LAS VEGAS (AP) — The federal government isn't going to tap the Missouri River to slake the thirst of a drought-parched Southwest, the government's top water official said Wednesday.

But rising demand and falling supply have water managers in the arid West considering a host of other options to deal with dire projections that the Colorado River — the main water supply for a region larger than the country of France — won't be able over the next 50 years to meet demands of a regional population now about 40 million and growing.

Interior Secretary Ken Salazar issued what he termed "a call to action" with a three-year study of the river, its flows and its ability to meet the future needs of city-dwellers, Native Americans, businesses, ranchers and farmers in seven Western states.