TAUNTON, Mass. — Few who travel along a quiet residential street in this southeastern Massachusetts city would ever give much thought to what lies below: a rotting culvert built decades ago to convey water from a narrow, unnamed stream that empties into the scenic Taunton River.

But environmentalists are paying attention. And they are dismayed by what they are seeing.

“It’s wicked nasty in there,” says Alison Bowden, freshwater program director for The Nature Conservancy, as she slogs through mud and debris to inspect the culvert, a circular pipe bracketed by a decaying stone wall.

“If you were a fish, you would probably die,” she said.

Bowden and her organization are among those participating in an ambitious project to survey and evaluate river and stream crossings around New England – with an emphasis on culverts – using standards adopted in recent years by the Army Corps of Engineers.

The standards seek to reduce the threat of flooding from aging or structurally unsound culverts and to minimize barriers to fish and other wildlife as they navigate through culverts.

Organizers could not estimate how much the effort would cost, but said they have received some state grants and funding from environmental trusts. Much of the survey work is being done by volunteers and college interns.

On first glance, the task appears overwhelming. Scott Jackson, a wildlife biologist at the University of Massachusetts Extension, said the exact number of crossings in New England isn’t known, but he estimates there could be about 250,000 in the six-state region.

To date, teams have inspected and graded about 6,000 culverts and are compiling a database that project managers hope to make available to the public online by the end of the month.

The early data gathered so far show fewer than 15 percent meet general or optimum standards. Nearly half were found to have blockages or other obstacles that caused moderate or severe impediments to the passage of fish and wildlife.

The enormity of the problems would make any kind of quick fix unlikely, said Jackson, who helped compile the standards and spearheaded creation of the database.

“What it screams out for is some kind of prioritization process, where you don’t try to do everything at once,” he said.

Culvert failures can have devastating consequences for people. During torrential rains in October 2005, a wall of water smashed through a culvert in Alstead, N.H., sweeping away a dozen houses and leaving four dead. While such catastrophic events are rare, defective culverts can cause flooded roads or even threaten local drinking water supplies, Bowden said.

Bill Napolitano, environmental program director for the Southeastern Regional Planning & Economic Development District, said many culverts were constructed without foresight of modern-day traffic and development impacts, and have become functionally obsolete.

“They can’t possibly handle the volume of (stormwater) that comes off the road anymore,” said Napolitano. The Taunton culvert, for example, sits only about a half-mile downstream from Route 44, a major thoroughfare dotted with strip malls, car dealerships and big-box stores.

Nor can many culverts properly handle the movement of wildlife, experts contend.

Ideally, they say, culverts should be designed so that the depth and velocity of the water inside them is virtually the same as in the waterway itself. If the culvert opening is too small, the constricted water may flow too strongly; too wide, and the flow could be weakened.

“If you’re a turtle or a fish, you should be able to move through the structure without really noticing that the structure is there,” said Bowden.

The Taunton culvert is estimated to be at least 50 years old. It will likely be entered into the database as posing a potentially “severe” barrier to wildlife.