Seattle Wash. Portland Mont. Minn. N.H. N.D. Vt. Maine Ore. Idaho N.Y. Wis. Boston Minneapolis S.D. Mass. Mich. R.I. Wyo. Detroit Conn. Iowa Chicago New York Pa. Neb. Ohio N.J. San Francisco Nev. Ind. Md. Del. Ill. Denver Washington, D.C. Utah Kansas City W. Va. Colo. Mo. Va. Kan. Las Vegas Ky. Calif. Raleigh Ariz. N.C. Okla. Los Angeles Tenn. Ark. N.M. Phoenix S.C. Atlanta Ala. Ga. Miss. Texas Fla. Alaska Houston La. Anchorage Honolulu Age of Major Dams Miami Hawaii 0 yrs 50 100 150 Seattle Portland Wash. N.H. N.D. Mont. Vt. Maine Mich. Idaho Ore. Wis. N.Y. Boston S.D. Minn. Mass. R.I. Wyo. Detroit Conn. Iowa New York Chicago Pa. Neb. Ohio N.J. Nev. Ind. Md. Del. Ill. Denver Washington, D.C. Utah W. Va. Colo. Va. Las Vegas Kan. Ky. Raleigh Mo. Calif. Ariz. N.C. Okla. Tenn. Los Angeles Ark. N.M. Phoenix S.C. Ala. Ga. Miss. Texas Fla. Alaska Houston La. Anchorage Honolulu Age of Major Dams Miami Hawaii 0 yrs 50 100 150 Wash. N.D. N.H. Vt. Mont. Mich. Maine Idaho N.Y. Wis. Ore. Mass. S.D. Minn. Wyo. R.I. Iowa Pa. Conn. Neb. N.J. Ohio Ind. Del. Ill. Md. Nev. Colo. Utah W. Va. Va. Kan. Mo. Ky. Calif. Ariz. N.C. Okla. Tenn. Ark. N.M. S.C. Ala. Ga. Miss. Fla. Texas Alaska La. Age of Major Dams Hawaii 0 yrs 50 100 150 Maine N.D. Mont. Vt. Mich. Ore. Idaho Mass. N.Y. S.D. Minn. Wyo. Iowa Pa. Neb. N.J. Ind. Nev. Ill. Colo. Utah Mo. Kan. Va. Ky. Calif. Ariz. N.C. Okla. Ark. N.M. S.C. Ala. Ga. Miss. Texas Fla. La. Age of Major Dams Alaska Hawaii 0 yrs 50 100 150 Me Vt ND Mt Id NY Mi Ma Or SD Wy Ia Ne NJ Nv Il Co Ut Mo Ks Ca Az NC Ar NM Ms Al Ga Fl La Tx Ak HI Age of Major Dams 0 yrs 50 100 150 Vt ND Mt ID Mi NY Or SD Wy Ia NE NJ Nv Il UT Ks MO Ca NC Az NM Ar AL Ga Ms Fl LA Tx Ak HI Age of Major Dams 0 yrs 50 100 150 Source: U.S. Army Corps of Engineers

After two weeks that saw evacuations near Oroville, Calif., and flooding in Elko County, Nev., America’s dams are showing their age.

Nearly 2,000 state-regulated high-hazard dams in the United States were listed as being in need of repair in 2015, according to the Association of State Dam Safety Officials. A dam is considered “high hazard” based on the potential for the loss of life as a result of failure.

By 2020, 70 percent of the dams in the United States will be more than 50 years old, according to the American Society of Civil Engineers.

“It’s not like an expiration date for your milk, but the components that make up that dam do have a lifespan.” said Mark Ogden, a project manager with the Association of State Dam Safety Officials.

The United States Army Corps of Engineers keeps an inventory of 90,000 dams across the country, and more than 8,000 are classified as major dams by height or storage capacity, according to guidelines established by the United States Geological Survey.

Dam failures can have serious consequences.

After the Twentyone Mile Dam in northern Nevada burst this month, floods forced delays or rerouting for more than a dozen freight and passenger trains on a main rail line that runs through the area. Stuart Johnson/The Deseret News, via Associated Press

Two weeks ago, heavy rains caused the Twentyone Mile Dam in Nevada to burst, resulting in flooding, damaged property and closed roads throughout the region.

The earthen dam, built in the early 1900s and less than 50 feet tall, is one of more than 60,000 “low hazard” dams, according to the Army Corps of Engineers. Typically, failure of a low hazard dam would cause property damage, but it would most likely not kill anyone.

Twentyone Mile Dam Reno NEVADA Las Vegas Hazard Potential of Dams Low Significant High Undetermined Twentyone Mile Dam Reno NEVADA Las Vegas Hazard Potential of Dams Low Significant High Undetermined Source: Nevada Division of Water Resources

Last week, 180,000 people downstream from Oroville Dam in California were evacuated after an emergency spillway showed signs of failing.

