The Colorado River, dubbed the hardest working river in the West, is one of the best examples of how human demands and climate change can bring about challenges for meeting the water needs of Western communities, farms and ranches, recreation, and the environment. The Colorado River has been engineered to store upwards of 60 million acre-feet of water, taming its powerful flows while providing significant flood control and a more consistent water supply to irrigators and communities in times of water scarcity. Taking a closer look at one of its most significant reservoirs, Lake Powell, provides an opportunity to examine how different stresses have impacted the Colorado River over time, and what those impacts mean for water supplies in the Western United States.
Reservoirs are designed to capture and store water during wet years, which can be used to supplement natural precipitation during dry years. Following dry years, a series of wet years can help refill the reservoirs. There are 10 major reservoirs on the Colorado River and its tributaries. Lake Powell, on the border of Arizona and Utah, and Lake Mead, on the border of Nevada and Arizona, are the two largest in the United States.
What Can The Amount Of Water Flowing Through Lake Powell Tell Us About Water In The West?
Lake Powell began to fill in 1963 with the construction of Glen Canyon Dam in northern Arizona. When full, it is designed to hold more than 26 million acre-feet (MAF) of water that originates from snowpack in the Upper Colorado River Basin states of Colorado, Utah, Wyoming, and New Mexico. Lake Powell acts as the primary storage account for water that originates in the Green, Gunnison, San Juan, and Colorado rivers and is eventually released to Lake Mead and the Lower Colorado River Basin states of Arizona, Nevada, and California, and eventually, Mexico. The storage provided in Lake Powell is absolutely critical for the ability of the entire Colorado River Basin system to supply water for upwards of 40 million people in these seven basin states and Mexico. The lake is also a major recreation area, known for magnificent sandstone canyons and rock formations.
Lake Powell operates under multiple agreements collectively called the “Law of the River”, which require a minimum amount of water be distributed from the Upper Basin states of Colorado, Utah, Wyoming, and New Mexico to Lake Mead and the Lower Basin states. Since the early 1970s, 8.23 MAF has been the “minimum objective release” from Lake Powell to Lake Mead, where it is distributed to meet requirements for water allocations under the Law of the River (more on the Law of the River below). Under the current operating guidelines, in above-average snowpack years, releases can be greater than 8.23 MAF, and in below-average snowpack years, releases can be less, although this is very rare and happened for the first time in 2014. The amount of water released from Lake Powell stays relatively consistent (typically between 8.23 and 9 MAF) so, in theory, the elevation of Lake Powell should respond in kind to the amount of water flowing into the reservoir from annual snowpack and rain events (i.e. inflow).
But since 2002, the water elevation in Lake Powell has not gone above its historical 50-year annual average of 3,639 feet above sea level. At an elevation of 3,639 feet, Lake Powell contains about 15.8 MAF of water. So why aren’t years that experience above-average annual snowpack enough to help Lake Powell bounce back? What has happened since the early 2000s that has made it so difficult for Lake Powell to recover elevation? And what role did the multi-year drought play?
Elevation of Lake Powell (1968-2018). The graph shows each year’s difference from average annual elevation in Lake Powell from 1968 to 2018. Lake Powell started filling in 1963, but took much longer than anticipated. The greater differences from average elevation seen from 1968 to 1972 are a result of the reservoir filling once construction was complete.
Adjust the years filter to see the average over different time periods.
Data source (elevation): https://www.usbr.gov/rsvrWater/HistoricalApp.html
Won’t Big Snowpack Years Save Us? Notable Trends In Snowpack And Elevation In Lake Powell.
We can’t help but get excited when we have a great snowpack year. Ski resorts stay open longer, whitewater sports get an economic boost, and farmers can hope for a long and steady runoff season. But big snowpack years have been fewer and farther between. When we do see an above average year for snowpack in the West, it can be hard to keep the bigger picture in mind. So what are the long-term trends in snowpack and the amount of water in Lake Powell?
