Central and northern California, I thought, get much of their water from reservoirs in the Sierra Nevada (Hetch Hetchy, et al.), not the Colorado River. In fact, IIRC, that's the source of many water disputes in California: people in the south see the wetter north as a source of fresh water, while people in the north want to protect their own water supplies for drinking and agriculture. San Francisco and Sacramento probably wouldn't be in a state of emergency. Hell, even Los Angeles, while it would face a water emergency, would probably be able to make do with strict water rationing, given that they only got a fraction of their water from the Colorado River in 1983; most of it came from the Owens Valley and northern California. That said, were the Glen Canyon Dam to fail and take the rest of the Colorado River's dams with it, I'd imagine those fights getting even more heated and bitter than in OTL, with serious calls in northern California to split the state in two rather than drain the Central Valley the way the Owens Valley was by the LA Aqueduct. Los Angeles wouldn't dry up and vanish like Las Vegas and Phoenix, but its future growth would suffer badly.
I'd also imagine, with a calamity like this, that the "sagebrush rebellion" would take on a very different cast. In OTL, it came to be driven heavily by anti-environmental concerns, led by ranchers, sportsmen, and miners who saw federal management of the land as more concerned with protecting nature than supporting the livelihoods of the people who worked the land. Here, however, I can see a lot of environmentalists in the Edward Abbey mold getting involved in the movement, seeing the government as caring only about development for its own sake without regard to either nature or people. Even in OTL, the Glen Canyon Dam was a rallying cry for the environmental movement in the '70s and '80s, and here, with its collapse grabbing the world's attention, they've seemingly been vindicated in their concerns about its construction. The new narrative for the sagebrush rebellion would be similar to OTL's in terms of its calls for more state and local control over federal lands, but very different in its message on environmental protection, seeing Washington as being obsessed with development for its own sake without considering the consequences.