Central Washington's Yakima Valley sits on the dry side of the Cascades, where just eight inches of precipitation falls each year. But the arid valley has rich volcanic soils and an accommodating climate of warm days and cool nights, and after the U.S. Bureau of Reclamation built six dams in the early 20th century, farms and orchards flourished here. Thanks to the water brought via pipes and canals, the Yakima region became known as the nation's "Fruit Bowl." Today, apple, pear and cherry trees cover 92,000 acres, and wine grapes spread over another 10,000. The valley also produces three-fourths of the country's hops, a $100 million crop.

In recent years, though, the region has reaped a different bumper crop. Since 2000, the city of Yakima has added 12,000 new residents  -- a 17 percent jump in population  -- and the valley is thirstier than ever. That's a familiar story in the growing West. But the era of big dam building has passed, and water developers have turned to a new round of mid-sized structural solutions to increase water supply.

In 2003, Congress ordered Reclamation to look into building a 10-mile-long reservoir in a dry valley about 20 miles east of Yakima, where it won't block the Yakima River. The proposed $6.7 billion Black Rock Reservoir would hold 1.6 million acre-feet of storage behind a 755-foot-high dam, making it one of the largest reservoirs built in the U.S. in recent decades.

Similar proposals are under review throughout the West. Northern Colorado is considering an off-channel project that would tap the Cache la Poudre River. California wants off-channel storage near the Sacramento River, as part of a $9.3 billion water initiative, and Wyoming is exploring off-channel sites in the Green River Basin. And in Utah, water managers want to build a 140-mile-long, $1 billion pipeline to bring water from Lake Powell to the city of St. George and the surrounding area. To help sell the projects, water developers have figured out ways to mitigate some resource damage and help low-flowing rivers and fish in need of habitat.

But river advocates say Black Rock and other projects billed as environmentally friendly promise far more than they can deliver. And the high costs involved may put an end to this new dam era before it even begins.

The Bureau of Reclamation and other dam builders have already blocked and diverted most of the West's major rivers, flooding the deep valleys and canyons best suited for big reservoirs. Black Rock is a comparative puddle next to the 150-mile-long water hole behind Grand Coulee Dam (although Black Rock's dam would be taller). And with national environmental laws making it harder to get monumental projects approved, the glory days of dam building are history.

As a result, new dam proposals tend to demonstrate an environmental sensitivity that was seldom seen in last century's water planners. Off-channel dams and reservoirs, though not a new engineering feat, have become increasingly common among water proposals in recent years. They are built in dry canyons or valleys, instead of on the main course of a river.

In the Yakima Valley, peak flows (above target flows set for salmon) would be diverted from the Columbia River and pumped over a ridgeline into Black Rock Valley. The new reservoir would supply growing municipalities and existing irrigation canals to help drought-plagued farmers, freeing some of the water stored behind existing dams in the Yakima River Basin to be released into the river system when it would most benefit salmon.

The off-channel construction would neither block fish passage on the Yakima nor inundate biologically rich riparian habitat and floodplains. According to University of Montana river ecologist Jack Stanford, the basin offers one of the Northwest's best opportunities for salmon restoration. The increased flows could support     1 million salmon in a river system where only about 3,000 now survive.

"Looking at the broad range of benefits, I don't see any environmental downside to this," says Sid Morrison, a former congressman who now chairs the Yakima Basin Storage Alliance, a coalition of Black Rock supporters.

There are no salmon in Colorado's Cache la Poudre River. But Glade Reservoir's supporters are equally enthused about the project's environmental and agricultural benefits. Former U.S. Congressman Hank Brown, R-Colo., who mediated a fight over a main-channel dam on the Poudre in the 1980s, praises Glade as "an enormous plus for the environment."

Glade would hold 177,000 acre-feet of water behind a 250-foot-tall dam in a hogbacked valley off the Poudre River channel. The stored water would be pumped from the river during spring high flows to meet the demands of 15 fast-growing rural towns and bedroom communities between Fort Collins and Denver. Those communities would shoulder the $420 million estimated cost.

Glade Reservoir is a sign of evolution among water managers, says Brian Werner, a spokesman for the Northern Colorado Water Conservancy District. The district began pursuing the project after a previous attempt at a dam led to the Poudre's designation as Colorado's only wild and scenic river.

Werner says that Glade is a cautious  -- and necessary  -- first step that could return flows to the Poudre even as it protects local agriculture against buyouts by booming communities and helps meet regional water demands for the next few decades.