Lake Mead and Lake Powell, which is also down somewhat, get most of their water from the "Upper Basin." The lower part of the Colorado River also gets water from the "Lower Basin." This rain below the latitude of Hoover Dam is greatly helpful, but it is true, it won't do much for Lake Powell and just a little for Lake Mead. It's helping the Salt River, which helps Phoenix and irrigation, and it certainly is helping with comfort and yard water.

Here's a map of the collection areas: https://www.usgs.gov/media/images/colorado-river-basin-map