I just learned about this yesterday. The reason Lake Mead is going dry is because they are diverting the head waters of the Colorado River from the west side of the Rockies to the east side. It goes through a tunnel under the great divide that was built in the 1940s called Alva B Adams tunnel. It's also sent through the Grand Ditch.

https://en.wikipedia.org/wiki/Alva_B._Adams_Tunnel

https://en.wikipedia.org/wiki/Grand_Ditch