A glimpse into the history of the Colorado River Basin system, how it was designed and the impacts of climate change shed light on why it was destined to fail

By Becky BolingerToday at 9:14 a.m. EDT75

For the first time, the U.S. Bureau of Reclamation issued a water shortage for Lake Mead starting in 2022. Located between southern Nevada and northwestern Arizona, Lake Mead provides water and generates electricity for the more than 20 million people in the lower Colorado River Basin.

This shortage isn’t a surprise. Water levels at Lake Mead and Lake Powell to the northeast have already reached historic lows amid the summer drought. By January, the bureau projects water levels at Lake Mead to fall to 1,065.85 feet — nine feet below the first shortage trigger elevation. Levels on Lake Powell, which stores water for the Upper Colorado River Basin, are only marginally better, projected to be just 45 feet above the required elevation to produce hydropower.

The overall situation is not good, but why? This whole reservoir system along the Colorado River Basin was designed to get us through the drought years. Why isn’t it working? A glimpse into the history of the system, how it was designed and the impacts of climate change sheds light on why it was destined to fail — and why it may never recover.

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A crash course on the Colorado River Basin

As Americans began moving west, they found that Western rivers behaved very differently from those found in the Midwest and East Coast.

Western rivers were fed by snow from the peaks of the Rocky Mountains. During the winter, river flows would decrease, sometimes even freeze over. As spring and summer arrived, the warmer temperatures melted snowpack that accumulated on the mountains over the winter. Then the melt would run off at exactly the perfect time — the beginning of the growing season. Water would be abundant for farming and other needs during the warm season.

But issues arose with this “perfect” system. People learned less snowfall in one winter would result in less water flowing in the spring and summer. Water might not be as abundant as desired.

Reservoirs are drying up as consequences of the Western drought worsen

Then came an issue of who could use the water. Consider a farmer named Joseph. He and his family would settle on their land and pull from the river during the warm season. It had been a good winter so they expected high river flows that spring. Instead, the flows were really low. Where was his water?

He would go upstream to find that another farmer named William had settled his family there, and he was taking the water. Joseph told William that he couldn’t have the water. But William said it flowed through his land and therefore it was his. Joseph argued that it would actually flow through this land, and he was here first — it was his.

Thus was born the idea of water appropriation, albeit this is an extremely simplified and embellished version of the story.

Later, the Colorado River Compact of 1922 determined the river belonged to all parties where the river and its tributaries flowed. Everyone would share it equitably. This would include the upper basin states (Wyoming, Utah, Colorado and New Mexico) and the lower basin (Arizona, Nevada, and California).

The compact stated the upper basin would share 7.5 million acre-feet per year and the lower basin would also share 7.5 million acre-feet per year. Since the majority of this water originates in the Rocky Mountains of the upper basin states, those states must ensure the consistent delivery of water to the lower basin.

Lake Mead (initially formed by the Hoover Dam in 1935) was designed to hold water for the lower basin states. As an “insurance policy,” the upper basin had Lake Powell, which began filling in 1963. If drought meant the upper basin states couldn’t deliver their promised amount to the lower basin, they could deliver it with water in the savings account of Lake Powell.

While this plan initially seemed to work well, it was doomed from the beginning, for three reasons.

1. The water was already overallocated

How did the compact come up with the number 15 million acre-feet? Well, the number wasn’t just picked out of hat. There was a bit of analyzing of annual precipitation and runoff to come up with the estimate. In the early 1920s, data from the previous 10 to 20 years would be used to calculate the estimate. Unfortunately, the 1910s was a relatively wet decade and skewed the estimates higher than they should have been.

The chart above shows the basin-averaged runoff since 1920, as recorded by the U.S. Geological Survey. There is a lot of year-to-year variability, which is why building reservoirs to store during the wet periods became essential.

The gray line shows what the average runoff was when the compact was written in the early 1920s. Extending the gray line beyond its initiating point, you can see it’s much more common for total annual runoff to come up short. And with each passing year, reaching that gray line becomes even more improbable. The upper basin states consistently owed water to the lower basin that couldn’t realistically and consistently be achieved.

2. Population increases

Today, the Bureau of Reclamation estimates 40 million people rely on water from the Colorado River Basin. When the compact was signed in 1922, the total population of the seven basin states was not even 6 million people.

While the majority of the water from the Colorado River is used for agriculture, the smaller percentage of municipal use can’t be ignored when considering significant population increases. The old rule of thumb is that one acre-foot of water is enough for two households for a year. An increasing number of households throughout the Southwest puts further strain on the already overallocated Colorado River system.

3. Climate change will further reduce water availability in the basin

Temperatures throughout the Colorado River Basin are increasing, with particular “hot spots” in the Rocky Mountains. Scientists are still parsing out precipitation and snowpack trends on the mountains, but higher temperatures alone will reduce the water supply provided by the Colorado River.

For one, an earlier peak snowpack and earlier melt because of a warmer environment reduces runoff efficiency. Higher temperatures allow more water to evaporate into the atmosphere. This increased evaporative demand also means the same level of crop production requires more water.

Climate change is also increasing the frequency and severity of droughts in the Southwest. We’ve seen this quite obviously play out in the 21st century — repeated and prolonged droughts have chipped away at the available water supply while fewer opportunities for recovery have occurred. These trends will continue.

