The Future of the Colorado River in Grand Canyon

The Colorado River is the centerpiece of the Grand Canyon region and is the focus of sublime natural wonder and recreational thrills by millions who stand on the canyon rim or float the river. Despite the appearance of unspoiled naturalness within a national park, the Colorado River today is one of the most extensively regulated and managed rivers on Earth, and its stream flow, sediment transport, temperature, and ecosystems are primarily a product of societal decisions of how to manage the water supply provided by the river. The American West is getting drier, with low water storage in Lake Powell and Lake Mead reservoirs, and a very dry winter likely to cause more problems.

To understand how the Colorado River in Grand Canyon fits into the puzzle of the plumbing and management of western water, I’ll provide a bit of simple math, using a few simple quantitative measures: the rate of stream flow in cubic feet per second (ft3/s) and the volume of flow in acre feet per year (af/yr).

Approximately four basketballs have a volume of a cubic foot (ft3). The mean annual flow of the Colorado River through Grand Canyon between 2001 and 2021 was approximately 13,200 ft3/s, as measured at the U.S. Geological Survey gaging station just upstream from Diamond Creek in western Grand Canyon. Think of 52,800 basketballs full of water whizzing past the Diamond Creek takeout every second. During the course of a year, approximately 9.58 million acre feet of basketballs would accumulate.

Is that a large number? It is a very tall stack of basketballs if you stack them in your front yard because most of our yards are less than an acre in size. Although there is wide variation in how much water a typical American family uses in a year, an oft quoted number is about 0.5 af/yr. How many families can be supported by 9.58 million acre feet of water? You do the math.

Approximately 92 percent of the water flowing past Diamond Creek and into Lake Mead came from Lake Powell, almost all of it released from Glen Canyon Dam, although about 200 ft³/s seeped through the sandstone around the dam and back into the river between the dam and Lees Ferry. Downstream from Lees Ferry, additional flow was contributed from spring-fed tributaries such as the Little Colorado River, Bright Angel Creek, Tapeats Creek, Deer Creek, and Havasu Creek, springs that directly flowed into the river, and runoff from the larger watersheds of Paria River, Little Colorado River, Kanab Creek, and Havasu Creek.

Despite these local sources, releases from Lake Powell are the dominant control on the Colorado River’s flow through Grand Canyon, and those releases are determined by three factors:

• Natural runoff, primarily snowmelt, from the Rocky Mountains;
• The amount of water consumed by users in Colorado, New Mexico, Utah, and Wyoming; and,
• The amount of water released from Lake Powell to meet the demands of users in Arizona, California, Mexico, and Nevada.

Between 2001 and 2021, the estimated natural runoff of the entire watershed that drains to Lake Powell was approximately 14 percent less than the natural runoff for the period 1930-1999. Natural runoff is the runoff that would occur if there were no upstream human uses or reservoirs. The geophysical research community agrees that this decline resulted from changing precipitation patterns in spring and in the North American monsoon season and the decreasing proportion of the winter snowpack that becomes spring runoff.

Of the total 12.4 million af/yr of natural runoff between 2001 and 2021, a full 30 percent was consumed by Upper Basin irrigated agriculture, municipalities, industry, or was transferred out of the watershed to the Colorado Front Range, Wasatch Front, central New Mexico, or southeastern Wyoming.

Overall, the natural runoff minus the millions of af/yr consumed by humans, minus the evaporation from reservoirs equals 8 million af/yr available to be stored in Lake Powell. But, average releases from Lake Powell between 2001 and 2021 were 8.8 million af/yr. It is no wonder that Lake Powell was nearly full in 2000 and is only 30 percent full today. It is hard to keep a bathtub full if the drain is wide open.

None of us knows precisely what the future climate will bring in winter snow, spring runoff, or monsoon rain, but the increasing aridification of the climate now leads water supply managers to intensely debate how to allocate the declining flow of the Colorado River. These are the difficult and painful debates of today—how can human uses throughout the watershed be reduced to match the declining supply, and where should the primary decreases in use occur? In the Upper Basin? In the Lower Basin? In Mexico?

