SIBA, Iraq — The Shatt al Arab, the river that flows from the biblical site of the Garden of Eden to the Persian Gulf, has turned into an environmental and economic disaster that Iraq’s newly democratic government is almost powerless to fix.

Withered by decades of dictatorial mismanagement and then neglect, by drought and the thirst of Iraq’s neighbors, the river formed by the convergence of the Tigris and the Euphrates no longer has the strength the keep the sea at bay.

The salt water of the gulf now pushes up the Faw peninsula. Last year, for the first time in memory, it extended beyond Basra, Iraq’s biggest port city, and even Qurna, where the two rivers meet. It has ravaged fresh-water fisheries, livestock, crops and groves of date palms that once made the area famous, forcing the migration of tens of thousands of farmers.

In a land of hardship and resignation and deep faith, the disaster along the Shatt al Arab appears to some as the work of a higher power. “We can’t control what God does,” said Rashid Thajil Mutashar, the deputy director of water resources in Basra.

But man has had a hand in the river’s decline. Turkey, Syria and Iran have all harnessed the headwaters that flow into the Tigris and Euphrates and ultimately into the Shatt al Arab, leaving Iraqi officials with little to do but plead for them to release more from their modern networks of dams.

The environment problem became particularly acute last year when Iran cut the flow entirely from the Karun River, which meets the Shatt south of Basra, for 10 months. The flow resumed after the winter rains, but at a fraction of earlier levels.

In the 1980s Iran and Iraq fought over the Shatt al Arab, which forms the southernmost border between the countries and is still littered with the rusting hulks of sunken ships from that war. Now, despite improved relations after the fall of Saddam Hussein, the river has once again become a source of diplomatic tension.

“The water is from God,” said Mohammed Sadoon, a farmer and fisherman in the village of Abu Khasib, who sold two water buffaloes last year because he could no longer provide them with potable water from the Shatt. “They shouldn’t seize it from us.”

Iraq’s minister of water resources, Abdul Latif Jamal Rashid, said that the environmental problems and the disputes over water rights were a lingering legacy of dictatorship.

Mr. Hussein diverted the southerly flow of water into a trench during the war with Iran and drained the marshlands of southern Iraq in the 1990s. His belligerence toward Iraq’s neighbors also left the country isolated — and then weakened — when those countries built their dams, siphoning off what for millenniums flowed through Mesopotamia, the land of the two rivers.

“Iraq was in a position neither to reject nor to cooperate with them,” he said in an interview in his office in Baghdad. “They did what they wanted to do.”

In Basra and in the villages that cling to the Iraqi shore of the Shatt, the impact of the disaster has been profound. The fresh waters that once flushed the canals of Basra — the Venice of the Middle East, it was called, though long ago — are fetid and filled with garbage.

The encroaching salt has so polluted supplies of drinking water that the government has scrambled to dig canals from the north that bypass the Shatt — Prime Minister Nuri Kamal al-Maliki inaugurated one ahead of this year’s national election — and to truck in fresh water to much of the region. Anyone who can afford it avoids tap water, which is salty enough to leave spots on a glass when it dries.

Mr. Mutashar said that Iraq’s acceptable level of salt in the Shatt’s fresh water was 1,500 parts per million; last year the level reached 12,000.

Faris Jassim al-Imara, a chemist at the University of Basra’s Marine Science Center, said he had recorded levels as high as 40,000 parts per million, as well as heavy metals and other pollutants flowing from the north and from Iran’s oil refinery at Abadan, where enormous pipes steadily discharge waste water.

“It’s killing the river and the people,” he said. Here in Siba, across the river from Abadan, the salt water is slowly destroying agriculture, the primary source of income other than oil.

Jalal Fakhir, who with his brothers farms a plot of land that has been in his family for decades, lost his grape vines, five apricot trees, and his entire crop of okra, cucumbers and eggplants. The new date palms he planted two years ago have died; the older ones have held on, but their branches are yellowing, while the annual crop of dates has become meager.

Walking in his emaciated groves, he said, “This used to be paradise.”

Iraq’s leaders, struggling first with the post-Saddam Hussein strife and now with a political impasse that has delayed the formation of a new government, have so far been unable to do much to avert the catastrophe unfolding here, let alone reverse it.

Efficient water management throughout the country remains more a goal than a reality. The government is drafting plans to build its own dam on the Shatt — to keep the sea water out — but the cost and complexity of the idea remains prohibitive, according to Mr. Mutashar.

Iraq has held repeated talks with neighboring countries to increase the river’s flow, resulting in pledges of cooperation, but with a drought hitting the region in recent years, not much more water.

“If our government was good and strong, we would get our rights,” said Hassam Alwan Hamoud, the 71-year-old patriarch of a Bedouin family that lives in reed huts on the marshlands adjoining the Shatt near Abu Khasib. Instead, they move with their water buffaloes as the salt water dictates. “Our government just talks. They are weak.”

Mr. Rashid, the minister of water resources, said the problem was decades in the making and would take decades to address.

One benefit of the country’s democracy, he said, was that the problems had become public, something that did not happen under Mr. Hussein’s rule. “It has come to the surface now,” he said, “because Iraq is a free country.”

Zaid Thaker contributed reporting.