A short summary of the key issues related to water releases...some of this is recycled from an earlier post I made to a different thread, but thought it might be useful here too...
...As you might suspect, it's more complicated than the LP Database indicates. If you want to read the entire set of moving parts that goes into the "Law of the River", here's the link. If you have time, go for it--pretty interesting:
The short version is that the Upper Basin needs to deliver 75 million acre feet in any given 10-year period to meet the delivery requirements to the Lower Basin, or a yearly average of 7.5 MAF. In addition, the Upper Basin has to deliver its share (half) the 1.5 MAF to be delivered to Mexico, since the river ends up in Mexico. Under the current protocol followed by the Bureau of Reclamation, that means the usual "minimum" release requirement is 8.23 MAF, not 7.5 MAF, to account for release requirements to the Lower Basin and Mexico. There is some flexibility in that. The Bureau can release up to 9.0 MAF in order to achieve a better operational balance between Mead and Powell. There's a few other nuances there, but that's the idea.
So all this means that means the LP Database is a bit oversimplified/misleading. For example, instead of saying we are at "10% of the minimum required 7.5 MAF", it should really say we are at "9% of the projected release of 8.23 MAF", if the intent of the USBR is to release that much during a given water year.
Of course, the critical component in this arrangement right now is Mead, not Powell. If you get below 1075 in Mead, then some real changes in the Lower Basin happen, mostly that not everybody gets their water allocation--specifically, AZ and NV do not get their full share (CA has the senior water right among the three states, a separate topic--CA doesn't have to cut back until it falls to 1025). So there's a very strong effort to keep it above 1075, with especially loud voices in AZ and NV since they'd be affected first. Today we're at 1082, only 7 feet above that critical level.
Powell, on the other hand, is not really tied to any level in terms of Upper Basin water delivery, because nobody in the Upper Basin really draws directly from Powell except the City of Page (the Navajo Power Plant did too, but now it’s being shut down). And if it ever happens, the proposed pipeline to St. George would draw off a relatively small amount annually--about 0.86 MAF (although hydrologically St. George is really in the Lower Basin, legally it's considered in the Upper Basin because it's in Utah). For Powell, right now it's mostly about acting as a storage bank for Mead and for power generation through Glen Canyon Dam, and the minimum power pool for Powell is 3490. We're at 3613 right now--not close. So from a management standpoint, Mead is under way more stress than Powell, so it makes sense to send as much as possible in the allowed range (8.23-9.0) down to Mead as long as water delivery requirements downstream are in jeopardy. And in a big water year, it makes sense to send as much as possible downstream. That's what happened during Water Year 2018-19, but even with that, Mead has been slowly dropping from the most recent peak of 1090 on March 31... and that's with the Lower Basin states staying within their allowed usage per their water rights.
So that's the reality from a legal framework...
Of course, the whole thing is based on the assumption that the river system can deliver 16.5 MAF to all users on an average annual basis. The reality is that average flows over the past century are much less than that, more on the order of 11 MAF +/- since 1981. Last year—which was a very good year—only saw about 13 MAF inflow into Powell. So while some years may exceed 16.5, the vast majority do not (since 1964, only 5 years have experienced inflow into Lake Powell that exceeds that figure), which means in the long run, water rights exceed supply, and the reservoir levels will inevitably trend downward if everyone exercises their water rights, even with the occasional up years. And the long-term forecast in terms of climate change is that the SW will experience less precipitation in general over the next century, not more. And more people continue to move to the West, so water demand continues to rise. Of course, the elephant in the room is "what if the Upper Basin exercises its currently unused water rights?" Then there's a big fight, of course. What we're seeing now with regard to the St. George pipeline is just the beginning of that conversation...
That's why cutting back demand is the critical approach if a sustainable balance is ever going to be achieved, which is hard to do within an established water rights framework... but that's the whole point of the recent agreement among the seven basin states...
