It took more than 17 years for Lake Powell to finally reach its full elevation of above sea level, which it crossed on June 22, 1980. One of the main reasons for this slow rise, in addition to the need to meet obligations to the Lower Basin, was the leakage of vast amounts of water into the porous Navajo Sandstone aquifer. Between 1963 and 1969, as much as leaked into the reservoir banks each year. Conversely, some of this "bank storage" flows back into the reservoir as springs and seeps when Lake Powell is low. Exactly how much of this water has potential to return to the reservoir, and how much "disappears" into the ground, is subject to debate.