DETROIT (AP) - Some environmental observers say that increases in pollution threaten to erode a generation of progress in curbing the flow of harmful chemicals into the Great Lakes, a crucial source of fresh water.

Thousands of companies flush more toxic chemicals into the Great Lakes and other waterways than laws allow with little fear of punishment, The Detroit News reported yesterday.

State and federal agencies have chronicled violations at three-quarters of the nation's largest 6,500 industrial and sewer plants over the past two years, according to the newspaper's review of federal enforcement records. Fewer than 25 percent of the facilities were punished.

"You don't really think about it when you're boating. It looks clean. But I'd like to see it cleaned up more," Dennis Moore, of Riverview, said while recently putting his boat into the Detroit River.

The launch he used is near two companies that dump more than a half-million pounds of toxins each year and is separated from a hazardous waste dump only by a chain-link fence. "You never know what's being dumped in there," Moore said.

The violations are among the causes of a six-year increase in toxic water pollution in Great Lakes waterways. And while the pollutants rarely pose an immediate health threat, some add to continued warnings against eating fish.

Over two years, 14 of the 25 largest U.S. polluters of the Great Lakes exceeded pollution limits set by the Clean Water Act.

"That's intolerable," said Eric V. Schaeffer, the former head of the Environmental Protection Agency's enforcement division, who now runs the Rockefeller Family Fund's Environmental Integrity Project.

"You're looking at people violating limits sometimes by 1,000 percent and they're doing it for years and years. It threatens the whole Clean Water Act."

State and federal watchdogs acknowledge the problem, but say limited resources keep them focused on pollution problems they contend pose larger risks.

"We can't get rid of everything. With our current technology, it's impossible," said Gail Krantzberg, who runs the Windsor, Ontario, office of the International Joint Commission, set up by the United States and Canada to monitor the lakes.

"But obviously we haven't done enough," she said. "We need to look toward the elimination of other discharges that have the potential of causing a human health threat."

Companies say they are doing what they can to cut pollution and stay within legal limits. But they say limited technology means it is impossible to get rid of toxins altogether and what they release is in concentrations that aren't harmful.

Known releases of toxic chemicals into the network of lakes and rivers that feed the Great Lakes grew from 12.5 million pounds to 15.7 million pounds between 1996 and 2001.

"We know that some of these chemicals are active at very low levels," said Jack Manno, director of the Great Lakes Research Consortium, which studies the lakes.

"I think there's a devastating impact that we don't fully understand yet."