The toughest question I got was about how long it would take the tamarisk, a notoriously invasive exotic, to recover. I suddenly felt about 30 years younger. One of the strongest selling points comes from the Bureau of Reclamation itself. In 1996, the Bureau found that almost a million acre-feet, or 8 percent of the river's flow, disappeared between the stations recording the reservoir's inflow and outflow. Almost 600,000 acre-feet were presumed lost to evaporation.
Nobody knows for sure about the rest.
Some bank storage is recoverable, but all too likely the region's downward-slanting geological strata are leading some of Powell's waters into the dark unknown. It takes only one drain to empty a bathtub, and we don't know where, when, or how the Powell tub leaks. But these numbers are moving upward. What has happened in the last decade or two is interesting, but what will happen in the next century or two is critical. Whatever the final details of Lake Powell's water losses turn out to be, the draining of the lake simply has to happen.