IN THE MISSISSIPPI RIVER DELTA — Service canals dug to tap oil and natural gas dart everywhere through the black mangrove shrubs, bird rushes, and golden marsh. From the air, they look like a Pac-Man maze superimposed on an estuarine landscape 10 times the size of Grand Canyon National Park.

There are 10,000 miles of these oil canals. They fed America’s thirst for energy, but helped bring its biggest delta to the brink of collapse.

They also connect an overlooked set of dots in the Hurricane Katrina aftermath: The role that some say the oil industry played in the $135 billion disaster, the nation’s costliest.

The delta, formed by the accumulation of the Mississippi River’s upstream mud over thousands of years, is a shadow of what it was 100 years ago. Since the 1930s, a fifth of the 10,000-square-mile delta has turned into open water, decreasing the delta’s economic and ecologic value by as much as $15 billion a year, according to Louisiana State University studies.

The rate of land loss, among the highest in the world, has exposed New Orleans and hundreds of other communities to the danger of drowning. Katrina made that painfully clear.

“I remember when I was a young boy we had a camp out in the marsh,” said Don Griffin, a grocer and seafood dealer in the delta town of Leeville, which became an oil-drilling center for decades. “The same places you used to have to get around with a pirogue and a push pole now you can go with a 25-foot outboard. There’s no more marsh, which is your first barrier of defense for hurricanes.”

In Katrina’s wake, the Army Corps of Engineers has gotten the brunt of the criticism for the disaster.

Besides building suspect levees, the Corps’ mission to control waterways with spillways, floodgates, and other measures has played havoc with nature by restricting the Mississippi’s sediment and fresh upriver water from replenishing the delta’s wetlands.

There are other reasons for the disastrous wetlands loss: Human development, cypress logging, ill-advised farming on the coast, hurricanes, slipping-and-sliding geologic faults, and even a South American semi-aquatic rodent called nutria imported to Louisiana in the 1930s.

But many scientists say the oil industry’s 10,000 miles of canals — enough to stretch nearly halfway around the world — and the drilling they supported played a decisive role.

Some scientists say drilling caused half of the land loss, or about 1,000 square miles.

“The whole thing was manifest destiny written large on a marshy landscape,” an LSU professor emeritus who specializes in delta ecologies, John Day, said.

The industry denies that and points to disagreement among scientists over who or what caused damage, and how much. “I’ve got duck leases out there, and I remember when they were covered in grass. They’re all ponds now,” Don Briggs, president of the Louisiana Oil & Gas Association, said. “It’s not gone because of drilling. It’s because nutria ate all the grasses.”

However, a substantial body of evidence points to oil’s heavy toll. The canals, most dug to access wells by bucket dredges between the 1930s and 1970s when restrictions and mitigation requirements were lax to nonexistent, crisscross the marshy coast like a liquid maze.

In many places, they run perpendicular to the Gulf of Mexico shoreline, allowing salt water to intrude far inland. One spot is dubbed “The Wheel” because a series of canals looks like a wagon wheel from the air.

The muddy slop dredged from the canals had to go somewhere. Oil companies piled it where they found it, creating an estimated 13,000 miles of tide-blocking spoil banks.

R. Eugene Turner, an LSU oceanographer, has calculated that every square mile of the delta is bounded on three sides by oil-canal ridges. Mr. Turner has spent more than 30 years studying the oil industry’s footprint on the delta.

“If the water is blocked from going in, the wetlands on other side is drier for a little longer and also stays flooded longer than it otherwise would be,” Mr. Turner said. “By drying it, the land oxidizes and dries out; and if it’s wetter, it’s like leaving a lawn sprinkler on and the plants are going to die.”