They have a lot of farm land that they buy alfalfa from but water rights are a bit different. Water rights are based on the "doctrine of prior appropriation" which means the water from the river goes to the body that is able to claim first "beneficial" use for that water, beneficial as defined as economically beneficial. Los Angeles and many Imperial Valley farmers primarily used that to build massive water infrastructure projects to divert massive amounts of water from the Colorado for projected growth. Those rights to that water are locked, however because of the crises on the Colorado river system the entire Colorado River compact is coming into question which has the doctrine of prior appropriation as a foundation to the water legal system in the west.

Just for some additional context the Colorado River Compact is essentially the West's version of the constitution. Its a water treaty governing commerce and political power in the West. Keep in mind the American West is a desert and the only real thing of true value in a desert is freshwater. There is an old west saying "whiskeys for drinking, waters for fighting over", and people did fight over water out here.