Officials confirmed that part of the Three Mile Bridge in Pensacola, which was being reconstructed after being damaged by previous hurricanes, is missing.
Sally, now a tropical storm, brought "life-threatening" 105mph winds and rain across the American South when it made landfall as a category two storm at 4.45am local time on Wednesday.
Parts of Pensacola have been swamped with floods as high as 1.5m (5ft).
By Wednesday night, Sally was just north of the Florida Panhandle with winds of up to 45mph, spreading heavy rain into eastern Alabama and western Georgia.
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The storm is moving at speeds of just 3mph - about as fast as a person can walk - and officials have warned that thousands of people will need to flee rising waters in the coming days.
David Morgan, the sheriff of Escambia County in Florida, said: "There are entire communities that we're going to have to evacuate. It's going to be a tremendous operation over the next several days."
Although the storm is expected to weaken as it moves farther inland, heavy rainfall is expected to continue well into Thursday.
One forecaster in the Alabama city of Mobile, David Eversole, said: "It's not common that you start measuring rainfall in feet. Sally's moving so slowly, so it just keeps pounding and pounding and pounding the area with tropical rain and just powerful winds. It's just a nightmare."
Forecasters have nearly run through the alphabet of storm names, with two-and-a-half months left in the season, with Hurricane Teddy, currently in the Atlantic, heading for Bermuda.
"We've only got one name left," said Jim Foerster, a chief meteorologist. "That's going to happen here soon, Wilfred, and then we'll be into the Greek alphabet."
Like the wildfires raging on the West Coast, the onslaught of hurricanes has focused attention on climate change, which scientists say is causing slower, rainier, more powerful and more destructive storms.
More than 377 people have been rescued from flooded areas in Escambia County (which includes Pensacola) by Wednesday afternoon.
More than 40 people trapped by high water were brought to safety within a single hour, including a family of four found in a tree, Sheriff Morgan said.
In some areas, such as Orange Beach in Alabama, rescue operations were hampered by the conditions.
The city's mayor, Tony Kennon, said: "We got a few people that we just haven't been able to get to because the water is so high. But they are safe in their home, as soon as the water recedes, we will rescue them."
The National Hurricane Centre has likened Sally's slow pace to that of Hurricane Harvey, which inundated the Texas city of Houston back in 2017.
President
Donald Trump issued emergency declarations for parts of Florida, Alabama, Mississippi and Louisiana.
White House press secretary Kayleigh McEnany said on Fox News that Trump was in contact with the states' governors and ready to help "in every way possible".
More than 540,000 homes and businesses are without electricity in south Alabama and the Florida Panhandle.
