> I wonder why Colorado and Utah were not involved in the deal? If one of them had a problem with it, would they have standing to sue?

The Colorado River Compact effectively divides states into two buckets, the Upper Basin and Lower Basin states. The Upper Basin states already came to an agreement a while ago; the Lower Basin states only came to an agreement today.

The upper basin states have already hit their targets (and then some) in terms of water usage. The problem is that California has been consuming the surplus, and there is no longer enough water flow to sustain the surplus.

I'm sure all seven states in the compact were involved in this deal. They're all signatories on the compact. The way the compact is structured, the upper basin states (Colorado, Utah, Wyoming, New Mexico, and Arizona (a smidgen)) are supposed to deliver 7.5 million acre feet per year to the lower basin states (California, Nevada and Arizona (bulk)), but they can use whatever they want other than that amount; this amount was supposed to be about even between upper and lower, but it isn't. It sounds like this deal is reducing the required delivery by 1 million acre feet per year, although I don't know if that's from the original target or the lowered targets from the 2007 interim agreement, so somewhere between 6 and 6.5 million acre feet; and the upper basin states get the rest.

The laws you want to learn about are called "water rights". Western water rights laws (which are the predominate legal framework in the US West) basically say "first in time, first in right". The people who used that water first have the ownership rights of that water flow.

So it would be illegal in Colorado to build such a dam because you would be blocking the water that belongs to someone else.

In Colorado, it is illegal to collect the rainwater that lands on your roof because that water would normally flow into a river and that river water belongs to someone else. I used to live in Colorado and about 10-15 years ago, there was a change in the state law that the media proclaimed made it legal for you to collect the rainfall that landed on your roof. They left out the conditions that the statute required: you had to have an existing well permit, you could only use the rainwater for the same purposes that your well permit allowed (so if the permit only allowed watering livestock, you could not use the water for your garden), then there was another stickler - there could not be a water utility adjacent to your property (even if you were not connected to that water company). There were 3 other requirements, but those 3 were so narrowly focused that the law basically upheld the illegality of collecting rainfall.

Interesting. But I don't understand why "first in time" applies to states downstream of the source. The water started as ice melt in Colorado's mountains. So why isn't Colorado "first in time?" Is it because in the case of Colorado building a dam, they'd be utilizing the water (instead of just letting it flow), but Arizona was utilizing it before them (by pumping from it to drink)? And if that's the case, then surely Colorado can apply the same logic so long as they have some water facilities powered by the upstream flow? Also, Colorado was settled as a state before Arizona (IIRC), so they must have been using the water before anyone in Arizona was.

The vast majority of Utah relies on snow melt for water as the Colorado bypass most of the population so it's not as much of an issue.

These states follow prior appropriation doctrine for water rights, which states that upstream owners can use/divert water for "beneficial use". Building a dam to intentionally stop the flow to downstream consumers is not a beneficial use.

I don’t see how they have standing being up river. The deal can’t affect them thanks to gravity.

If Colorado wanted to build a dam, that’s completely different and the other states would likely challenge it in court.