It’s been done before (though not intentionally at first), at the Salton Sea.

And the results weren’t that bad (granted possibly a smaller scale than some are imagining when they forecast doom).

ppic.org/…/the-troubled-history-and-uncertain-fut…

But the problem is any deserts are deserts for a reason: lack of rainfall and/or natural inflow from rainwater upstream. The result is that you have to keep pumping in tons of water and/or rely on agricultural runoff which is nutrient-depleted and usually full of chemicals.

Read the rest of the linked article for what’s going on with that one.