October 2021 Colorado River 24-Month Studies Shift to a More Realistic, but Troubling Future for Lakes Mead and Powell

A shrinking Lake Powell

By Eric Kuhn

The latest Bureau of Reclamation monthly Colorado River modeling runs show an even bigger drop over the next year in Lake Powell’s elevation that previously projected. But this is not an example of bad news getting worse. Instead, a change toward a drier baseline hydrology more accurately reflects the drying of the Colorado River basin in the 21st century.

Usually, during the non-snow accumulation and non-run-off forecast seasons (August – December), the Bureau of Reclamation’s 24-month studies change very little from month to month. Today, however, the basin’s nerds (I’m on that list) that closely follow these studies were in for a sobering surprise. All three of the October studies (most probable, minimum probable, and maximum probable) showed a consistent shift to a drier future. The reason for this shift is disclosed in footnote #1 – “The October 2021 24-Month Study includes the Colorado Basin River Forecast Center shift to the 1991-2020 period of record.”  In simple terms, out are the wet years from the 1980s and in are the more recent dry years from the 2010s.

This shift to the more recent years is important for several reasons. First, the 1991 -2020 period is likely to be much more representative of the future under climate change driven aridification of the basin.  Second, given the depleted state of the system reservoirs, seemingly small changes in storage can have big operational impacts. For example, the September most probable study showed Lake Powell ending Water Year 2022 at elevation 3545’, the October most probable study now shows elevation 3534’. The change is only 11’, but it’s enough to push Lake Powell into the Lower Elevation Balancing Tier for 2023. This means that the projected most probable release from Glen Canyon Dam for Water Year 2023 has gone up from 7.48 maf to 7.818 maf. When Lake Powell is in this bottom tier, the goal is to release between 7 and 9.5 maf/year so as to equalize end of water year storage between Powell and Mead.

For another example, look at the minimum probable studies. The September version shows Lake Powell bottoming out at 3482’ while in the October version it’s 3467’.  Again, only a 15’ difference, but since the minimum power elevation is 3490’ it has a big impact on how long the power plant will be unable to produce power. Which in turn has an enormous impact on the Upper Basin Fund cash flow.  The September minimum probable showed Glen Canyon Dam not producing power for about 21/2 months. See the graph below, under the October version a Glen Canyon Dam power plant outage could be as long as ten months (if not all year).  From a cash flow perspective, the difference is about $200 million.

The shift in the period of record traditionally only happens once per decade. It is a good thing it happened this year. Given what we’ve seen over the last 20 years, the shift will add needed conservatism and urgency to the Basin’s efforts to bring our water use in balance with the available supply.

What might planning for an 11 million acre foot or 10 million acre foot Colorado River look like?

One of the central questions dimly visible in the early discussions around the upcoming renegotiation of the Colorado River’s water operations and allocations rules is the question of how bad a “worst case” scenario should be considered.

This is crucial, because it constrains what sort of questions must then be confronted. The lower the future flows considered, the more likely it is that the negotiators will have to stare down the third rail question of how much water the Upper Basin can delivery hydrologically, and must deliver legally, at Lee Ferry, the dividing point between the Upper and Lower Basins.

For the century since the Colorado River Compact was signed, we’ve avoided dealing with that central question – what happens if the river’s flows are so low that the Upper Basin cannot deliver the 7.5 million acre feet per year (or 8.25 million acre feet, we can’t even agree about which number to argue about) contemplated by the compact’s Article III.

This question is so untouchable that in work done for the 2012 Basin Study, the Bureau of Reclamation’s modelers famously added what came to be called “miracle water” at Lee Ferry every time one of their model runs dropped below the threshold that might have otherwise triggered this legal argument.

Under the low flows possible under climate change, we face a stark choice – either we reduce the Upper Basin’s Lee Ferry deliveries below 7.5/8.25 maf, or we will have to curtail existing Upper Basin uses. Advocates of modeling such low flows in the planning scenarios are essentially saying – Let’s have that conversation now.

