JFRCalifornia
Keeper of San Juan Secrets
Well, we’ve looked at the best case for the coming El Niño in a separate thread. So what about the flip side? What’s the worst case? I mean, how bad could it really be in an El Niño year?
USBR published their version of “worst case” in August, and it does indeed paint a grim picture. In that reading, you’ve got Lake Powell bottoming out at 3502 in April 2027, then barely rising to nearly 3510 by the end of June. After that, the lake goes into a slow freefall. That forecast also assumes a runoff year that looks like a near carbon copy of 2026 unregulated inflow of about 3.6 maf (one of the worst years on record), and a release through Glen Canyon Dam of 6 maf. It also assumes a net release from Flaming Gorge of about 0.5 maf and about 0.25 maf from Blue Mesa to help Lake Powell. They see Navajo Reservoir in a holding pattern. That means overall, they see 0.75 maf added to Powell from the already-depleted upper reservoirs. In this scenario, Glen Canyon Dam stops generating power in October 2027.
Can it really get that bad?
Once again, let’s look at the historical data about El Niño conditions. By nearly all accounts, we are about to experience one of the strongest El Niños on record. So I want to see what the worst natural flow of the river was during any strong El Niño event. And for that, let’s look at the seven strongest such events of the past 70 years. And when we do, the worst natural flow of any of those happened in 1992—about 11.1 maf. That’s pretty bad, below the 12.1 maf average since 2000, but nowhere near the worst we’ve seen. It’s a lot like 2024. Not good. But nothing like 2025, which was closer to 7 maf, or this past year, which was similar. The next worst “strong El Niño” year was 1988, which produced a natural flow of 11.7 maf.
So to me, unless the dynamics of El Niño have fundamentally changed (and I haven’t seen the studies), it’s fair to guess we should see at least 11 maf of natural flow this coming year before any diversions.
Once again, let’s do the math. Let's assume the natural flow for 2027 ends up at 11 maf. What would that mean for Lake Powell? Start by peeling off the 4.5 maf the upper basin will use this year, unencumbered by any cutback requirements from the feds (although with less snow in the Rockies, those states may have no choice but to cut back… let’s say their total might be 4 maf, not 4.5). Then let's assume about 1.0 maf in evaporation and seepage from Lake Powell and the river and reservoirs above that point (less than in a big year, since there is less water in the system). And since we know USBR is already committing to releasing a net 0.75 maf from the upper reservoirs in a bad runoff year, let’s include that. We also know that USBR would release 6 maf from Lake Powell in a bad year, but under the fed’s recent directive, they might be able to drop that to 5 maf. Add it up, and you get a net increase of up to 1.75 maf to Lake Powell. But of course that comes at a big cost—the smaller upper basin reservoirs take a big hit, there’s much less water delivered to Lake Mead, and less available for the lower basin users. And then there’s absolutely no reserve left over for WY 2028.
But in this scenario, here’s what Lake Powell looks like in summer 2027. If you add 1.75 maf to the October 1 (beginning of WY 2027) live storage of 5.1 maf, you end up with about 6.9 maf in live storage in 2027… or a surface elevation of 3546. That’s not bad, about 35 feet higher than what USBR presents as a worst case. How could my result be so different? Am I way off base here? Possibly. I made a lot of assumptions, most importantly about what the worst natural flow in a strong El Niño might be. But we’ve got the past data, and I’m picking the absolute worst of those years. If this turns about to be a reasonable worst case (or close to it), that would mean USBR could ease back on releases from the upper reservoirs, and go on with its planned release of 6 maf through Glen Canyon Dam. If they did that, that effectively would wipe out the entire projected net increase of 1.75 maf to zero. And that would mean the lake tops out next summer close to where it is right now—about 3518. But it would hold the upper reservoirs steady.
So there it is. Depending on how USBR manages the reservoirs, and to some extent on whether the upper basin states are forced to cut back their use (by about 10%) because of poor snowfall, we could end up with a range anywhere from 3518-3546 in a worst case El Niño snowfall/runoff event. That’s a huge range because USBR still has some control on the levers between now and then, and we really don't know what the upper basin states would use. But even the low end of that range is considerably higher than the USBR’s “minimum probable scenario” published in August--which was a peak of 3510.
The USBR undoubtedly knows more about all this than I do, but that’s how I read the “worst case” tea leaves. Also remember that the “worst case” is not the likely case. But it’s certainly possible.