The first question to ask is whether trends in snowpack, in fact, impact the amount of water that flows into Lake Powell on any given year. We compared the amount of water that enters Lake Powell annually (inflow) and the average annual snowpack of the Colorado River Basin based on the percent median of snow water equivalent (SWE, or the amount of water contained within the snowpack), on April 1 each year over the past 50 years. As we see in the graph below, the amount of water flowing into Lake Powell largely follows trends in snowpack.
Notably, the Upper Colorado River Basin has recently experienced several significantly dry years, but there have been two exceptionally dry years in the last two decades: one in 2002 and the other in 2012. However, 2002 and 2012 are not the lowest snowpack years on record: 1977 and 1981 saw two of the lowest years for snowpack in recorded history. We can see that the 20th century relationship between snowpack, streamflow, and Lake Powell storage began to change in the 21st century. So why were 2002 and 2012 different from 1977 and 1981, and what happened to that relationship? As we discuss below: a lack of recovery.
Despite exceptionally dry years in 1977 and 1981, most of the 1970s and early 1980s were above-average years for snowpack in the Colorado River Basin, and inflows bounced back. Since 1987, snowpack in the Colorado River Basin has rarely been above the 50-year average more than two consecutive years in a row.
The prevailing narrative regarding Lake Powell’s ability to recover, or failure to do so, is long-term persistent drought. It’s true that a majority of the Intermountain West has not been experiencing drought conditions only in recent years, but actually is shifting long-term to a more arid climate (known as “aridification”). Drought implies a temporary condition, but the Colorado River Basin has already experienced many years of drought (e.g., 2000-2014 was the worst 15-year drought in the Basin in recorded history). So is a decreasing amount of water flowing into Lake Powell the driving force behind Lake Powell’s inability to bounce back to its heydays of the 80s and 90s? Is Lake Powell simply receiving less total water from snowpack over time?
Snowpack & Inflow into Lake Powell (1968-2018). The graph shows the difference from average of annual snowpack in the Colorado River Basin on April 1 and the difference from average annual inflow in Lake Powell from 1968 to 2018 as a comparison.
Adjust the years filter to see the average over different time periods.
Data source (snowpack): https://www.nrcs.usda.gov/wps/portal/wcc/home/
Data source (elevation): https://www.usbr.gov/rsvrWater/HistoricalApp.html
Examining the graph below reveals several interesting trends for snowpack and elevation levels in Lake Powell. One important trend is revealed by looking at when the Lower Basin began to fully utilize its full apportionments under the Colorado River Compact (discussed in more detail below). Specifically, the Central Arizona Project (CAP), which allowed Arizona access to its entire 2.8 MAF apportionment, came fully online in 1994 during a relatively wet year for snowpack – however, as snowpack decreased in the following years (similar to 1987 to 1992), the reservoir did not recover in elevation as well after the turn of the century as it did in the 1980s. Another interesting trend is seen with the years following 1997 and 2011; 1997 was a relatively wet year with more than 140% of annual average snowpack, while 2011 was a similar year for snowpack at 136% above average. The wet year of 1997 helped Lake Powell’s elevation recover following dry years from 1987 to 1994; however, Lake Powell’s elevation did not rise from 2012 to 2018 after the wet year in 2011.
A final important trend to note: following the historically dry 2002 water year, Lake Powell’s elevation has not yet recovered above the historical 50-year average, despite above-average snowpack years like 2008, 2011, and 2014.
These two graphs reveal interesting trends as they relate to snowpack and inflows, and snowpack and elevation levels in Lake Powell. They suggest that since the late 90s something has changed in how Lake Powell’s elevation levels respond to annual fluctuations in snowpack.
Snowpack & Elevation of Lake Powell (1968-2018). The graph shows the difference from average of annual snowpack in the Colorado River Basin on April 1, and the difference from average annual elevation in Lake Powell from 1968 to 2018 as a comparison.
Adjust the years filter to see the average over different time periods.