Built in the 1960s and more than 16 times the height of the Nevada dam, Oroville was listed as a high hazard dam. Had it not been for the speed of the response last week, there could have been severe flooding of the surrounding area.

“The larger dams are being watched very carefully. The smaller dams don’t enjoy that level of scrutiny,” Mr. Ogden said.

The U.S. would need to spend billions to repair public and private dams.

Seattle Wash. Portland Mont. N.H. N.D. Vt. Minn. Maine Ore. Minneapolis Idaho N.Y. Boston Wis. S.D. Mass. Mich. R.I. Wyo. Conn. Detroit Iowa Chicago New York Pa. Neb. N.J. Ind. Columbus San Francisco Ill. Md. Del. Utah Nev. Ohio W. Va. Denver Washington, D.C. Kansas City Colo. Va. Kan. Las Vegas Mo. Ky. Raleigh Calif. Ariz. N.C. Okla. Tenn. Los Angeles N.M. Phoenix Atlanta S.C. Ark. Miss. Ala. Ga. La. Texas Fla. Alaska Houston Anchorage Honolulu Ownership of Major Dams Miami Hawaii Private Public Unknown Seattle Wash. Portland N.H. N.D. Mont. Vt. Minn. Maine Ore. Wis. Idaho N.Y. Boston S.D. Mass. R.I. Mich. Wyo. Detroit Conn. Iowa Chicago New York Pa. Neb. Ohio N.J. Ind. Ill. Md. Del. Nev. W. Va. Denver Washington, D.C. Utah Colo. Va. Las Vegas Kan. Ky. Raleigh Mo. Calif. Ariz. N.C. Okla. Tenn. Los Angeles Ark. Phoenix N.M. Atlanta S.C. Miss. Ala. Ga. Texas Fla. Alaska Houston La. Anchorage Honolulu Ownership of Major Dams Miami Hawaii Private Public Unknown Wash. N.H. N.D. Mont. Vt. Maine Ore. Idaho N.Y. Mass. S.D. Minn. Wis. Wyo. Mich. R.I. Conn. Iowa Pa. Neb. N.J. Ohio Ill. Ind. Del. W. Va. Md. Nev. Utah Colo. Kan. Va. Mo. Ky. Calif. Ariz. N.C. Okla. Tenn. Ark. N.M. S.C. Ala. Ga. La. Miss. Texas Fla. Alaska Ownership of Major Dams Hawaii Private Public Unknown Wash. N.H. N.D. Vt. Mont. Maine Ore. N.Y. Idaho Mass. S.D. Minn. Wyo. Wis. Mich. R.I. Iowa Conn. Pa. Neb. N.J. Ind. Ill. Ohio Del. Utah W. Va. Colo. Md. Nev. Mo. Kan. Va. Ky. Calif. Ariz. N.C. Okla. Tenn. N.M. S.C. Ark. Miss. Ala. Ga. La. Texas Fla. Ownership of Major Dams Alaska Hawaii Private Public Unknown WA Vt ND Mt Id Or NY Wi Ma Mn SD Wy Mi Ia Ne NJ In Il Co Ut Nv Mo Va Ks KY Ca Az NC Tn Ar NM SC Ms Al Ga Tx La Ak Fl HI Ownership of Major Dams Private Public Unknown ME ND Mt MN Or NY SD Wy Mi Ia PA NJ OH Il Nv MO VA Ks Ca NC Az NM Ms Ga Tx Fl Ak HI Ownership of Major Dams Private Public Unknown Source: U.S. Army Corps of Engineers

In 2016, the Association of State Dam Safety Officials estimated that it would cost $60 billion to rehabilitate all the dams that needed to be brought up to safe condition, with nearly $20 billion of that sum going toward repair of dams with a high potential for hazard.

In 2015, Representative Sean Patrick Maloney, Democrat of New York, introduced the Dam Rehabilitation and Repair Act to provide grant assistance to rehabilitate publicly owned dams that fail to meet minimum safety standards. A version of the legislation that covered private dams — which account for more than half of the dams in the United States — was incorporated into a larger water infrastructure bill and signed into law by President Barack Obama in December.

Oroville Dam is owned by the state of California, but the Twentyone Mile Dam is owned by Winecup Gamble Ranch, a cattle operation in northeastern Nevada.

While most legislation involves inspection and rehabilitation, hazardous dams that have outlived their usefulness can also be removed.

1920 1940 1960 1980 2000 60 40 20 0 dams removed per year 1920 1940 1960 1980 2000 60 40 20 0 dams removed per year Source: American Rivers

“The very fact that you are blocking a river and allowing a reservoir to fill up with millions of gallons of water presents a risk,” said Bob Irvin, the president of American Rivers, an environmental conservation and advocacy organization. The group has partnered with the State of Maryland to remove the Bloede Dam, built in 1907, where several people have drowned over the years.