Des Moines

Levees are not alone in their vulnerability to pests. In Iowa, engineers say the appetites of gophers and muskrats can endanger dams across the state. The dam at Fort Des Moines Park, for example, has a large hole created by rodents, along with overgrown vegetation blocking an emergency passageway. It is one of 31 dams in the state that authorities consider deficient. In July, a 92-year-old dam near Delhi, Iowa, failed, destroying a beloved lake. Another dam at Lake Ponderosa, surrounded by 730 homes, was also deemed unsafe.

Simple mechanical wear and tear can threaten enormous floods, according to Iowa’s Department of Natural Resources. The gates used by dams to regulate water flow are vulnerable to a Catch-22: dam operators are reluctant to raise the gates, which resemble king-size garage doors, fearing they will seize up, but failing to move them allows gears to rust and become inoperable.

Pittsburgh

The Monongahela River, a major inland shipping route, houses two of the nation’s oldest continually operated locks; each is a half-century past its intended lifespan. “It’s the largest, oldest and arguably most fatigued inland waterway system in the United States,” said Jeff Hawk, a spokesman for the Army Corps of Engineers.

Everyday detritus, like car rims, tires and logs floating in the river, can damage the underwater wooden seals used by the lock’s gate system, which allows vessels in and out. “Steel cables fall out of commercial tugboats and smash the wood seals,” said Charlie Weight, the lockmaster. The seals are original: wood has been shunned by lock manufacturers since the 1950s. Steel bolts at important locations also have a tendency to become loose.

The lock at Elizabeth, Pa., dates from 1907, and through it travels about 12 million tons of cargo a month. Maintenance costs $2 million annually; Mr. Hawk compared maintaining it to “working on an old junker in the yard.” A breakdown would snarl the region’s coal distribution, clogging highways with extra tractor-trailers and pushing up gas and electricity rates throughout the Pittsburgh region. Federal funds to replace the lock have not been forthcoming. “This thing was supposed to be blown out of the water several years ago,” Mr. Hawk said cheerfully.

New York City

In 2005, at a kitchen-size relay room in the Chambers Street station in Lower Manhattan, a fire destroyed hundreds of antique switches and circuits, nearly crippling two subway lines for months and disrupting the commutes of 580,000 New Yorkers. It could happen again tomorrow. The subway system has about 480 relay rooms, 25 of which still use technology that was in place when the subway opened in 1904. Only two companies in the world can repair the antiquated signals, which help locate trains in the tunnels. In 2005, it could have been worse: officials said the room that caught fire was one of the least critical in the system.

Anchorage

Surprise: This fossil lives. In Anchorage, home to an estimated 285,000 people, parts of the subterranean water system still use wooden pipes, carved of white cedar and wrapped in wire, a relic of pioneer days when hollowed-out logs were the water conveyance of choice. But Alaskans, apparently, need not worry about splinters in the throat. “What we’ve found, kind of unbelievably, is that the wooden water mains wrapped in that wire actually perform better than ductile iron,” said James Weise, who manages the state’s drinking water program. “They are less susceptible to fracture due to earthquake activity, and they are more flexible.” Although Mr. Weise said the Depression-era wooden mains never leak, they are gradually being replaced with ductile iron pipes. “They pull them up, and the wood looks incredibly fresh,” he said. It could be another century before we know if those new pipes are as durable.