Editor’s note: Today (Oct. 11, 2023) is the 75th anniversary of the signing of the Upper Colorado River Basin Compact. The following is an excerpt from Revisiting the Upper Colorado River Basin Compact on its Diamond Anniversary, a forthcoming analysis by Eric Kuhn and John Fleck, co-authors of the book Science Be Dammed: How Ignoring Inconvenient Science Drained the Colorado River.

By Eric Kuhn and John Fleck

The Upper Colorado River Basin Compact was signed by representatives from Arizona, Colorado, New Mexico, Utah, and Wyoming on October 11, 1948, after over two years of negotiations. It was an attempt to resolve the allocation of water among the five states, and for three quarters of a century it performed that task well.

But as we approach the middle of the third decade of the 21st century, the challenges of overallocation of Colorado River, over-appropriation of the water we have, and climate change reducing the river’s flows, the Upper Basin Compact and the extended body of rules in which it is embedded are showing their age.

At its simplest, the Upper Basin Compact divided the water use available from the 7.5 million acre-feet per year apportioned to the Upper Basin by the 1922 Colorado River Compact. The compact accomplished two major tasks:

• It apportioned the consumptive use of water among the Upper Basin states using percentage allocations. Colorado received 51.75%, New Mexico 11.25%, Utah 23%, and Wyoming 14% of the water available for use in the Upper Basin. Arizona received a fixed 50,000 acre-feet per year.
• It defined the obligations of the Upper Division states (Colorado, New Mexico, Utah, and Wyoming) to deliver water to the Lower Basin at Lee Ferry to satisfy the requirements of the Colorado River Compact.

In pursuing a new set of post-2026 Colorado River Operating rules, major water agencies and state leaders have insisted that the “Law of the River” – the suite of rules dating to the 1922 Colorado River Compact and including the Upper Basin Compact – should be a fundamental guiding principle of future river management. “The Post-2026 Operations should reside in a framework consistent with a reasonable interpretation of the Law of the River,” the Central Arizona Project wrote, to cite one example among many.[1] But a careful review of the history of the Upper Basin Compact shows how tenuous a foundation the Law of the River provides, and how uncertain any attempt at “reasonable interpretation” might be, because of fundamental uncertainties about what the Law actually says.

• When the Upper Basin compact was signed there was agreement on the definition of the “what” to which the percentage allocations apply. Water use in the Upper Basin was limited by water availability after meeting the Colorado River Compact’s Lee Ferry delivery requirements. Today, because of the impacts of climate change on flows, there is no such agreement and there are claims that the intent of the compact was to provide an equal amount of water for use to each basin. This creates deep uncertainty in the actual volumes of water available to each state.
• There is still no consensus on how to measure consumptive use basin-wide. The Upper and Lower Basins use different methods, and Lower Basin tributary use is neither well understood nor quantified. This makes managing the river system challenging.
• The Upper Division States claim overuse by the Lower Basin based by using one measurement method, while using a different method for their own uses. There is valid dispute over these theories and methodologies.
• Tribal water rights remain unresolved and limited in some cases by provisions aimed at preventing tribes from using their full legal entitlements.

In negotiating the Upper Basin Compact, the states made key decisions on critical compact issues that continue to echo through 21^st century water management.

Stream Depletion

Colorado River management has always suffered under controversy and ambiguity around the question of how to measure consumptive use. The Colorado River Compact did not include a definition of “beneficial consumptive use.” In the century since it was signed, two competing (and conflicting) methods have been used: diversions less return flow, and stream depletion. On some scales, they may look the same. But on large enough scales, they do not, in ways that have profound implications for 21^st century river management decisions.

Under the stream depletion theory, each basin’s consumptive use is measured as the net reduction in natural flows caused by man-made activities. For example, the Upper Basin’s consumptive use would be calculated as the amount that upstream uses deplete the natural flow of the river at Lee Ferry.

During the Upper Basin Compact negotiations, Colorado and Arizona were the main proponents of this theory. It was ultimately adopted in Article VI of the Upper Basin compact as the method for measuring consumptive use.

But the stream depletion theory is not universally used in river management today. It is, for example, used to quantify reservoir evaporation in the Upper Basin, but not the Lower Basin. It is not used to measure Lower Basin mainstream uses, where the “diversions minus return flows” method is used instead. Uses on the Lower Basin tributaries, which are included in the compact definition of “Colorado River System” are currently not measured at all – using either theory.

So, while the stream depletion theory survives in the Upper Basin Compact, inconsistencies and lack of measurement means there is still no consensus on how to quantify consumptive use basin-wide under the Law of the River. This remains a source of conflict and uncertainty and if not corrected or finessed could lead to interstate litigation among the compact states.

Allocating state water by percentages rather than absolute amount

The Upper Basin Compact is frequently praised for state-by-state allocations based on percentages (except Arizona), rather than absolute numbers, thus avoiding the mistake in the Colorado River Compact that over-allocated the river’s water.

But modern policy discussions are unsettled on a central issue – percentage of what? On their own, the percentages are meaningless without reference to some sort of underlying total amount of water available to be shared among the states.

When negotiating the Upper Basin Compact, the states’ representatives were clear on what they intended as the basis for using the percentages. They intended to apply the percentages to the amount of water available for consumptive use in the Upper Basin after meeting what they viewed as their compact “delivery obligations” at Lee Ferry.

Today, there is no such consensus. Climate change has altered the river’s hydrology, putting the burden of impacts on the Upper Basin. Its leaders have responded by arguing that the compact’s negotiator’s intention was to equally divide the water available to each basin for use. Since climate change is causing a decline in natural flows, whatever Lee Ferry obligations the Upper Division States have must now be adjusted to reflect the new hydrologic reality.

Resolving this issue requires either litigation, negotiated settlement, or collectively agreeing on a modified approach – one that appropriately factors in climate change and maintains the benefits of the 1948 flexible percentage allocations.

Tribal Water

While large Native American water needs and legal entitlements were identified before the Upper Basin Compact was negotiated, Tribal communities were excluded from the negotiations. Instead, Indian water use, which the negotiators knew was legally perfected long before 1922, was lumped into state allocations, with each state being responsible for meeting tribal needs from its share of the water. This gamble set up a potential conflict between the apportionments made by the Upper Basin Compact and the protections provided Indian rights under the Colorado River Compact.

A decade after the compact was signed, this conflict became real. In response, Upper Basin leaders took steps to limit tribal water rights and prevent full use of tribal entitlements, by inserting provisions in project authorizing legislation. The implications today are a legacy of intentional discrimination against tribes, unresolved legal questions around tribal water rights, and provisions that treat Native Americans as second-class citizens.

[1] Brenda Burman letter to Bureau of Reclamation, Aug. 15, 2023. See also comments by the state of Wyoming, the Salt River Project, the state of Colorado and the Upper Colorado River Commission.