Carrying 'off the chart' amounts of moisture, Barry crawled ashore today in Louisiana and quickly weakened to a tropical storm that promised to dump heavy rains that could last for days, posing a test of the flood-prevention systems built after Hurricane Katrina 14 years ago.
The storm made landfall near Intracoastal City, Louisiana, about 274km west of New Orleans, and its winds fell to 113kph, the National Hurricane Center said.
The Coast Guard rescued more than a dozen people from the remote Isle de Jean Charles, south of New Orleans, where water rose so high that some residents clung to rooftops. But in the city, locals and tourists wandered through mostly empty streets under a light rain or stayed indoors.
Video showed water overtopping a levee in Plaquemines Parish south of New Orleans, where fingers of land extend deep into the Gulf of Mexico. Officials were still confident that the levees would hold firm.
More than 70,000 customers were without power this morning, including nearly 67,000 in Louisiana and more than 3000 in Mississippi, according to poweroutage.us.
Hours earlier, the storm had strengthened into a Category 1 hurricane, with maximum sustained winds of 120kph, just above the 120kph threshold to be a hurricane. Barry was expected to continue weakening and become a tropical depression tomorrow.
The system threatened disastrous flooding across a swath of the Gulf Coast.
Why this hurricane could devastate the local environment
Impact could range from accelerating runoff of farmland nutrients to toppling trees and damaging wildlife habitat and fisheries, scientists say.
But the extent of the damage – and whether it will be at least partially offset by benefits such as disruption of the notorious Gulf of Mexico 'dead zone' – is hard to predict, they say.
That's because the region faces a rare one-two-three punch: the storm's anticipated tidal surge and torrential downpour, combined with record-high water levels in the Mississippi River.
'We don't know how the system is going to respond to all this because it's so unusual,' said Melissa Baustian, a coastal ecologist with the Water Institute of the Gulf in Baton Rouge, Louisiana.
Multiple forces are at play
One of the wettest-ever springs in the nation's heartland engorged the Mississippi, sending massive volumes of water southward toward the Gulf.
Levees and dams were breached and millions of acres of cropland flooded in the Midwest.
Barry threatens to hurl a storm surge of up to one metre onto coastal regions. And forecasters said the hurricane could stall inland and dump up to 61cm of rain.
Evacuations ordered as Tropical Storm Barry lands first blow on the US on Louisiana
How this could create an oversized 'dead zone'
Rainfall washes manure and chemical fertilisers from Midwestern corn and soybean fields into streams, smaller rivers and eventually the Mississippi.
The nutrients – especially nitrogen – overfeed aquatic plants that eventually die and decompose, leaving a large section of the Gulf with little or no oxygen each summer.
The National Oceanic and Atmospheric Administration predicted that this year's dead zone will be roughly the size of Massachusetts.
Large storms can shrink the zone by churning the water column and replenishing oxygen levels in deeper areas. That could be a positive, if short-lived, outcome of Barry's rampage, Ms Baustian said.
Yet even if this year's oxygen-depleted area winds up smaller than expected, historical data suggests that Barry – like other big storms – will flush additional nutrients into the Mississippi and other Gulf tributaries, leading to bigger dead zones in the long run, said Anna Michalak of the Carnegie Institution for Science in Stanford, California.
'If you get a few sprinkles over time, the water has time to infiltrate and there's not much runoff,' said Ms Michalak, who studies effects of climate change on water quality.
'But a single extreme event overwhelms the capacity of the soil and the ecosystem to absorb it, and much of it ends up flowing down the rivers and ultimately off to the coast.'
Even if Barry's winds remain barely powerful enough for the storm to qualify as a hurricane, they could topple lots of trees, their roots weakened by the saturated ground, said Tim Carruthers, coastal ecology director with the Water Institute.
What Barry could do to local waterways
Also vulnerable are Louisiana's famed coastal marshes, already hammered by development and flood control measures that prevent natural coastal shoreline replenishment.
Scientists also will keep watch for stranded dolphins. About 290 have been found along the Gulf coast since February 1 – triple the usual number – and nearly all have died.
It's unclear why, although one possibility is exposure to fresh water coursing into the Gulf from flooding rivers and a Louisiana spillway that diverts overflow from the Mississippi, said Teri Rowles, head of a NOAA program on stranded marine mammals.
The spillway, built in 1938 to provide a safety valve during flooding, has been open for a record number of days this year, said David Muth of the National Wildlife Federation's Gulf Restoration Program.
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