With the sun out and Washington’s cherry blossoms at peak bloom, visitors on Wednesday thronged the Tidal Basin, eating picnic lunches and taking selfies beneath some of the Mall’s 3,800 cherry trees.

A few yards from the Jefferson Memorial, a pathway that rings the basin was under nearly a foot of water and parts of its concrete wall were crumbling — a reminder that the landmark is falling prey to increased foot traffic, the growth of the D.C. region and a changing climate.

“In 2019, it’s not functioning the way we need it to,” said Teresa Durkin, the senior project director for the nonprofit Trust for the National Mall. “There is a certain amount of urgency.”

Officials from the trust as well as the National Trust for Historic Preservation and the National Park Service launched an effort dubbed “Save the Tidal Basin” in a partnership that aims to bring nonprofit organizations and the federal government together to reimagine the area and develop improvement plans that they say could cost up to $500 million.

Anyone not distracted by the blossoms could see possible targets for a makeover. Advocates say walkways around the 107-acre basin are too narrow for the 36 million annual visitors, forcing them off the paths to tread on the roots of the trees whose beauty they come to celebrate. The trees are also being harmed by brackish water during floods.

“We are in a continual cherry tree replacement mode around here,” said Sean Kennealy, the chief of the professional services division for the Park Service’s National Mall and Memorial Parks.

Meanwhile, the entire basin, built on landfill in the Potomac River in the 19th century, is slowly sinking. Parts of the site are flooded twice daily at high tide, and water gets even closer to Thomas Jefferson after heavy storms — indications that the man-made reservoir cannot handle increased storm water runoff from a rapidly urbanizing region and increases in sea level.

“Climate change will be catastrophic,” Durkin said. “This whole area will be underwater.”

To find solutions, officials started with a $750,000, three-year brainstorming period sponsored by the American Express Foundation, a philanthropic arm of the financial company. This “ideas lab,” as officials have named it, calls for design firms to rethink the Tidal Basin and then present options to federal officials and the public.

“There’s a price on simpler repairs, but we don’t want to do that,” said Catherine Townsend, the president and chief executive of the Trust for the National Mall. “We want the focus to be on re­imagining the space. How can we transform this area so it’s sustainable long term?”

There’s a lot to think about. A flurry of monument building in recent decades, including shrines to President Franklin Delano Roosevelt, civil rights leader Martin Luther King Jr. and Founding Father George Mason, have brought more people to the basin even as parts of it are being reclaimed by the river.

Security and motor traffic are additional impediments, officials said. Vehicles are barred from parts of the area by jersey walls advocates say are incompatible with the cherry blossoms’ beauty or a site intended to celebrate the third president and founder of the University of Virginia.

“That’s not our preferred solution,” Kennealy said.

If the Tidal Basin is reinvented, it won’t be the first time. In the early 1900s, the area was home to floating baths and, eventually, a whites-only bathing beach. That beach was closed in 1925, partly in response to pressure to build a beach for African Americans in a different part of the area.

About a decade later, legislators debated whether a memorial to Jefferson should be built, citing concerns about the expense and the need to remove cherry trees given as a gift by Japan in 1912.

According to a 1937 Washington Post story, a congressional report on the proposed site sought “to protect the Congress from subscribing to a project which would of necessity remain forever an eyesore and an impediment to traffic or occasion the spending of money in sums at the present time unpredictable, but certainly very large.” (The memorial was completed in 1943 for $3 million.)

Officials say it is too early to know what large sum is needed to protect the Tidal Basin and its cherry trees from rising waters and an onslaught of tourists, or who will pay it. But officials said the time for action — or, at least, thinking about action — is now.

“We can’t wait 100 years,” Durkin said. “We want this place to be here in 100 years.”

Michael Ruane contributed to this report.

Dig Deeper: Environment + U.S. Cities

Want to explore how climate change is affecting cities across America? Check out our curated list of stories below.​​

California is hotter and drier than ever before because of climate change. The result: More, bigger and faster wildfires that leave little time for evacuation and cost billions to fight and recover from.​​

The Jefferson Memorial in Washington sits on a landfill and it’s sinking. Some walkways flood twice a day.​​

Many cities have created “heat islands,” urban areas with little vegetation but blocks of surfaces that absorb heat all day, then release it slowly into the night. ​​