In late April 1996, Lake Powell sat at an elevation of 3,673 feet — just 27 feet below its maximum capacity. At that time of plenty, Arizona lawmakers worried that the state wasn’t using its full share of Colorado River water. Instead of potentially ceding those flows to California, the state opened a kind of liquid piggy bank, storing away a share of its water for an uncertain future. In the first year of operations, the Arizona Water Banking Authority set aside 300,000 acre-feet of water. After 25 years, its savings balance — stored underground in facilities across the state — has grown to 3.75 million acre-feet.