Abysmal math on the Colorado River

Feds look to avoid de facto deadpool at Glen Canyon Dam

🐟 Colorado River Chronicles 💧

The Central Arizona Project canal, which carries Colorado River water to Phoenix and Tucson, as it runs past fields in the desert (that are irrigated with groundwater, not CAP water). The CAP is not likely to see new cuts this year beyond the levels already imposed. Source: Google Earth.

With each passing April day without major snowfall, we gain more clarity on the Colorado River situation and what things might look like this summer, which is, in a word, grim. Or, as Arizona’s top water officials put it: “The winter and spring snowpack and runoff projections in the upper basin are abysmal.”

The Colorado River Basin Forecast Center is putting a number to that term by predicting that the Colorado River system will deliver about 1.4 million acre-feet¹ of water to Lake Powell from April 1 through July 31. That’s about 23% of the median for the spring runoff season, which is when flows are most abundant, and just over half of last year’s not so great figure of 2.6 MAF.

This year’s Upper Colorado Basin spring runoff is forecast to be about 1.4 million acre-feet. That isn’t as low as 2002, which was just below 1 million acre-feet, but if conditions don’t improve it could fall even lower than that. Source: Colorado River Basin Forecast Center.

Believe it or not, that figure — the official 50% forecast, made by an actual person — may be optimistic. Over the last two weeks, the Ensemble Streamflow Prediction model (which is a constantly updating automated forecast) has come up with an even more dire outlook, downgrading the forecast to 1.16 MAF during that same time period.

Abysmal, indeed.

We’re also getting a little more information as to how the feds plan to address the crisis, at least in the near-term. Most significantly, they tentatively plan to “defend” minimum power pool at Glen Canyon Dam, which is to say they will do what it takes to keep the surface level of Lake Powell at or above 3,500 feet in elevation to avoid relying on the lower river outlets, which are not engineered for sustained use. The weapons they will use for this defense include:

• Reducing Lake Powell releases from the planned 7.48 million acre-feet to 6 million acre-feet.

• Releasing up to 1 MAF from the “Upper Initial Units,” which includes Flaming Gorge, Blue Mesa, and Navajo Reservoirs. Hydrology may make this impossible, however, meaning that these releases could be as low as 650 MAF .65 MAF (or 650,000 acre-feet).

• For now, Interior is not asking for larger cuts from the Lower Basin (beyond the 1.5 MAF cuts they’ve already taken), which presumably means the feds will not reduce Lake Mead releases through Hoover Dam.

But will it be enough to avoid dipping below what I call de facto deadpool at Lake Powell? We won’t really know until later this summer, but a fairly simple calculation can help predict that future. Keep in mind that I’m no hydrologist, I’m just working with the numbers that are available to see whether potential inputs (Lake Powell inflows) are at least equal to planned outputs (Glen Canyon Dam releases).

I put together this little diagram to help visualize things. I know the text is tough to read in the email version, and especially if you’re reading this on your phone. So I’d suggest clicking on the image (or the headline of this post) and viewing it in the web version.

Simplified diagram of Glen Canyon Dam with inputs (on the right) and outputs (on the left). *Fish pool is the surface level scientists have deemed necessary for minimizing the potential of non-native bass escaping through the dam and propagating downstream, where they can compete with endangered native fish. Infographic by Land Desk using data from Bureau of Reclamation and the Colorado River Basin Forecast Center.

Here are the figures for the equation.

Inflows:

• 1.5 MAF: Lake Powell Storage available above 3,500 feet.

• 1.1 MAF to 1.4 MAF: Forecast Lake Powell inflows April-July

• .65 MAF to 1 MAF: Planned releases from upper basin reservoirs.

TOTAL INFLOWS: 3.25 to 3.9 MAF

Outflows:

• 2.9 MAF: April 1 - Oct. 1 releases to reach 6 MAF for the water year (3.13 MAF has already been released)

• .3 MAF: Rough estimate of evaporation from Lake Powell for the remainder of the water year.

