Anyone who wants to know more about the situation on the colorado river should read Science Be Damned: How Ignoring Inconvenient Science Drained the Colorado River [1]. The Tl;DR is that water is doled out by a multi-state compact that used several years of historically high water flow as it's basis. Over time, normal river flow rates were bound to deplete the reservoirs.

[1] https://www.amazon.com/Science-Be-Dammed-Ignoring-Inconvenie...