- Data source (snowpack): https://www.nrcs.usda.gov/wps/portal/wcc/home/
- Data source (elevation): https://www.usbr.gov/rsvrWater/HistoricalApp.html
Before diving into the recovery issue, here is a quick primer on Colorado River Basin management and how the River is allocated among the Basin states:
In 1922, six of the seven Colorado River Basin states agreed to allocate water to the Upper and Lower Basins in an interstate agreement called the Colorado River Compact (Arizona held out for several decades but eventually agreed). However, the data used to calculate how much water could realistically be divided between the basins reflected an uncharacteristically wet time period. Water managers at the time decided that the Upper and Lower basins each would receive 7.5 million-acre-feet (MAF) annually. In the coming decades as the Law of the River was further developed, though, it became clear the Colorado River could not consistently deliver 15 MAF annually to the Upper and Lower basin states, and the problem is further compounded when including Mexico’s eventual 1.5 MAF apportionment, reservoir evaporation, and the ecological health of the Colorado River and its tributaries. Put succinctly, the Colorado River is over-allocated.
The Colorado River is managed and operated under numerous compacts, federal laws, court decisions and decrees, contracts, and regulatory guidelines collectively known as the “Law of the River.”
Among other things, the Boulder Canyon Project Act of 1928 apportioned the Lower Basin’s 7.5 MAF based on a fixed quantity of water. So, each year the Lower Basin states receive the following amounts:
- Arizona = 2.8 MAF
- California = 4.4 MAF
- Nevada = 0.3 MAF
In 1948, the Upper Colorado River Basin Compact created the Upper Colorado River Commission. It decided to apportion the Upper Basin’s 7.5 MAF, or the amount available in any given year, by percentages, so that:
- Colorado would receive 51.75%,
- New Mexico would receive 11.25%,
- Utah would receive 23%, and
- Wyoming would receive 14%.
In 1944, the United States and Mexico negotiated a treaty allocating Mexico’s Colorado River apportionment based on a fixed quantity. So, each year Mexico receives the following amount:
- Mexico = 1.5 MAF
Colorado River Basin Apportionments. It is important to note that these are the allocations on “paper”. Actual annual use in both the Upper and Lower basin varies with hydrologic conditions and state and local decision-making (e.g., voluntary cutbacks). While the Lower Basin has developed its full apportionments, the Upper Basin has only been using between ~4 and ~5 MAF (including evaporation and other losses) since the 1980’s. See the graphs below for a breakdown of each state’s annual use (MAF) for several recent years (2015-2017).
For more information on Colorado River Basin water use, please see the Bureau of Reclamation’s Consumptive Uses and Losses Reports and Lower Colorado River Water Accounting Reports.
Why aren’t Lake Powell elevations recovering like they were 30 years ago?
Three factors help explain why Lake Powell has lost the ability to fully recover following dry years:
One
Lower Basin States’ Consumptive Use
Despite the Colorado River being over-allocated, demands from the river were largely met throughout the 20th century, primarily because uses in Arizona, Nevada, and the Upper Basin states were well below their Colorado River Compact apportionments. With the completion of the Central Arizona Project in 1994, however, Arizona was able to fully divert and utilize its entire Colorado River apportionment of 2.8 MAF.
What is consumptive use?
Water use is divided into two broad categories: consumptive use and non-consumptive use. When water is consumptively used, that water is taken out of the river or reservoirs and is no longer available to other users in the system. When water is non-consumptively used, that water is returned to the system and available for downstream users.
Almost every year from 1953 to 2003, California exceeded its 4.4 MAF annual apportionment, drawing on the unused apportionment of Arizona and Nevada and the abundant storage in Lake Mead; in 24 of those years, California consumed more than 5 MAF. California has since limited uses to 4.4 MAF, but the Lower Basin’s overall use, in combination with evaporation and system losses and downstream delivery obligations to Mexico, has caused Lake Mead to operate at a loss for the past 20 years. Specifically, average annual inflows to Lake Mead are about 9 MAF (8.23 MAF from Lake Powell releases and the rest from downstream tributaries), while average outflows are 9.6 MAF (Lower Basin states and Mexico). Add in the additional average annual evaporative losses from Lake Mead of 0.6 MAF, and you have a net negative balance of 1.2 MAF.
In other words, in most years, 1.2 MAF more water is taken out of Lake Mead than flows in. In what has become known as the “structural deficit”, Lake Mead storage levels continue to struggle because of this 1.2 MAF annual imbalance. It’s important to note that since 2016, Lower Basin states’ demands have been less than their annual apportionments because of voluntary reductions by each state, but the structural deficit still exists to this day.