At the tale end of yesterday’s (Friday 10/15/2021) House Natural Resources Sucommittee on Water, Oceans, and Wildlife, it was California Rep. Jim Costa, a congressman from outside the Colorado River Basin, who asked the question pointed at the heart of the matter – how do we redo water allocations that make no sense in a river much smaller than contemplated in our hallowed Law of the River?

He was addressing a panel of representatives from each of the Colorado River Basin states (his comments start around 2:30 here):

The Law of the River and the quantification of the Upper and Lower Basin states amounted to some 17 million acre feet of water that was determined at that time was the annual flow of the Colorado River, and we know that in the last two decades its been more like 12.4 million acre feet, and that doesn’t account for other Native American tribes that have reserved water right claims that have yet to be resolved. So there’s just a tremendous amount of demand. And with climate change, we know the yield is only going to decline.

This is the question I’d like to submit to all of you, and if you want to provide written statement to your answer I think we would appreciate that.

Let’s say the annual yield over the next 30 years is 10 million acre feet. I don’t know, with climate change, maybe it’s plus or minus. How do we take into account how we got to the original allocation, with the Upper and Lower Basin States and the Native tribes, the sovereign nations, and then reallocate that on a lot less water.

At this point all we can see through the public windows into discussions about next-step Colorado River management guidelines is shadow boxing on this question.

But testimony yesterday from Southern Nevada’s John Entsminger suggests the public shadowboxing we’re seeing on this question is representative of disagreements in the private discussions. (I quote here from John’s written testimony.)

Despite the fervent warnings from internationally renowned scientists like Jonathan Overpeck and Brad Udall that urge us to plan for a future with even less than 12.3 million acre-feet, the river community is far from consensus about how dry of a future to plan for. And, while this panel was asked to talk about drought, on-the-ground evidence suggests the Colorado River basin is not experiencing drought but aridification – a permanent transition to a drier future. If we are to build upon the river’s many successes over the last 25 years, we must confront the magnitude of the challenge in front of us and quickly reach agreement on what future scenario we’re willing to plan for. (emphasis added)

Speaking two weeks ago at this year’s Getches-Wilkinson Center conference in Boulder, Entsminger put a number to Southern Nevada’s thinking. In the next iteration of its long range water resources plan, Entsminger’s Southern Nevada Water Authority will include a “what if” planning scenario for how the agency would deal with an 11 million acre foot per year Colorado River. This is not to say that Southern Nevada expects an 11 million acre foot river, but rather than it believes it needs to have a plan in place should that happen.

I could be wrong, but so far I’ve seen no public evidence that any of the states of the Upper Basin are willing to entertain flows that low in the planning scenarios to be considered in the modeling done to support the upcoming negotiations. I look forward to seeing the written answers the basin states’ representatives submit to Costa’s question.


The challenge of meeting a legal and moral obligation to Colorado River Basin tribes

At last week’s Getches-Wilkinson Center conference in Boulder, attorney Jay Weiner, who represents tribes (but was careful to say he was not speaking on any particular tribe’s behalf) made an important point, which is repeated in this excellent piece by Mark Armao this week in Grist:

“The basin is free-riding off of undeveloped tribal water rights,” said Jay Weiner, an attorney for the Quechan Indian Tribe. Weiner said there is a “fundamental tension” between tribes’ desire to fully develop their water rights and the overarching need for everyone in the basin to consume less water overall.

Kathryn Sorensen on getting real in the Colorado River Basin

High Horses

At last week’s Getches-Wilkinson Center conference on Colorado River stuff, I had the privilege of moderating a panel with the provocative title “Time to Get Real”.

The opening remarks from Kathryn Sorensen of Arizona State University seemed worth repeating, and she kindly gave me permission to post on the blog (the pictures were her slide deck, accompanying the remarks):

By Kathryn Sorensen

John Fleck asked each of us to consider the following:

What does getting real in the Colorado River Basin mean?

I’ve got three things:

First, with humility and respect, it means that we all need to get off our high horses.  Everyone.  Me, you, everyone in the basin.  Thinking that your water use is justified, and no one else’s is, is not helpful.  Thinking that you know how water should be allocated and everyone else has it wrong, is not helpful.  And it’s going to push us into camps at a time when we need to focus on collaboration.

