Lake Mead is so low because the Colorado River Water Compact allocates more water than has historically been in the river, based on tree ring data. There have b

Lake Mead is so low because the Colorado River Water Compact allocates more water than has historically been in the river, based on tree ring data. There have been a number of years of historically average flow in a row, and keeping the reservoirs full requires roughly a decade of above average flow. * Edit * For clarity the compact divides up 17 million acre/ft per year and the real average historic flow is closer to 13 million acre/ft per year. The compact also double dips in it's accounting for some return flows.