PHOENIX (AP) -- Nearly a month after water filled the Warren H. Brock Reservoir near the Arizona-California border 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.
Now, that water can be shunted into the reservoir and held until the farmers ask for it.
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 structure's 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," Jennifer McCloskey, manager of the bureau's Yuma-area office, told The Arizona Republic. "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 a farmer and agricultural researcher in California's Imperial Valley, 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.