Well, losing the mouth of the Colorado might have some impact on SW Utah. If that Baja senator (I forget his name) hadn't filibustered in the 30s due to the perceived damage to the Colorado River delta, the Colorado Dam would have been built, and I suspect there might actually be some civilization in central Alta. For sure, the Southern California region wouldn't have near as bad a water problem as it does now.
The whole area is a rocky desert. Sure there are the tourist cities of Tijuana, Los Angeles, and the San Diego Naval Station, but the American South West is mostly unexploited scrubland for a good reason, the Colorado is the only major river and, for better or worse, wasn't exploited for much during the height of the US's industrial expansion and is now pretty untouchable aside from some minor draws for city usage (which is nothing compared to what it would have been if the water was used for agriculture), thanks to the various Wild Lands Acts (OOC: National Parks and etc.)
The only reason Southern ACA, BCA, and Sonora are becoming populous is thanks to air conditioning, which is the same reason Florida went from swamp filled green hell to the most populous state in the Southeast.
Still, Baja and Sonora, especially, had an enormous impact in North American politics with Santa Anna losing just enough land to be cast aside for the ascendancy of Benito Juarez and his implementation of the Mexican Miracle of the 1850's.
And then, of course, there was the 'technical' implementation of slavery in Baja and Sonora in the Compromise of 1850 despite the overwhelming hatred of the institution by the few who lived there (and it being a simple spit in the face for those people since slaveholding held no purpose in the desert.) The ambition of slaveholders in supporting William Walker's disastrous 'filibusters' that antagonized Mexico and CentroAmerica for nothing other All culminating in Mexico's alliance with the Union during the US Civil War and the 'Invasion' of Texas and Louisiana.