LOS ALAMOS, N.M. (Reuters) - Scourged by fire, the town that gave birth to the atomic bomb now faces a threat from water after New Mexico's worst wildfire left the mountains around Los Alamos prone to flash floods ahead of the rainy season.

As firefighters worked on Sunday to contain the blaze's last front line, the town of Los Alamos and the adjoining U.S. nuclear weapons laboratory struggled to recover from the fire, which scorched research facilities, destroyed more than 200 homes and caused other widespread damage.

Los Alamos National Laboratory, where the first atomic bomb was built in 1945, plans to reopen gradually starting on Monday, two weeks after it closed as the fire drew near. The town that grew up as a bedroom community for the lab allowed its 11,000 residents to return in stages last week after being evacuated on May 10.

``We're all just kind of numb. The reality is just sinking in,'' Los Alamos resident Pat Minor said as he picked up cleaning supplies and snacks from a Red Cross emergency station. Minor's house suffered smoke damage but the one next door was burnt to the ground.

Fire Leaves Flash Flood Threat In Its Wake

Even as residents were beginning to clean up or lay plans to rebuild, emergency officials warned the wildfire had created a new danger -- summer flash floods in late June or July, when heavy rains usually fall on northern New Mexico.

The town and the laboratory are perched on a high plateau nestled in the pine-forested Jemez Mountains, which rise above them to the west and north.

The wildfire burned much of the forest underbrush that normally slows water running off the mountainsides and glazed the top layer of soil into a surface that sheds water, said Ken Palmrose, spokesman for a multi-agency Burned Area Emergency Recovery team surveying the damage.

As a result, the summer rains could send powerful torrents down the canyons and arroyos that cut through Los Alamos, potentially flooding roads, undermining bridges and carving new watercourses through property.

Los Alamos county has warned fire-weary residents of the flood danger and urged people to keep a three-day survival kit at home in case their neighborhoods are cut off.

``We do face the possibility of getting parts of town isolated for a day or two until we get back across the arroyo or canyon where the road used to be,'' said Capt. Robert Repass, the county emergency manager.

``My understanding is that the normal types of summer monsoons we get, a good hard thundershower, nothing necessarily out of the ordinary, could cause problems,'' he said.

Palmrose said the burn recovery team is mapping the expected danger spots and will draw up recommendations as early as this week for preventing floods, including erecting barriers of hay bales or mesh and crisscrossing fallen trees on the mountainsides to slow the rush or rainwater.

``The heavy clouds that were over the mountains on Friday were a stark reminder of what could happen,'' he said.

The clouds were part of a cool front with spotty showers over the weekend that helped firefighters make progress in the final stage of containing the blaze, which has consumed 47,650 acres (19,060 ha) since it started.

The National Park Service set the blaze on May 4 to burn away underbrush as part of its fire-prevention measures. The fire raged out of control the following day.

Los Alamos Residents Work To Clean Up

By Sunday, officials said the fire was 90 percent contained, with the final front expected to be hemmed in by Wednesday, although it will continue smoking and burning in spots within the perimeter until the summer rains fall.

Smoke rose from behind one of the peaks over Los Alamos as people in the worst-hit neighborhoods that abut the mountains drove by a Red Cross emergency truck to pick up free lunches and gloves, goggles, buckets and brooms for clearing away smoke and fire damage.

``Nobody has time to worry about the rains,'' said Rosemary O'Connor, who has lived in Los Alamos since 1947. Her one-story brick house escaped damaged on a street where others were charred, but she was collecting cleaning supplies anyway because everything in her refrigerator and freezer had gone bad after days without power.

Her neighborhood was spotted with big signs in front yards thanking firefighters for saving the homes they could. People around town wore green ribbons or flew them from their cars and trucks as a sign of thanks to all emergency workers.

``Most people are in a 'let's hurry up and get this done' mode,'' said Patricia Graves, a Red Cross volunteer staffing one of several emergency aid trucks parked in the neighborhoods.

One house down from the truck, as if in agreement, an elderly man with a three-legged walker used his free hand to rake burned grass from his lawn.