In answer to all the below talking about Lake Mead and the Colorado river, note that much (perhaps most) of the water supply in Las Vegas is groundwater (think:

In answer to all the below talking about Lake Mead and the Colorado river, note that much (perhaps most) of the water supply in Las Vegas is groundwater (think: large municipal wells), and the water level in these is dropping as well. There is some significant subsidence in the Northeast section of town.