Second, it means that it is not helpful to continue to talk about closing the gate. There is a long history of people moving out here to the West and then wanting to turn around and close the gate.  Unless you are a Native American and your family has been here since time immemorial, you do not have the moral high ground to close the gate.  There’s something like 8 billion people on the planet.  Our cities are going to continue to grow.

Our energy is better focused on making sure all families that live in this basin, whether they have lived here since time immemorial or moved here yesterday, have equitable access to safe clean drinking water and effective sewer service.

The Arizona Navy

Third, it means that the basin is aridifying, the Lower Basin is using too much water, and the system is draining.  We need to cut water use in the Lower Basin very significantly.  Like Senator Sinema, I’m not going to give you a number.  Everyone knows we Arizonans are a difficult bunch.  If you are not convinced of this, I am happy to convince you further.  But on a long-term, sustained basis, the Lower Basin needs to cut by more than 500,000 AF, and I don’t see how, politically, we can cut 3 million. That being said, if we get another year or two of terrible runoff, we may not have a choice.

But have some faith.  Save a little optimism.  solving for water scarcity is really hard, and the problems we face are enormous.  I don’t want to minimize those. But still, solving for water scarcity is technologically easier and less expensive than solving for flooding and seawater intrusion.  So we got that going for us.

Thank you.


Taking climate change seriously: the Colorado River “stress test”

Courtesy Dave Kanzer and Eric Kuhn, from Eric’s 2013 Colorado River Water Users Association presentation


The Bureau of Reclamation Colorado River team did something remarkable in yesterday’s release of its new 5-year reservoir levels analysis – the “stress test”, a methodology pioneered a decade ago by an Upper Colorado River Basin technical team that included John Carron of Hydros and Eric Kuhn and Dave Kanzer of the Colorado River District is now the “new normal”, to borrow a terrible phrase. From the “5-year projections approach tab” here:

The method used to generate future inflows in the current projections includes resampling a subset of the historical natural flow record (1988-2019) using the Index Sequential Method (ISM), referred to here as “Stress Test” hydrology. In the past, the full historical record (1906-2019), known as the “Full” hydrology, was used to provide 5-year probabilistic projections. The Stress Tests hydrology scenario applies ISM to a shortened period of the natural flow record, 1988-2019, which removes the earlier portion of the natural flow record and focuses on the recent (approximately 30 years) hydrology. This period has a 10% drier average flow than the Full hydrology. Use of the Stress Test scenario is supported by multiple research studies that identified a shifting temperature trend in the Colorado River Basin in the late 1980s that affected runoff efficiency and resulted in lower average flows for the same amount of precipitation (McCabe et al. 2017Udall and Overpeck 2017Woodhouse et al. 2016).

The idea is that the traditional approach – using the entire period of record to model the probabilities of future river flows – is not longer valid because climate change is changing the river.

John, Eric, and Dave reasoned nearly a decade ago that using a shorter record, focused on our climate-changed Colorado, might better help managers think about and plan for what to expect next. (Dave also famously provided the memorable Homer Simpson image for Eric’s CRWUA presentation).

The “stress test” has been creeping into basin management discourse for a while, and Reclamation had already begun publishing stress test scenarios alongside. But the new 5-year flow and reservoir level estimates now are all in on the stress test.

The stress test may not be stressful enough, which was one of the implicit messages in the editorial Brad Udall and I published in Science magazine in May, and which Brad and I made more explicit here. But this use of the stress test is nevertheless hugely important, kudos to the Reclamation technical and management team for this important step.

Dry in all my river basins

odds favor a dry autumn 2021 in the watersheds that matter to me

Getting ready for class class discussion this afternoon about “drought” (“I get to see my students in person!” he exclaimed nervously.), I had occasion to check the latest Climate Prediction Center long lead forecast. It’s a few weeks old, but I don’t expect it’ll have changed much.

The brownest blob captures both river basins I care most about – the Rio Grande and Colorado, with odds tilted toward drier than average conditions through November. (And it doesn’t get any better if you look at the longer leads.)

