Widening the Pearl River near Jackson, Mississippi, is the best way to prevent flooding in the area, a Mississippi levee district concluded in a long-awaited draft environmental impact statement released last week.

But the controversial project has drawn heated opposition from southeast Louisiana and the Mississippi Gulf Coast, where government officials, environmentalists and fishing interests fear that the project — which would create a reservoir or "lake" several miles long — will cause a decrease in water flow to the lower Pearl River basin.

Some opponents contend the project is being driven by development interests in Mississippi that see the potential for waterfront property along the new lake.

The project was authorized by Congress in 2007 but has not been funded. The estimated cost is $350 million, with a three-year construction timeline.

The more than 300-page draft document doesn't say much about the project's impact on the lower Pearl River system. But the Rankin-Hinds Pearl River Flood & Drainage Control District, the project's local sponsor, says on its website that the issue has been studied "extensively" and that there will be no discernible variance in the amount of water flowing into the new lake from the existing Ross Barnett Reservoir and the amount flowing out of the lake to the downstream river.

Flow requirements set for the Ross Barnett Reservoir will apply to this project, the site says, and emergency gates have been added to the design of a proposed underwater dam that can be opened to allow the minimum flow to continue during extreme drought conditions.

About two-thirds of the Pearl River watershed relies on runoff from south of the project location, the site says, "so the Jackson area is not the only driver of water levels in the southern parts of the watershed."

But St. Tammany Parish officials, who have opposed the project for years, worry that changes in the Pearl's flow during critical times of the year could alter the fragile wetland ecology in places like the Honey Island Swamp.

Andrew Whitehurst, of the Gulf Restoration Network, said the project's sponsors chose to look at evaporation rates from the lake on an annual basis, losing what he called the nuance of looking at the low-flow months of June through September.

"We've said we know it's not a problem year-round, but they didn't pull those months out and treat them differently," he said, despite requests by environmental groups that they do so.

Changes in salinity also require a more nuanced examination, he said. "Every month of the year matters," he said.

Whitehurst called the project an experiment. And while the levee district says there won't be a significant loss of water that will hurt the river south of Jackson, "we don't have the modeling to back that up," he said.

The Rankin-Hinds flood control district has been pushing for the project for years. According to the environmental impact statement, the district initially considered 16 alternatives for reducing the flooding threat around Jackson posed by the Pearl. It narrowed the list to four: doing nothing, relocating structures from flood-prone areas, building additional levees and widening the Pearl's channel — the "lake" alternative.

The project will entail moving an existing underwater dam 4 miles to the south and excavating 25 million cubic yards of material from the river's channel. The levee district calls it the most technically feasible, environmentally sound and cost-effective alternative.

At $350 million, it's by far the least expensive of the alternatives, except for doing nothing.

Relocating threatened structures — including homes, businesses, government buildings, hospitals and schools — would be the costliest option, topping $2 billion, an amount the district called cost-prohibitive, according to the draft statement.

Adding levees would cost an estimated $730 million, but the risk of overtopping and levee failure would remain.

Flood control in the Jackson area has been studied for decades, with levees constructed in the 1960s. But much of the area remains unprotected, and the study cites the record flood on Easter Sunday of 1979, which placed most of Jackson underwater and would cause more than $1 billion in damage if it happened today. Another significant flood happened in May 1983.

Environmental issues addressed in the study include the potential impact on two endangered species — the gulf sturgeon and the ringed sawback turtle. 

However, environmental groups are criticizing the draft statement for failing to include a biological assessment, as required by the National Environmental Policy Act. The document says that assessment is "to be added."

Jill Mastrototaro, of Audubon Mississippi, said the omission means her group, other stakeholders, natural resources agencies and the public are unable to do a science-based review of the project's impact, including on the hydrology downstream.

"Such huge deficiencies send a signal that the proponents are not taking this environmental review process very seriously," she said.

Groups like the Gulf Restoration Network are pushing to extend the public comment period on the draft document from 45 days to 60, Whitehurst said, and they don't want the clock to begin until the biological assessment is added.

The current deadline for public comment is Aug. 3.

The Rankin-Hinds district has said it will hold a public meeting in the downstream area, but the only meeting now scheduled is in Jackson on July 24.

State Sen. Sharon Hewitt, R-Slidell, has invited the district to hold a meeting in St. Tammany Parish.

Whitehurst said he expects a "tussle" over the time limit. But there's also going to be a larger fight over the science.

"Its going to be a battle of experts," he said.

Follow Sara Pagones on Twitter, @spagonesadvocat.