Approximately 1,450 miles long, the Colorado River has a drainage basin that covers 246,000 square miles and encompasses seven western U.S. states and two Mexican states. Water from the Colorado river is used for agricultural, municipal, hydroelectric, recreational, and habitat purposes, among other uses. A growing population of 40 million people rely on a water supply sourced from the Colorado River Basin.
How the Colorado River’s water gets used – and who gets to use it – is a century old conversation that’s becoming more complicated in the face of long-term drought, increased water use, and climate change. With less water to go around, the risk of curtailment to Colorado River diversions grows.
Backstory: The Colorado River Compact
In 1922, the seven U.S. states in the Colorado River Basin negotiated the Colorado River Compact, dividing themselves into Upper Basin states (Colorado, New Mexico, Utah and Wyoming) and Lower Basin states (Arizona, California and Nevada). The Compact allowed each basin (Upper and Lower) to consume 7.5 million acre-feet per year (MAF/yr) of the river’s water on a 10-year average basis – approximately half of the river’s 10-year average annual flow, as estimated at the time the Compact was entered.
Language in the Compact states that Upper Basin states will “not cause the flow of the river at Lees Ferry [Colorado River stream gage now located just below Lake Powell] to be depleted” below the Lower Basin’s share. The intent of this language – the conditions under which it places Upper Basin states in a position of obligation – is still under debate.
Regardless of the reason for average flows at Lees Ferry falling below the Lower Basin’s share of 7.5 MAF/yr – whether due to over-appropriation by the Upper Basin states, inappropriate calculations of availability at the time of the Compact based on wetter hydrological conditions, or to a natural cause like a 20-year headwaters drought — there is a possibility that Lower Basin states could place a call on the Upper Basin states, requiring the Upper Basin states to cut back their own uses and send their water downriver, whether they “caused” the shortage or not.
Challenges: A Warmer, Drier Basin
The Colorado River today is the primary water source for an expanding population and 90% of the nation’s winter vegetable production. This makes it one of the most over-allocated rivers in the world, with water taken out each year exceeding natural flows from rain and snow.
Climate change and a warmer, drier basin compound the challenges of over-use.
In the period since 2000, air temperatures in the Colorado River Basin have averaged 2 degrees Fahrenheit warmer than the twentieth century average – this creates “hotter” droughts that deplete river flows.
The river’s more recent average annual flow (between 2000 and 2018) has been only 12.4 MAF/yr — 16 percent lower than the 1906-2017 average of 14.8 MAF/yr. Thus, the Colorado River is experiencing a diminished supply in addition to increased demands.
Curtailment Risk: Unable To Meet the Lower Basin’s Demands
In 2019, U.S. Congress passed the Drought Contingency Plan in an attempt to prevent Lake Powell and Lake Mead reservoirs from falling so low that they cannot deliver water or produce hydropower. New operating guidelines are due by 2026, but the risk of shortages before that due date make it prudent to assess – and plan for – curtailment risks now.
In a recent paper by Anne Castle and John Fleck, they noted that even with the recently adopted Drought Contingency Plan, a repeat of hydrology experienced in the 2001 to 2007 period could cause Lake Powell to fall below the minimum power pool of 3,490 feet, as shown in the graph below. Below this level, hydropower generation is not possible.
Additionally, water supply simulations commissioned by a group of western Colorado water agencies found a greater than one in three chance that flows could drop so far in the next decade that the ability of the Upper Basin to meet their potential legal obligations to downstream users in the Lower Basin would be in grave jeopardy, possibly causing Lower Basin states to demand curtailment in the Upper Basin.
Even if the risk of curtailment was judged to be low, the consequences aren’t. Experts are urging communities to prepare for possible curtailment now to mitigate loss of economic activity, jobs, income, and community benefits that depend on the water.
Analyses of the impacts of reduced water supplies caused by drought suggest losses to Colorado’s economy of tens of millions of dollars, as well as thousands of job losses. Tackling the complicated issue of demand management could be a prudent precautionary measure when insufficient water exists in Colorado streams.