Today’s class teaching goal: there’s no one thing called “drought”, it has many different definitions depending on who and where you are. For me, the most important measure is soil moisture (now) and snowpack (over the coming winter). Those are the things that determine available water supply for the communities of interest to me. There’s some overlap between my “drought” and a forest’s, or a fish’s, but they’re not necessarily the same thing. (If any of my students are listening in here, that’s a clue to the “what did Prof. Fleck leave out of the recorded lectures” discussion question.)

After some time last week in the mountains of northern New Mexico and southern Colorado, where Albuquerque gets its water, I had occasion to pull the latest soil moisture modeling from the Colorado Basin River Forecast Center. CBRFC only does the Colorado River Basin side of the continental divide, but there’s no reason to think it’s any different on our side of that line.

I cringed:

Dry soil moisture heading into fall 2021 in the Colorado River Basin





On the importance of gathering stones

Round rocks of the San Juan-Chama project

I had the joy of sharing a goofy group text thread yesterday evening with a couple of friends exchanging pictures of the round rocks we each collected yesterday morning on a field trip to see the plumbing of the San Juan-Chama Project, which diverts Colorado River Basin water beneath the continental divide to bring drinking and irrigation water to New Mexico’s parched middle Rio Grande Valley.

The San Juan-Chama Project’s 25 miles of tunnels are the thread that connects us in central New Mexico’s Rio Grande Basin to water and its management across the Colorado River Basin. That makes the tunnels the thread that holds together the work I’ve been doing for the last decade trying to understand the relationship between the water in my little town of Albuquerque, New Mexico, and the water challenges of the greater West. But I’d weirdly never actually visited this vital bit of plumbing.

So when a friend invited me to tag along Thursday and Friday on a workshop and facility tour for San Juan-Chama Project water users to talk about infrastructure management and governance, I said “yes”.

We hung out Thursday afternoon at a lovely picnic shelter along the Rio Chama talking about concrete maintenance priorities and the crazy-sounding Law of the Colorado River governance rules. (The concrete discussion was fun, but I didn’t understand a lot of it. There were a lot of totally legitimate “What, what? Why did they do it that way?” questions about the crazy governance structures.)

Friday we toured the dams and diversions.

the mouth of the Azotea tunnel

When I told WT, one of my former students, that I was headed up to Chama for a facility tour, he told me I had to pick up some round rocks at the Azotea Tunnel. Azotea is where the water emerges from beneath the continental divide after miles of concrete tunnel. Rocks that make it through the intakes have been skittering and rolling for a good long while, like time in a multi-million dollar rock tumbler, before they emerge to be deposited as the water slows and begins its trip down Willow Creek and into the Rio Chama on its way to my Albuquerque faucet.

The group had swollen to about 30 by the time we got to Azotea, with about a dozen mostly white government rigs parked on the apron above the tunnel. And within moments of our arrival, the senior managers from central New Mexico’s largest municipal water agencies had scampered like kids down the rocky channel embankment to begin hunting for the best round rocks.

The headwaters snow is long gone, and the tunnel’s been dry save for a couple of late summer rainstorms since the second week in August, which sucks for water management purposes, but was great for round rock hunting!

The rocks were cool, but the real value was the earnest goofiness of the endeavor.

I write a lot about the importance of “social capital”, the personal bonds among the water management community’s problem solvers. That’s what this was about.

We sat out by the Rio Chama Thursday evening until well after dark, long after the grill had cooled, talking water until it was so dark you could see the Milky Way and the evening star (Venus?) next to a setting sliver of a moon. We artfully arranged the car-sharing arrangements so we had time to talk during the shuttling from one tunnel and diversion dam tour stop to the next. We clustered in the shade eating our salads and PB&J talking about the Green Book and water bypass rules and Sec. 11 of Public Law 87-483, the San Juan-Chama Project’s 1962 authorizing legislation.

We collected round rocks. And then gleefully texted pictures of them with one another after we got home.





