GORDONS WELL, Calif. - Twenty-eight days after water filled the Warren H. Brock Reservoir for the first time, the project's builders got the news they wanted: It didn't leak.

So, they pulled the plug and let all the water out.

Emptying the reservoir, dug out of the sand dunes about 25 miles west of Yuma, was as much a part of the final construction test as filling it and watching for leaks. This reservoir was built to be in motion: Get the water in, wait a few days, get the water out.

The $172 million project is an attempt to seal decades-old leaks in the Colorado River's water-delivery system by capturing the dribbles lost downstream to Mexico when farmers in Arizona and California don't take water they ordered, usually because rain filled the need.

That water can now be shunted into the reservoir and held until the farmers ask for it again. The U.S. Bureau of Reclamation estimates the project could save as much as 70,000 acre-feet of water a year, water that can remain in Lake Mead as a hedge against drought.

With testing complete, the reservoir could begin operating any time, although officials plan to inspect the two structure's two basins and the site to be certain no hidden problems emerge. The water used in the test was returned to the system and used by farmers.

Conservation groups say the water that escaped the Colorado's system of canals was never wasted because it helped sustain the river's few remaining riparian areas on the river's delta in Mexico. Those areas still need a source of water to survive, the groups say.

But state and federal officials, although sympathetic to the environmental issues, say the new reservoir helps water users better manage their own supplies and could delay drought-related rationing on the river, restrictions that would cut into Arizona's allocation.

"We want to capture every drop we can," said Jennifer McCloskey, manager of the bureau's Yuma-area office. "The water we could save is enough for a city the size of Yuma. We think that's worth it."

The new reservoir, known during construction as Drop 2, grew out of a 2007 drought-management plan adopted by the seven states along the Colorado River. The plan identified opportunities to add water to the river by eliminating inefficient practices that led to system losses.

One of those opportunities was in the system near Yuma, where billions of gallons of water allocated to U.S. farmers but never used by them flowed into Mexico, where it could be used without counting against that country's annual allocation.

Nevada, at the time the state most at risk of running out of water, offered to pay much of the construction tab in exchange for a share of what was conserved.

As the idea developed, Arizona and California agreed to contribute money for their own shares of water.

In the end, Nevada paid $115 million for 400,000 acre-feet and Arizona and California added $28.6 million each for shares of 100,000 acre-feet. The states can use the water in increments over about 20 years or leave it stored in Lake Mead to delay drought restrictions.

An acre-foot of water is 328,851 gallons, enough to serve two average households for one year.

The reservoir itself is not that big. Full, its two basins can hold up to 8,000 acre-feet. By comparison, Canyon Lake, the smallest reservoir on the Salt River, can hold more than 57,000 acre-feet.

But Brock Reservoir (named for an Imperial Valley farmer and agricultural researcher) was not built to store water long-term. On any given day, it could be the largest body of water for miles in any direction, or it could be two empty holes in the ground.

It will operate most often after a rainstorm, when farmers on the lower river decide they don't need water they had ordered several days earlier. That water, which had previously flowed south into Mexico, will now be diverted into Brock and stay there until it can be returned to the system.

The project sits along Interstate 8, just past the popular Imperial Sand Dunes. It is hidden from motorists by earthen berms and will be operated remotely by the Imperial Irrigation District, which supplies water to farmers in California. The district can open and close the inlet and outlet gates and regulate the amount of water diverted into the reservoir and returned to the main system.

The two basins are lined with polyethylene and a 9- to 10-inch layer of soil-cement. Water filled it in mid-October for testing and, over 28 days, no serious leaks were discovered, project manager Mike Vandevelde said. Less than 6 inches of water was lost to evaporation.

"We haven't had any structural problems," he said. "We even had an earthquake on Easter Sunday, and there was no damage."

Water flows into the reservoir from the All-American Canal, which distributes river water to farmers in Southern California. A short outlet allows the water to be diverted back into the main system.

Once water is scheduled to be released from the reservoir, the same amount will be held upstream, which will prevent water levels from dropping at Lake Mead, which is at its lowest point in more than 70 years. Low levels at Lake Mead trigger rationing, and, under the 2007 agreement, Arizona suffers the largest losses.

The water saved in one year would result in less than 1 foot of added storage at Lake Mead; but as the reservoir's elevation nears the drought triggers, even a small amount could buy the states time.

The water that will now flow through Brock had flowed down the Colorado into Mexico. It did not count toward that country's annual share of 1.5 million acre-feet, and although it could be used by farmers, it also remained in the river, feeding riparian areas.

Conservation groups worry that the loss of those flows could hinder efforts to find a dedicated source of water to help repair the river's fragile delta, which has shrunk and dried up as water users divert most of the Colorado.

"I am the first person to say we need to be as efficient as we can be with Colorado River water," said Jennifer Pitt, a senior resource analyst for the Environmental Defense Fund, a national advocacy group.

"I don't think the answer is to say we don't need these projects. The answer is to say we need dedicated flows for those resources," Pitt added.

U.S. and Mexican water managers have discussed finding sources of water for the delta, but most ideas - desalinating water, lining canals - require a financial investment in addition to a commitment by water users.

"We can propose solutions on our own, but it's hard to get traction, to get the politicians to focus on environmental issues," Pitt said. "We are here to find solutions that everybody can live with."