As I said before, we'll know better where this is all headed once we get closer to spring...
USBR published their version of “worst case” in August, and it does indeed paint a grim picture. In that reading, you’ve got Lake Powell bottoming out at 3502 in April 2027, then barely rising to nearly 3510 by the end of June. After that, the lake goes into a slow freefall. That forecast also assumes a runoff year that looks like a near carbon copy of 2026 unregulated inflow of about 3.6 maf (one of the worst years on record), and a release through Glen Canyon Dam of 6 maf. It also assumes a net release from Flaming Gorge of about 0.5 maf and about 0.25 maf from Blue Mesa to help Lake Powell. They see Navajo Reservoir in a holding pattern. That means overall, they see 0.75 maf added to Powell from the already-depleted upper reservoirs. In this scenario, Glen Canyon Dam stops generating power in October 2027.
Can it really get that bad?
Once again, let’s look at the historical data about El Niño conditions. By nearly all accounts, we are about to experience one of the strongest El Niños on record. So I want to see what the worst natural flow of the river was during any strong El Niño event. And for that, let’s look at the seven strongest such events of the past 70 years. And when we do, the worst natural flow of any of those happened in 1992—about 11.1 maf. That’s pretty bad, below the 12.1 maf average since 2000, but nowhere near the worst we’ve seen. It’s a lot like 2024. Not good. But nothing like 2025, which was closer to 7 maf, or this past year, which was similar. The next worst “strong El Niño” year was 1988, which produced a natural flow of 11.7 maf.
So to me, unless the dynamics of El Niño have fundamentally changed (and I haven’t seen the studies), it’s fair to guess we should see at least 11 maf of natural flow this coming year before any diversions.
Once again, let’s do the math. Let's assume the natural flow for 2027 ends up at 11 maf. What would that mean for Lake Powell? Start by peeling off the 4.5 maf the upper basin will use this year, unencumbered by any cutback requirements from the feds (although with less snow in the Rockies, those states may have no choice but to cut back… let’s say their total might be 4 maf, not 4.5). Then let's assume about 1.0 maf in evaporation and seepage from Lake Powell and the river and reservoirs above that point (less than in a big year, since there is less water in the system). And since we know USBR is already committing to releasing a net 0.75 maf from the upper reservoirs in a bad runoff year, let’s include that. We also know that USBR would release 6 maf from Lake Powell in a bad year, but under the fed’s recent directive, they might be able to drop that to 5 maf. Add it up, and you get a net increase of up to 1.75 maf to Lake Powell. But of course that comes at a big cost—the smaller upper basin reservoirs take a big hit, there’s much less water delivered to Lake Mead, and less available for the lower basin users. And then there’s absolutely no reserve left over for WY 2028.
But in this scenario, here’s what Lake Powell looks like in summer 2027. If you add 1.75 maf to the October 1 (beginning of WY 2027) live storage of 5.1 maf, you end up with about 6.9 maf in live storage in 2027… or a surface elevation of 3546. That’s not bad, about 35 feet higher than what USBR presents as a worst case. How could my result be so different? Am I way off base here? Possibly. I made a lot of assumptions, most importantly about what the worst natural flow in a strong El Niño might be. But we’ve got the past data, and I’m picking the absolute worst of those years. If this turns about to be a reasonable worst case (or close to it), that would mean USBR could ease back on releases from the upper reservoirs, and go on with its planned release of 6 maf through Glen Canyon Dam. If they did that, that effectively would wipe out the entire projected net increase of 1.75 maf to zero. And that would mean the lake tops out next summer close to where it is right now—about 3518. But it would hold the upper reservoirs steady.
So there it is. Depending on how USBR manages the reservoirs, and to some extent on whether the upper basin states are forced to cut back their use (by about 10%) because of poor snowfall, we could end up with a range anywhere from 3518-3546 in a worst case El Niño snowfall/runoff event. That’s a huge range because USBR still has some control on the levers between now and then, and we really don't know what the upper basin states would use. But even the low end of that range is considerably higher than the USBR’s “minimum probable scenario” published in August--which was a peak of 3510.
The USBR undoubtedly knows more about all this than I do, but that’s how I read the “worst case” tea leaves. Also remember that the “worst case” is not the likely case. But it’s certainly possible.
As I said before, we'll know better where this is all headed once we get closer to spring...