This morning’s operations missive from the federal-state-local Middle Rio Grande operations group (by “Middle” here we mean central New Mexico) notes a release of ~100 cubic feet per second of imported San Juan-Chama Project water for environmental flows, an effort to help the struggling Rio Grande silvery minnow.

SJC water is removed from the Colorado River Basin in the San Juan mountains of southern Colorado, through a series of tunnels beneath the continental divide, for use in the Rio Grande Basin.

So – water from one troubled basin (the Colorado) to provide environmental benefits in another (the Rio Grande).

The driest on the Rio Grande since….

One measure of dryness on the Rio Grande

I’ve been thrashing about this year thinking about ways of visualizing and measuring how dry this year has been on my home river, the Rio Grande through Albuquerque. Above is my latest stab at it.

A flow of 100 cubic feet per second at the USGS gage at Albuquerque’s Route 66 bridge in the middle of town has always been a threshold I watch. That’s in part because for a time it was used as a criteria for Endangered Species Act compliance (no more), and in part because when it drops that low it just looks dry – a shallow braided channel meandering through muddy sand.

This particular bit of code counts the days each year below a specified threshold, and the graph above was created by setting that to 100 cfs.

The flows here are a combination of climate-driven inflows and human management. All those dry days back in the 1960s, ’70s, and ’80s were driven in significant part by river management – much larger diversions from the river for irrigation. The relative lack of dry days across much of the 21st century was also driven by river management – an ESA requirement to keep the river wet here.

In both cases that combines with hydrology.

Unfortunately we only have data for the Route 66 bridge gage since 1965. I’ve got some more work I’m doing (back later for that, gotta get to the day job) looking at other gages with longer records, but the interplay of gage history and diversion points makes that more complicated than I have time for this morning, still ingesting first coffee.

Yesterday was the 14th day this year with a daily average flow less than 100 cfs. Last year we had 14 such days. The last time we had more was 1989.

Flow this morning, as I write this, is < 80 cfs.

Brad Udall on Upper Colorado River Basin climate change risk

By Brad Udall, posted with permission, from his Aug. 18, 2021 presentation to the Colorado Water Conservation Board

Colorado State University’s Brad Udall has been doing some really interesting thinking about how to conceptualize and communicate climate change risk to water supplies in the Colorado River Basin.

Shown above (and shared with permission) is one of Brad’s “selected averages” graphs. The horizontal lines show the average river flow value for a period of interest – the entire period of record, for example, or the 21st century. (It’s from a presentation Brad gave to the Colorado Water Conservation Board earlier this month.)

While the general pattern will likely be familiar to people working on Colorado River issues – wetter in the long run, drier in recent decades – the value being graphed may not be. It’s “unregulated inflows”, which is the number calculated by the Bureau of Reclamation and the Colorado Basin River Forecast Center to estimate the actual inflows into Lake Powell, minus the impact of upstream reservoir operations.

It’s the best number to use if you want to look at the the actual water flowing into Lake Powell, and therefore available to meet compact-driven Upper Basin commitments to deliver water downstream to the Lower Basin at Lee Ferry. It combines both natural flows, and Upper Basin consumptive uses.

The key number to look at here is the one in the blue box. Since 2000, the average unregulated inflows have been 8.38 million acre feet per year. The Upper Basin’s Lee Ferry compact delivery “obligation” (lawyers please don’t subpoena me on this point, I ) is 8.23 maf, which means that we’re right on the edge of trouble even now, in terms of the Upper Basin’s ability to meet the expected deliveries to users in the Lower Basin. Increased Upper Basin use, or decreased flow (see Brad’s 6.66maf for 2018-2021) would cross that trouble threshold.

What this means is that, based on 21st century hydrology, the Upper Basin is already caught in what Doug Kenney and colleagues dubbed “the Upper Basin climate change squeeze”.

This is what Brad and I were getting at in our May editorial in Science. Lower flow scenarios are entirely credible based on the best available climate science and the hydrology we are seeing. We need analyses – modeling runs – that consider them as we prepare for the next round of river management negotiations, so that we know what will do if Brad’s scary “6.66” is a harbinger of our future.