No going back

For many in the upper basin states, the situation became clear after the 2002 drought. The chart below shows how volume has changed in Lake Powell since it was built in the early 1960s. Wetter periods in the 1980s and 1990s helped bring it to “full pool” levels.

The 2002 drought was the most severe drought in the Upper Colorado River Basin in recorded history. To this day, cumulative flows on the Colorado River near the Colorado-Utah state line have not been lower.

While the lower basin states continued a business-as-usual path, it became clear in the upper basin that the system would not quickly recover from this drought. But with each step forward in recovery, another drought would take the system two steps back again.

Upper basin and lower basin states have worked on drought contingency planning, and new adjustments were written after the 2002 drought to prepare for a time when Lake Mead might get too low.

This year, we reached that point. Moving forward, we need to explore other solutions to meet our population and agriculture demands and preserve our forests, rivers and wildlife.

We all must accept that the question is not: “How do we recover Lakes Powell and Mead and get them back to good water levels again?” Instead, we need to ask: “How do we continue to meet the needs of the Southwest without Lakes Powell and Mead?”

Becky Bolinger is the assistant state climatologist for Colorado and a research scientist at Colorado State University.

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Creeks, rivers and lakes that are fed by melting snow across the U.S. West are already running low as of mid-July 2021, much to the worry of farmers, biologists and snow hydrologists like me. This is not surprising in California, where snow levels over the previous winter were well below normal. But it is also true across Colorado and the Rocky Mountains, which in general received a normal amount of snow. You’d think if there was normal amount of snow you’d have plenty of water downstream, right?

Over a century ago, snow scientist James Church at the University of Nevada, Reno, began examining how the amount of snow on mountains related to the amount of water in riversfed by the melting snow. But as hydrologists have learned over the many decades since, the correlations between snows and river flows are not perfect. Surprisingly, there is a lot researchers don’t know about how the snowpack is connected to rivers. 

Of course, a dry winter will result in meager flows in spring and summer. But there are other reasons snow from the mountains won’t reach a river below. One growing area of research is exploring how droughts can lead to chronically dry soil that sucks up more water than normal. This water also refills the groundwater below. 

But another less studied way moisture can be lost is by evaporating straight into the atmosphere. Just as the amount of snow varies each year, so too does the loss of water to the air. Under the right conditions, more snow can disappear into the air than melts into rivers. But how snowfall and loss of moisture into the air itself relate to water levels in rivers and lakes is an important and not well understood part of the water cycle, particularly in drought years.

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Losing moisture to the air

There are two ways moisture can be lost to the atmosphere before it reaches a creek or river. 

The first is through evaporation. When water absorbs enough energy from the Sun, the water molecules will change into a gas called water vapor. This floating water vapor is then stored in the air. Most of this evaporation happens from the surface of lakes, from water in the soil or as snow melts and the water flows over rocks or other surfaces.

Another way moisture can be lost to the atmosphere is one you might be less familiar with: sublimation. Sublimation is when a solid turns directly into a gas – think of dry ice. The same can happen to water when snow or ice turns directly into water vapor. When the air is colder than freezing, sublimation happens when molecules of ice and snow absorb so much energy that they skip the liquid form and jump straight to a gas.

A number of atmospheric conditions can lead to increased evaporation and sublimation and eventually, less water making it to creeks and streams. Dry air can absorb more moisture than moist air and pull more moisture from the ground into the atmosphere. High winds can also blow moisture into the air and away from the area where it initially fell. And finally, the warmer air is and more Sun that shines, the more energy is available for snow or water to change to vapor. When you get combinations of these factors – like warm, dry winds in the Rockies called Chinook winds – evaporation and sublimation can happen quite fast. On a dry, windy day, up to around two inches of snow can sublimate into the atmosphere. That translates to about one swimming pool of water for each football field-sized area of snow.

Sublimation is mysterious

It is relatively easy to measure how much water is flowing through a river or in a lake. And using satellites and snow surveys, hydrologists can get decent estimates of how much snow is on a mountain range. Measuring evaporation, and especially sublimation, is much harder to do.

Today researchers usually estimate sublimation indirectly using physics equations and wind and weather models. But there are lots of uncertainties and unknowns in these calculations. Additionally, researchers know that the most moisture loss from sublimation occurs in alpine terrain above the treeline – but snow scientists rarely measure snow depths there. This further adds to the uncertainty around sublimation because if you don’t know how much moisture a system started out with, it is hard to know how much was lost.

Finally, weather and snowpack depths vary a lot from year to year. All of this makes measuring the amount of snow that falls and then is lost to the atmosphere incredibly difficult. 

When scientists have been able to measure and estimate sublimation, they have measured moisture losses that range from a few percent to more than half of the total snowfall, depending on the climate and where you are. And even in one spot, sublimation can vary a lot year to year depending on snow and weather.

When moisture is lost into the atmosphere, it will fall to the surface as rain or snow eventually. But that could be on the other side of the Earth and is not helpful to drought-stricken areas.

It is hard to say how important loss of moisture to the atmosphere is to the total water cycle in any given mountain range. Automated snow monitoring systems – especially at high elevations above the treeline – can help researchers better understand what is happening to the snow and the conditions that cause losses to the atmosphere.

The amount of water in rivers – and when that water appears – influences agriculture, ecosystems and how people live. When there is a water shortage, problems occur. With climate change leading to more droughts and variable weather, filling a knowledge gap of the water cycle like the one around sublimation is important