If the federal government doesn't open for business Tuesday, park gates from Muir Woods to Yosemite National Park will slam shut.

Many of the 169,000 federal employees in the Bay Area will be furloughed without pay, after Congress failed to avert the first shutdown in nearly two decades Tuesday night. Those who rely on food programs for the poor could feel the effects within days.

The upshot of House Republicans' efforts to tie continued government funding to a delay in what they call Obamacare wouldn't technically be a shutdown - air traffic controllers would still guide flights into and out of Bay Area airports, the mail would be delivered, and the military would stay on the job. And the Affordable Care Act would still go into effect, with people able to sign up for health insurance through state exchanges.

But areas the government deems nonessential would be cut off, and national parks are at the top of the list.

Some of the most visible shutdown signs in the Bay Area would be the closure of landmarks inside the Golden Gate National Recreation Area. Alcatraz, Fort Point, Muir Woods and the Presidio Visitors Center would be off-limits, with parking lot gates padlocked and bathrooms and snack shops shut.

Yosemite National Park would also close. Campers and lodgers would have until Wednesday night to leave.

"If there are no federal appropriations to keep the national parks open, then it is illegal for us to keep them open," said Alexandra Picavet, a spokeswoman for the Golden Gate National Recreation Area.

And although it's easy to walk onto an open space like Crissy Field at the Presidio, even if the park is closed, "for health and safety reasons we would not recommend that people visit," Picavet said.

Time of uncertainty

For now, Social Security and Medicare payments will be paid, along with veterans' benefits. And other federal programs that benefit the disadvantaged would function normally at least for a week.

But for many recipients, the threat of a shutdown had been confusing and unsettling - and if a shutdown drags into a second or third week, they're likely to be hurt.

"When you think of these people, like the elderly or veterans, they don't need any more stress in their lives," said Jessica Bartholow, a legislative advocate for the Western Center on Law and Poverty, which works on behalf of low-income people. "And there isn't a one-stop shop where they can find out what is going to keep getting paid to them and what isn't. It's very stressful."

Take the Special Supplemental Nutrition Program for Women, Infants, and Children, otherwise known as WIC. It helps low-income pregnant and postpartum women and their children up to 5 years old get food and health care referrals.

The benefits will continue to recipients at least for a short time because state agencies that administer the program "can use up various existing funds and accounts to keep the doors open," said Laurie True, executive director of California's WIC agencies. "But beyond a week or 10 days, there will be significant problems."

The problems will be a little more immediate for those wanting to check out Bay Area landmarks run by the federal government.

At Alcatraz Landing on the Embarcadero, tourists continued to line up for ferry service Monday to the onetime prison. An Alcatraz Cruises spokeswoman said the boats shuttle about 5,000 people each day, but with the federal government shut down, the ferries will make their last runs late Tuesday. Those who have already bought tickets will get refunds.

Getting there now

On Monday, many visitors were unaware of the impending shutdown, while others made a point to take no chances and visit the island while they still could.

Bobby Piltu, 21, a Romanian who visited San Francisco to work at the America's Cup, said he got in line at 6 a.m. Monday to nab a standby ticket. Piltu, who said he is returning home Saturday, finally won a spot on the noon ferry.

"I was reading the news this morning and saw they're not moving forward," Piltu said. "So I figured I better get going to see it. Now or never."

Sometimes, in the heat of the partisan rhetorical battle over the shutdown, things get overstated.

A government shutdown "will have immediate and profound impacts" on San Francisco residents, threatening some of the $447 million in federal funding in the current fiscal year for housing, public safety and social service programs, Mayor Ed Lee said in a statement.

However, a significant portion of that money appears to be exempt from a shutdown, Lee's administration said. The exact amount that is exempt is unknown.

An internal memo from his administration on the shutdown said the city "is prepared to cover short-term cash flow challenges ... and thus we do not anticipate an immediate impact on city agency operations."

Housing agency at risk

At greater risk are the 31,000 people who rely on federally funded public housing at the San Francisco Housing Authority, which is technically not a city agency. The authority relies on monthly draws of federal funding, and while it should be able to receive money for October, "a prolonged shutdown could freeze funding" for November, according to the city memo.

The loss or delay of $3.5 million in operational and capital funding for the Housing Authority "could be devastating," the memo says. The agency is still struggling to right itself after being hammered by controversy earlier this year. It was on the brink of financial collapse, it was the subject of a series of highly critical audits, its executive director was fired, and many of its administrative staffers were laid off.