As the U.S. and China endure record-breaking floods this spring, there is a risk that is being overlooked amidst the inundated towns, evacuations and rising waters. Dams in the U.S. boast an average age of 50 years, and the American Society of Civil Engineers continues to give the nation's dams a D grade overall in terms of maintenance. Will it take the catastrophic collapse of a dam—like the five in the 1970s in the U.S. that killed hundreds—before the infrastructure is repaired?

The nation's more than 80,000 dams have served us well—restraining less-than-epic floods and generating billions of kilowatt-hours of electricity for regional grids. In fact, massive dams across the western U.S., like Grand Coulee in Washington state, still provide the vast majority of "renewable" electricity in the U.S., some 7 percent. At the same time, hydropower can help balance more intermittent renewable resources, such as wind power. For example, water can be held back water to cope with "wind droughts," prolonged periods of little or no wind such as an 11 day wind drought in the Pacific Northwest earlier this year.

But these dams of legend—that helped win World War II as the poster illustrating this post implies—are old. And old dams are in danger of failure—more than 4,000 in the U.S. alone are at high risk of imminent failure, according to the Association of State Dam Safety Officials.

Editor's note: The original broadcast incorrectly implied that the annual U.S. military budget is $60 billion. The actual figure is closer to $685 billion.

On a per kilowatt-hour basis, dams are the most dangerous source of electricity generation, followed by burning coal with its attendant mining accidents and deaths via heavily polluted air. A tsunami flood crippled Fukushima Daiichi in Japan—prompting the meltdown of three nuclear reactors that have, so far, killed no one. A series of dam failures in China in the 1970s killed more than 200,000 people. Similarly, more than 500,000 have been evacuated in central and southern China this month due to flooding and mudslides, whereas about 80,000 have been relocated due to the nuclear plant disaster in Japan.

In fact, the filling the reservoirs behind new big dams in China may have helped trigger the deadly 2008 earthquake in Sichuan Province. As a result, the Chinese government has admitted that its most massive dam—Three Gorges—has "urgent problems," ranging from "geological disaster prevention" to the ongoing relocation of millions of people.

Nevertheless, the world is embarking on a new renaissance of big dam building; just this month Brazil gave final approval to move forward with the massive Belo Monte dam in the Amazon region of Para state on the Xingu River, which will be able to produce more than 11 gigawatts of power. Meanwhile, the world's older dams are in dire need of refurbishment, lest the floods burst their bounds.