Neighbors Josh Brockhaus, left, and Cisco Gonzales, bring their boat ashore after checking their homes in after Hurricane Isaac came through the region, in Braithwaite, La., Friday, Aug. 31, 2012. Isaac crawled into the central U.S. on Friday, leaving behind a soggy mess in Louisiana. It will be a few days before the water recedes and people in flooded areas can return home. New Orleans itself was spared, thanks in large part to a levee system fortified after Katrina devastated the Gulf Coast in 2005.(AP Photo/Gerald Herbert)

New Orleans - The remnants of Hurricane Isaac caused headaches on Friday, bringing heavy rainfall and the threat of flash flooding to the Mississippi Valley as Gulf Coast residents began cleaning up.

The first hurricane to hit the United States this year will be remembered for striking New Orleans on the anniversary of 2005's deadly Hurricane Katrina - and providing a first, successful test of the city's new $14.5 billion flood controls.

Now a tropical depression, Isaac can still trigger tornadoes as it passes over the central U.S. Midwest states, the National Hurricane Center said - among the final acts of a storm that often confounded forecasters and punched above its weight in terms of damage.

One bright spot: rain that is a godsend to Midwest farmers suffering from the worst drought in more than 50 years, even if too late for many of this season's crops.

Isaac caused widespread flooding and property damage in the U.S. Gulf Coast region, mostly because of its heavy and persistent rainfall. The system lingered near New Orleans for the better part of two days, sometimes moving as slowly as five miles an hour (8 km per hour).

Through it all New Orleans sustained mostly cosmetic damage such as downed trees and street lights.

Life was beginning to returning to normal in New Orleans on Friday, although most of the city did not have power after what utility Entergy Corp described as the fourth-largest storm it had ever faced.

National Guard troops had opened three sites around the city to distribute water and ice to residents.

New Orleans International Airport reopened early on Friday morning with the first flights scheduled to resume by midmorning local time.

Downtown and in the French Quarter, businesses were opening, some with generators or without electricity. Most stores had removed boards from windows, and some commuters headed to work.

In residential areas outside the city center, streets were littered with downed limbs and some trees were uprooted. Residents were out clearing debris.

“I am surprised how much debris there is everywhere,” said David Doucet, 55, a member of the Grammy award-winning Cajun band Beau Soleil, as he walked his dog in downtown New Orleans on Friday. “The trees have had seven years to grow since Katrina, but they didn't grow all that strong.”

Republican presidential nominee Mitt Romney, fresh from his party's convention in Tampa, Florida, was to visit Louisiana on Friday to view storm damage.

The storm caused anywhere from $700 million to $2 billion in insured onshore losses, disaster modeler AIR Worldwide said late Thursday.

That would still leave Isaac, which came onshore as a Category 1 hurricane, well outside the 10 most costly U.S. hurricanes.

New Orleans' Audubon Park recorded 18.7 inches (47.5 cm) of rain in a 24-hour period during Isaac. That exceeded records dating back to 1871, said Jeff Masters of Weather Underground. Many other locations in Louisiana and Mississippi logged more than 10 inches (25.4 cm) of rain.

Surrounding areas, though, without the new protective federal flood barriers, did not fare as well from the relentless rain and huge storm surges.

Some of the worst flooding was in Plaquemines Parish, southwest of New Orleans, where flood waters overtopped at least one levee and left many homes under about 12 feet (3.6 meters) of water.

The state advised residents of flooded neighborhoods to boil tap water before drinking it.

Late on Thursday, local officials confirmed the death of a man and a woman in the parish town of Braithwaite. They apparently drowned in their kitchen as flood waters surged in. Earlier, a Mississippi man died after being hit by a falling tree.

Slidell, a town of about 27,000 people northeast of New Orleans, took the brunt of a storm surge from Lake Pontchartrain, which left some neighborhoods under about a foot (30 cm) of water.

“You'd have never made me believe a Category 1 would dump this much water,” said Sam Caruso, 71, a former mayor of Slidell who toured the town in his pickup truck on Thursday.

As the flood waters rose, some residents, including Caruso, wondered whether the new federal levee system had shored up New Orleans at the expense of low-lying neighboring parishes outside the system's protection - a debate that is likely to continue.

Isaac killed at least 23 people in Haiti and the Dominican Republic before taking aim at the United States.

Brent crude oil was up 55 cents to $113.20 a barrel on Friday, although major oil facilities on the Gulf of Mexico made it through Isaac mostly unscathed.

Louisiana oil refineries began to power back up and most other oil and gas companies in the U.S. Gulf Coast region were preparing to restart.

Storm watchers have turned to Tropical Storm Leslie, currently 940 miles (1,510 km) east of the Leeward Islands with wind speeds picking up. Leslie could become a hurricane later on Friday, posing a potential threat to Bermuda early next week. - Reuters