Well that's a great question with a complicated answer. But in short, all 50-foot rises are not created equal. Think of Lake Powell as something like a martini glass. When it's full, the surface area is much larger than when it's half full. And, as with a martini glass, most of the volume is in the top of the glass. That's why it's important to keep your martini glass full if you intend to get drunk.
In the case of Lake Powell, it can theoretically hold 24.3 million acre feet (MAF) when it's full, which is when the surface is at 3700 feet elevation. The base elevation below the dam is 3132, so when its full, the lake is 568 feet deep near the dam. Now you would think that that if you split the difference between 3132 and 3700, the lake would be half full. But in fact, if the lake were ever at that "midway" elevation point of 3416, it would only hold 1.2 MAF, or about 5% of its total volume when full. So when the lake was filling in the mid-60s, it rose very quickly because it didn't take much volume to get the initial rise. But once it started spreading out upstream and into the side canyons, it took a lot more volume just to get a small rise to happen.
Powell is currently 48% full by volume, but still 468 feet deep near the dam--and only 100 feet short of "full pool". So about half the volume of the lake is in that highest 100 feet, which is one reason we're having so much trouble filling it to the top. (It's the same reason it took from about 1970, when the lake was basically where it is today, till 1980 for the lake to fill completely.)
Last year, the lake was super low at this time of year, lower than it had been since spring of 2005, and before that you'd have to go back to 1969. In April 2019, the lake volume bottomed out at a shade under 9.0 MAF. The subsequent 53-foot rise brought the volume up to 13.9 MAF, or a rise of 4.9 MAF.
This year, we'll probably bottom out today or tomorrow at 3600.27 or so, which just under 11.8 MAF. If we were somehow able to add 4.9 MAF this spring, that would raise the lake to about 3645, which is a very respectable 45-foot rise, but not quite the 53 of last year. (By the way, the next 50 feet that would be the hardest climb--add another 4.9 MAF at 3645, and it only raises the lake about 33 feet.)
Okay, so the lake's depth profile is part of the answer. The other part is the inflow/outflow regime. The outflow is controlled by USBR, and we already know what that will be--8.23 MAF for the current water year. Best guess is that over the next 90 days, the lake will release about 2.2 MAF, so we'd need a gross inflow over those next three months of about 7.1 MAF just to get the same volume increase as we did last year, and that would only be a 45-foot rise. And what does 7.1 MAF over three months translate to? That would mean we'd need to sustain a daily inflow of 40,000 cfs on average every day for the next three months. That's a very tall order, especially since we don't have the same snowpack (at least according to USBR) as we did last year. Now, it's very true we had a very late snowpack and very late storms, which made last year unusual, so it's still possible to achieve that. But unless we have something similar happen as it did last year, we won't end up anywhere near a 45 foot rise, let alone 50 or 53. USBR is predicting a 15-16 foot rise. Seems conservative, but it's based on what historic inflows have been in April, May and June. Last year we had unbelievably above normal inflows in those months. Still, there is hope.
And just to put it all in perspective, to achieve a 50-foot rise this year would take an even greater net inflow volume than what we had last year, and last year's 4.95 MAF spring net inflow was the best since 2011, and the 6th best ever. It was a phenomenal year.
The top 10 spring net inflows at Lake Powell:
1979 - 7.48 MAF
1995 - 6.78 MAF
1993 - 6.67 MAF
1973 - 6.31 MAF
2011 - 5.91 MAF
2019 - 4.95 MAF
1997 - 4.64 MAF
2005 - 4.61 MAF
2008 - 4.61 MAF
1984 - 4.46 MAF
You might wonder why 1984 is so low on that list when it was such a huge runoff year, and why 1983 is not on the list at all (it had 4.14 MAF). That's because the lake was already almost basically full when the snowpack began to melt in those years, so they had to release huge amounts of water. In each of those two years, about 15 MAF gross inflow entered the lake during the spring and summer. That is a huge amount--almost 50% more than the typical annual flow of the entire Colorado River system!! Had the lake been as low as it is today, and releases were a normal 0.7 MAF/month, we'd have ended up with a net volume increase of somewhere around 12 MAF added to the lake. If we started at 3600, that would have basically filled the lake--we'd have ended up around 3697!!
So it's not impossible to add nearly 100 feet in one year, if we had another year like 1983 or '84... which won't be this year, but next year???