LEWISTON, Idaho — Cargo barges sail serenely between Portland and Lewiston, Idaho, the most inland seaport on the West Coast.

But rough waters may lie ahead, because some environmental activists oppose the federal government’s latest plan to dredge a shipping channel on the lower Snake River. Dredging is necessary every few years to keep the channel in front of the Port of Lewiston deep enough for barges.

Critics say the Lewiston seaport — located 465 miles from the Pacific Ocean in a deep Idaho gorge — suffers from declining business, and dredging amounts to a huge taxpayer subsidy to shippers.

“Costs are rising, use is dropping and taxpayers won’t continue to foot this bill,” said Sam Mace of Save Our Wild Salmon, one of a coalition of environmental groups which in late March submitted comments to the U.S. Army Corps of Engineers in which they opposed dredging.

Navigation from the Pacific Ocean to Lewiston is possible because the federal government in the 1960s and 1970s built four massive hydroelectric dams, equipped with locks, on the upper Snake River. The reservoirs keep water levels high enough for barging. Environmental groups for years have tried unsuccessfully to remove the dams to restore runs of wild salmon that were decimated when the huge concrete structures blocked migration routes.

But barge traffic has many supporters, including the Port of Lewiston, which receives grain from as far away as the Dakotas. The grain is barged to Portland, where it is loaded on cargo ships bound for Asia. The river highway carries other cargo, and even small cruise ships, up to Lewiston and adjacent Clarkston, Wash., which form a community of 50,000 people at the confluence of the Snake and Clearwater rivers.

The cargo barges are an incongruous sight on the Snake River, as they move past the bare brown hills of an arid and lightly populated farming region known as the Palouse.

The U.S. Army Corps of Engineers is preparing an environmental impact statement for the dredging. The Corps says it is required by law to maintain a shipping channel that is 14 feet deep and 250 feet across, spokesman Bruce Hendrickson said. Barges can get grounded on sediment if the channel is not dredged.

The coalition of environmental groups, which tried and failed to halt the dredging the last time it occurred in 2005-06, recently submitted comments in which they contended the Corps is not seriously looking at alternatives to dredging.

“Dredging is a foregone conclusion,” the environmental groups contended. The Corps should more seriously consider alternatives that don’t require dredging, such as loading barges with less cargo, stopping shipping during low water periods or diverting more cargo to rail and truck transport, the groups said.

The Corps will consider all comments before issuing a final decision this summer. The earliest the dredging could occur is next winter, Hendrickson said.

The Corps has not estimated how much dredging would cost, because it will be collecting bids to perform the work and federal rules prevent it from setting a price before that. Critics estimate the dredging will cost millions of dollars. The Corps wants to dredge more than 470,000 cubic yards of sediment near the confluence of the two rivers. The sediment would be used to create habitat for salmon and steelhead.

“Generally dredging is expected to keep the river open for five years,” Hendrickson said.

While critics argue that the cost of the dredging is not justified by the economic benefits of the barging, the Corps did not perform a cost-benefit analysis because that is not its mission, Hendrickson said.

“Dredging is the only effective, short-term tool available to restore the navigation channel,” Hendrickson said.