On the West Coast, four large dams on the Klamath River, which flows from Southern Oregon into Northern California, are slated for removal by 2022, streamlining some 400 miles of habitat for migratory fish.

On the East Coast, efforts are underway in Connecticut to eliminate obsolete dams from rivers that connect with Long Island Sound.

State officials are motivated partly by a concern for public safety, since aging dams can suddenly give way. “We have government regulations that say we are not going to let you let your dam collapse and kill people,” said Stephen Gephard, a supervising fisheries biologist for the Connecticut Department of Energy and Environmental Protection. “Hazardous dams have to be inspected every other year.”

Connecticut has about 4,000 dams, Dr. Gephard said, and the vast majority of those are obsolete. The state owns about 100 dams and is reviewing the list to determine which should be removed. But Dr. Gephard’s team has identified 20 to 30 privately owned dams that it would like to remove to allow fish passage.

It can take years, Dr. Gephard said, to educate a community or a private landowner about the merits of losing their dams, a process that often involves debunking myths. Some residents fear being left with a stinking mud flat once the dam is gone, but in fact, dormant seeds quickly become trees, shrubs and grasses.

“The most bizarre myth is the notion that if you remove the dam, there won’t be any water in the stream,” Dr. Gephard said. “It’s as if they think the water is coming from this concrete. So many Americans don’t understand the concept of a watershed or flowing water.”

To that end, conservation groups have enlisted engineering firms to create photo renderings showing how a river would appear without a dam, Dr. Gephard said.