A Western drought that began in 1999 has continued after the respite of a couple of wet years that now feel like a cruel tease. But this time people in the driest states are not just scanning the skies and hoping for rescue.

Some $2.5 billion in water projects are planned or under way in four states, the biggest expansion in the West’s quest for water in decades. Among them is a proposed 280-mile pipeline that would direct water to Las Vegas from northern Nevada. A proposed reservoir just north of the California-Mexico border would correct an inefficient water delivery system that allows excess water to pass to Mexico.

In Yuma, Ariz., federal officials have restarted an idled desalination plant, long seen as a white elephant from a bygone era, partly in the hope of purifying salty underground water for neighboring towns.

The scramble for water is driven by the realities of population growth, political pressure and the hard truth that the Colorado River, a 1,400-mile-long silver thread of snowmelt and a lifeline for more than 20 million people in seven states, is providing much less water than it had.