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?
What if, hypothetically, Colorado built a dam and stopped the flow of water to the downstream states?
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.
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.
If Colorado wanted to build a dam, that’s completely different and the other states would likely challenge it in court.