TOTAL OUTFLOWS: 3.2 MAF

That gives us a whopping .05 to .7 million acre-feet to spare. That is cutting it close, folks; a hot, dry summer could drive evaporation levels up, and/or bring inflows down, shaving off the sliver of breathing room this affords. But unless the outlook dims considerably, the BoR should be able to avoid a run-of-the-river situation this year, which is good news. And, since Arizona likely will not be required to take more cuts this year, the state will probably hold off on doing a compact call and dragging the Upper Basin to court.

These measures, however, will have a variety of consequences, including:

• The Upper Basin reservoirs (Flaming Gorge, Navajo, Blue Mesa) are also likely to see record low inflows this year. That, combined with up to 1 million acre-feet of additional releases to benefit Lake Powell, will draw them down considerably, affecting hydropower production, irrigation, and, especially, recreation.

• Non-native smallmouth bass are abundant in Lake Powell, but since they are warmer-water fish, they tend to stay near the surface of the reservoir, meaning under normal conditions they stay well above the penstocks, or the outlets in the dam that lead to the hydropower turbines. However, as the surface drops closer to the penstock openings, so do the fish, allowing them to get flushed through the dam into the Colorado River. And because the water released from the dam is warmer (since it’s nearer to the surface), that warms the river downstream, allowing the bass to thrive and compete with the endangered native fish downstream. This is likely to be exacerbated as the surface level nears 3,500 feet.

• This year’s 6 MAF release from Glen Canyon Dam will bring the ten-year aggregate flows at Lees Ferry down to about 79 million acre-feet. This potentially puts the Upper Basin in violation of Article III of the Colorado River Compact, which mandates that the Upper Basin “not cause the flow of the river at Lee Ferry to be depleted below an aggregate of 75 million acre-feet” for any 10-year period. A 1944 treaty added another 7.5 million acre-feet to this figure to cover half of Mexico’s allotment, making for a total of 82.5 MAF over ten years. Note: The interpretation of this provision is in dispute.

• The diminished reservoir levels, combined with the reduced releases, will lead to lower hydropower output from the dam. That will force tribes, communities, and utilities that buy the relatively cheap power to purchase it on the open market. And it will also cut into power-sale revenues, which help fund endangered fish recovery programs.

• Reduced dam releases will mean lower flows, on average, through the Grand Canyon, affecting riparian ecosystems and boating.

• Reduced dam releases equate to lower flows into Lake Mead. Since the BoR apparently does not plan to cut releases from Hoover Dam, that reservoir will likely see its levels drop considerably, diminishing hydropower output and affecting recreation. My rough calculation suggests Lake Mead’s surface level will drop from the current 1,060 feet to about 1,030 feet, which would be lower even than in 2022. The BoR has suggested it will “defend” a level of 1,000 feet. That would almost certainly lead to Lower Basin shortages.

It’s still a long ways out, but for now the NOAA is calling for above average precipitation in the Southwest later this summer.

A super El Niño appears to be forming, but the effects in the Upper Colorado River Basin are especially hard to predict because it sits right in between the “warmer, drier” and the “wetter, colder” zones, meaning it could go either way. Source: NOAAA.

There is potentially good news on the horizon. Conditions are ripening up for a “super” El Niño to begin forming this summer. It’s difficult to predict how that will affect the Upper Colorado River Basin, but for now, forecasts are calling for a strong monsoon in the Southwest, beginning in July. That probably would not do much to bring up Lake Powell’s levels, but it would provide relief to the many farmers who are almost certain to lose irrigation relatively early this summer and may help keep late-summer megafires at bay. And, you never know, El Niño might just bring a monster winter just when we need it most.

1. *The forecasts are for the “unregulated flow,” which means that it is an estimate of what the flow would be without upstream dams holding water back. This is not the same as “natural flow” which is a calculation of what the flow would be without upstream human consumptive use, dams, or diversions. In this case, actual inflow and unregulated inflow are almost the same.