The debate about how to achieve sustainability in water use and how to allocate the pain of drought is constrained by the structure of treaties, compacts, laws, Supreme Court rulings, and administrative agreements known as the “Law of the River.” In 2022, we celebrate the centennial of the signing of the Colorado River Compact, but that Compact is only one part of the complex Law of the River.

One view is that the general principal of the Compact was that the Upper Basin and the Lower Basin were to equally share the water supply provided by the river. The Lower Basin is now averaging 7.4 million af/yr of consumptive water use and the Upper Basin is averaging 3.7 million af/yr in use, not counting reservoir evaporation in either basin. The Upper Basin argues that the brunt of future reductions ought to occur in the Lower Basin where consumptive water use is twice that in the Upper Basin. Another view is that the priority should be given to the longstanding uses in the Lower Basin and that lower priority should be given to imaginary and controversial future proposed uses in the Upper Basin, such as the proposed Lake Powell pipeline. The delivery of water to Mexico is guaranteed by the bi-national treaty of 1944, and Mexico might continue to participate in reductions in the amount of water it receives. Indigenous nations have legally recognized water rights and have taken a relatively small proportion of their “federal reserved water,” further complicating negotiations about how to distribute the declining water supply.

But this debate about the long-term sustainability of consumptive water use of the Colorado River does not resolve the immediate crisis in how to manage the river today when Lake Powell and Lake Mead have shrunk in size and may soon reach minimum power pool—the crisis level when hydroelectricity can no longer be produced.

What does all this mean for the Colorado River in Grand Canyon? Grand Canyon connects the two largest reservoirs in the United States. All the significant consumptive water uses occur upstream from Powell or downstream from Mead. The Colorado River’s flow through Grand Canyon is merely the transfer of water from the Upper Basin to the Lower Basin. If more water is transferred downstream to meet the demands of the Lower Basin and of Mexico, then more water flows in the Colorado River through Grand Canyon National Park. If more water is consumed by the Upper Basin states upstream from Lake Powell, then less water flows in the Colorado River through Grand Canyon National Park. The math is simple, undeniable, and brutal.

So, what about an ultimate solution? What about constructing a permanent by-pass of flow around Glen Canyon Dam through new river diversion tunnels and abandoning water storage in Powell? In that circumstance, one thing can be clearly predicted—the Colorado River ecosystem in Grand Canyon would be drastically different than it is today. The future ecosystem would have much more fine sediment along its banks, a barren area free of vegetation during the low-flow season, and a fish community dominated by nonnatives much like the modern Green River. Many rapids and resources of lower Cataract Canyon and Glen Canyon might begin to reappear. Pearce Ferry Rapids in Lake Mead would be drowned, and Lake Mead would again begin to fill towards Separation Rapid. The warm summer flows of the Colorado River might cause water treatment problems for Southern Nevada Water Authority in its withdrawals from Lake Mead.

Is that imagined potential future a better one than today’s circumstance? Would the alternative of primarily storing water in Lake Powell, again releasing cool water from Glen Canyon Dam into Grand Canyon, and maintaining Lake Mead perpetually low, be a better solution? One’s opinion of how to distribute reservoir water between Powell and Mead partly depends on one’s opinion about the emergence of the long-inundated scenic wonders of Glen Canyon. However, an opinion about whether to primarily store water in Powell or in Mead also depends on what kind of an environmental future one wants for Grand Canyon.

Regardless, there is no question that consumptive water use must be reduced to match the dwindling supply. However, the geography of where reductions in consumptive water use occur is also of importance to Grand Canyon, because more water consumption upstream from Lake Powell or smaller releases from Lake Powell to the Lower Basin will result in a smaller Colorado River through the canyon.  

Once the societal crisis of achieving sustainability in water use is achieved, the secondary question is where to store the available water supply. Mostly in Powell? Mostly in Mead? Half and half as we do today? The answer to that question partly depends on what kind of Colorado River ecosystem we want to see in Grand Canyon’s future.