When I took a geography class focussing on the western US, one of the things the teacher mentioned (which I haven't verified independently, but it was his job) was that the Colorado River water rights were allocated based on how much the Colorado River was running in roughly 1920, which happened to be an unusually high flow rate period, so ever since then there hasn't been enough water to satisfy everyone. (Water rights are allocated by time priority: first person who used it gets to take the entire amount that person is entitled to, then second person, and so forth.) So it's 100% spoken for, forever. The shortfall is made up for by pumping out groundwater, and when they allocated the colorado river water rights, they also decided that they were going to make a 100 year plan for water usage, meaning that after 100 years they would have used up pretty much all the available aquifers. Since then we've discovered some more aquifers, and are willing to drill deeper and run more expensive pumps, but that's only somewhat covering the shortage. We're pretty much collecting exactly what we planned 95 years ago. There are still semi-serious proposals to divert and pump chunks of the Columbia River over into the upper Colorado River basin... which is sort of funny, as much of the original water projects in the upper Colorado River basin were, and are, pumping water from it through the Continental Divide over to the eastern slope to fulfill Colorado, Kansas, Wyoming, and Oklahoma water needs.
The same instructor also noted that depending on how you define your terms, the category of western state water rights was by quite a bit the most common lawsuit that ended up in the US Supreme Court, showing up every couple of years in one form or another.