Two
Climate Change
Climate change is another major driving factor. Numerous recent scientific studies have looked into the issue, and they generally find the same trend: heat-trapping greenhouse gas emissions are causing warming temperatures throughout the West. Specifically, increased temperatures are causing earlier snowmelt, increased rates of evaporation from rivers and lakes, and more precipitation falling as rain rather than snow. For example, according to noted climate experts Brad Udall and Jonathon Overpeck, there is a direct link between lower flows in the Colorado River and continually increasing temperatures. We could see as much as a 20% reduction in total flow in the Colorado River by 2050 due to higher temperatures. Higher temperatures also mean decreasing snowpack as measured in snow water equivalent, and earlier timing in snowmelt (which means the water runs off earlier in the season, leaving less water available in rivers and streams for communities, farmers, and recreationists later in the hottest part of the summer when it is needed most. This is especially harmful for the rivers and streams themselves, as low flows and warmer water can negatively impact those ecosystems).
Three
Operating criteria
Finally, changes to the policies governing the operation of lakes Powell and Mead in 2007 made a significant impact in Lake Powell’s ability to recover. These changes, known as the Interim Guidelines, specify how lakes Powell and Mead will be operated, dependent on specific elevation levels in each reservoir. Importantly, the Interim Guidelines prescribe that the two reservoirs be operated in a coordinated way. For example, if Lake Powell starts to get too low, less water will be released downstream to Lake Mead, thus helping prop up water levels in Powell.
While these guidelines were designed to better manage the system, and they represent an increase in collaborative governance for the Colorado River, this coordinated management – known as “equalization” – makes it extremely difficult for Lake Powell to ever fully recover.
Take 2011, for example, which was an above-average snowpack year, leading to increased streamflow in the Colorado River. Lake Powell inflows were given a big boost. But because Lake Mead had been falling in recent years (see the “structural deficit” above), those additional gains in Lake Powell were released downstream the following year (again, equalization).
This is not to say the Interim Guidelines are inherently problematic, but rather to point out that in an era of lower-than-average flows, any modest gains in Lake Powell do not necessarily stay in Lake Powell. It is important to note that the Interim Guidelines do not mean Lake Powell can never recover – certainly several consecutive years of exceptionally high snowpack (e.g., the mid 1980s) could see increased inflows and Lake Powell once again reaching capacity – but absent those near-record breaking snowpack years, the odds are of recovery are low.
Recovery is tough.
What is the Solution?
The good news is we can act now to address water supply challenges in the West. First, we need to tackle climate change by decarbonizing the West (avoiding carbon pollution from power plants, vehicles, and other sources). Second, we need to accelerate smart municipal reuse and conservation, integrating water and land use planning, and agricultural efficiency. Collaboration between stakeholders at the state level and across the West will be key to effectively implementing key water agreements like the Drought Contingency Plan, which was approved by the Upper and Lower basin states and Congress and signed into law in April.
The DCP enables all seven states to do their part to conserve water and protect the entire Colorado River system. In the short term, the plan will help prevent Lake Mead from falling to critically low levels and ensure water continues to reach users in the Lower Basin.The Upper Basin DCP also establishes a “demand management” program, one that would pay willing water users such as farmers, ranchers, industries, cities, and towns to temporarily reduce their water consumption, thereby keeping more water in rivers and reservoirs. Water conserved through this program would be delivered to the Colorado River water “bank account” in Lake Powell. If properly designed, a demand management program would ensure we have enough water during dry times while supporting the health of our rivers along the way.
There are a lot of factors and powerful players in this, and it can seem overwhelming, to be sure. But as a taxpayer and an interested citizen who cares about the future of the West, your voice matters as water decisions get made. From conserving water in your own home, to participating in local government water policy decisions (e.g., city councils, advisory boards), to making sure your local, state, and federal elected leaders know you want them to prevent carbon pollution and protect and conserve the Colorado River – you can make a difference. We (and the Colorado River) need you to stay involved!
