From 2000 through the end of 2018 (projected), Lake Powell’s elevation will have dropped approximately 94 feet despite Upper Basin consumption only averaging about 4.5 million acre?feet (maf)/year. Several particularly dry years—including 2018—in a process of continuing aridification contributed to the drop, but ultimately it is the operational rules that are slowly but surely draining Lake Powell. Through 2018, cumulative releases since 2000 from the reservoir will be approximately 11 maf higher than the 8.23 maf/year baseline traditionally utilized by Reclamation (see figure on page 3). Had those excess releases remained in Lake Powell, the lake level would not have declined. However, those extra releases—now governed by the 2007 Interim Guidelines—are the only thing that has kept Lake Mead from dropping into shortage conditions.
That’s from the a new white paper – It’s hard to fill a bathtub when the drain is wide open: The case of Lake Powell – from the folks at the Colorado River Research Group.