“There is no planning for a continuation of the drought we’ve had,” said one expert on the Colorado’s woes, who asked not to be identified to preserve his relationship with state officials. “There’s always been within the current planning an embedded hope that somehow, things would return to something more like normal.”

Unfortunately, the Colorado during most of Lake Mead’s 78-year history was not normal at all.

Studies now show that the 20th century was one of the three wettest of the last 13 centuries in the Colorado basin. On average, the Colorado’s flow over that period was actually 15 percent lower than in the 1900s. And most experts agree that the basin will get even drier: A brace of global-warming studies concludes that rising temperatures will reduce the Colorado’s average flow after 2050 by five to 35 percent, even if rainfall remains the same — and most of those studies predict that rains will diminish.

Already, the drought is upending many of the assumptions on which water barons relied when they tamed the Colorado in the 1900s.

The Colorado basin states tried in the 1920s to stave off future fights over water by splitting it, 50-50, between the upper-basin states of Utah, New Mexico, Colorado and Wyoming and the lower-basin states of Arizona, Nevada and California.

In fact, the deal underestimated how much water the fast-growing lower-basin states would need. During most of the wet 20th century, however, the river usually produced more than enough water to offset any shortage.

Now, the gap between need and supply is becoming untenable.

Lake Mead currently stands about 1,106 feet above sea level, and is expected to drop 20 feet in 2014. A continued decline would introduce a new set of problems: At 1,075 feet, rationing begins; at 1,050 feet, a more drastic rationing regime kicks in, and the uppermost water intake for Las Vegas shuts down. At 1,025 feet, rationing grows more draconian; at 1,000 feet, a second Las Vegas intake runs dry.

Lake Powell is another story. There, a 100-foot drop would shut down generators that supply enough electricity to power 350,000 homes.