apportioned (Arizona 2.04 maf, California 3.96 maf, Nevada 250,000 af; reductions 760,000, 440,000, 50,000 af); consultation below 1,010 ft and at 1,125 ft; 700,000 af of system conservation; no hydropower below 950 ft; Section 5.4: ICS consultation between 1,025 and 1,000 ft, none below 1,000 ft.
  12. Record of Decision: Decision Framework for Colorado River Guidelines, Coordinated Operations of Lake Powell and Lake Mead (2027–2036) · Department of the Interior, Bureau of Reclamation, 2026-08-21.
    Table 1, p. 28: up to 3.0 maf of Lower Basin shortage, allocated on applicable law including the Consolidated Decree, consultation below 1,000 ft; Section 4.6, p. 13: the FEIS modeled the Lower Division States' distribution up to 1.5 maf and applicable law above it; p. 17: the Department adopts no specific distribution; Section 8.4 and footnote 6: deliveries to Mexico not determined.
  13. Responding to Drought in the Colorado River Basin, Insight IN11982 · Congressional Research Service, 2026-08-03.
    Opening section: 16.4 maf assumed in 1922, 14.6 maf average 1906–2024, 12.4 maf average 2000–2024; Post-2026 Operations: FEIS maximum cumulative shortages Arizona 1.79 maf, California 1.01 maf, Nevada 206,670 af, Mexico 599,694 af (assumed).
  14. Hoover Dam: Lake Mead frequently asked questions · Bureau of Reclamation, 2026.
    The high-water line is at 1,229 feet above sea level.
  15. Lake Mead elevation, daily reading (Lower Colorado River operations hourly data), as shown on coloradorivermonitor.com · Bureau of Reclamation; read through Colorado River Monitor, 2026-10-18.
    Reading of October 18, 2026: 1,037.2 feet; updated by hand with the weekly refresh.