Congress is poised to approve the costliest water navigation and restoration project in U.S. history later this month, despite several studies suggesting the massive Mississippi River overhaul may not be needed.

The question of whether lawmakers should let the U.S. Army Corps of Engineers embark on a $3.1 billion construction and ecosystem project on the upper Mississippi and Illinois rivers has angered environmental and taxpayer groups as well as some Republicans and Democrats. But with a powerful coalition of midwestern senators, agricultural producers and barge operators behind it, the project is favored to make it into law.

Proponents of the plan, which the House is expected to take up this week, said it will help the nation's farmers, by speeding the shipment of grain and other goods down the Mississippi River. Locks operate as a staircase on the river, raising or lowering vessels so they can meet the river's level as they move upstream or downstream. Delays at the locks because of malfunctions can last hours.

Paul C. Rohde, who represents barge operators, corn and soybean farmers and other midwestern financial interests, said federal authorities have for years failed to adequately fund the vital shipping route. The Corps has projected river traffic could rise as much as 45 percent over the next two decades.

"It's an 80-year-old infrastructure that's been underfunded," said Rohde, president of Midwest Area River Coalition 2000, adding that last summer a lock failure just north of St. Louis tied up 60 barge tows for a few weeks. "In agricultural terms, we're milking the same cow and forgetting to feed her."

The driving force behind the plan is Sen. Christopher S. Bond (R-Mo.), a member of the Senate Environment and Public Works Committee, who included the project in a water project authorization bill headed for a floor vote by the August recess. Bond, who says river transport is cheaper and more environmentally friendly than rail or truck shipments, argues that the nation cannot afford a drop in river traffic.

"We need to modernize the system to keep us competitive in an ever-increasing global marketplace," Bond said. "Brazil is currently upgrading its transportation networks to feed its export ports, which are located well inland. We need to do the same before we fall behind and further erode U.S. transportation advantages."

But the project's critics question why the federal government would pour so much money into the upper Mississippi at a time when traffic is declining. Between 1990 and 2004, traffic through the five major locks dropped 40 to 45 percent, partly because midwestern growers sold an increasing amount of corn to nearby ethanol plants and shipped some goods by rail out West. Delays on the river are declining partly due to less traffic.

"When you're spending this kind of money, it's got to be justified," said Rep. Jeff Flake (R-Ariz.), who plans to offer an amendment this week to provide for the project's construction only if traffic rises 16 percent at seven major locks between 2007 and 2009. That would bring river traffic back to the level it was at during much of the 1990s, and it is the lowest level at which the Corps says the project would be financially justified.

The Corps has pushed for the project, suggesting that under the most likely scenario, river traffic between 2000 and 2025 would increase 30 percent.

"In today's economy we must take every opportunity to give our farmers the competitive edge of getting their products to market in the most efficient way possible," the Army Corps' Chief of Engineers Carl A. Strock said in a December statement endorsing its construction. But the Corps has an uneven track record on traffic projections: The agency's commercial traffic projections have materialized only two of 14 times on waterway projects it has constructed since World War II, environmental advocates say.

Analysts at the Congressional Research Service and the National Academy of Sciences have concluded that the Corps' projections are overly optimistic: A July 15, 2004, CRS report said, "There appears to be substantial potential for future agriculture-related barge demand to fall short of investment proponents' expectations."

Jerry Van Der Kamp, chief executive of the regional grain cooperative AGRI Industries, said with fewer bushels of grain going down the Mississippi, it makes less sense to invest in the waterway.

"It's hard to make a case based on solid data that there's a big increase in export grain along the Mississippi River that we can see in the future," Van Der Kamp said. "Given that, it's hard to be a strong proponent of building new locks."

That sort of analysis worries lawmakers such as Rep. Ron Kind (D-Wis.), whose district contains more miles (270) along the Mississippi than any other member's. Although Kind supports more funding for river ecosystem restoration and is concerned about occasional traffic bottlenecks, he hesitates at the idea of devoting 10 percent of the nation's water infrastructure budget to a single endeavor.

"There's going to be some questions about the project and the need for it," Kind said.

In the past few years, the Corps has come under fire from the Army inspector general and other critics for slanting project analyses in favor of construction. Scott Faber, a water resources specialist at the Environmental Defense Fund, said the upcoming House and Senate votes on the Mississippi plan may determine whether the Corps reassesses the way it justifies future construction projects.

"Either Congress approves the most expensive waterway project in history, or they'll set a new course for this remarkably troubled agency," Faber said.

An upper Mississippi River lock shows wear at its door seals.