“Get in your cars and leave this area at once or you will be underwater soon.”

It was a voice blasting from a police bullhorn near Roosevelt Island. A helicopter flew over the island broadcasting from a loudspeaker: “Get out of here now. This is the police. Get in your cars and leave the area at once. Your lives are in danger.”

At that moment, a boulder of ice pushed three feet out of the Potomac River and crashed onto the footbridge, shattering into pieces and causing one women to fall. The woman was helped to her feet and people decided it was time to heed the booming voices from the bullhorns.

It was Jan. 28, 1978 — a Saturday — and a large ice floe on the Potomac River had started to form a massive dam, preventing water from continuing downstream. As the river rose, it pushed large blocks of ice onto land and the footbridge, according to a report published by The Washington Post the next day titled, “Ice floe threat along Potomac.”

“For many people yesterday, the ice was the party,” The Post wrote. “Cars stopped at random along the parkways. MacArthur Boulevard, the Whitehurst Freeway and just about any other street near enough to the river to gawk, snap pictures and draw glares from any nearby police officers.”

When a long stretch of cold weather produces thick ice on a river and is then quickly followed by much warmer weather or heavy rain, the ice pack can break up and flow downstream in an ice floe. If the river narrows, or has obstructions such as bridge pilings or islands, the ice blocks can get stuck and accumulate to form an ice dam.

Behind the  dam, the water will rise and push other blocks of ice onto land. When the ice dam finally breaks, a flash flood of water and ice will rush downstream, which can sometimes break trees and crush buildings in its path.

Small ice floes on the Potomac River are common, but the large ice dams near Washington are somewhat rare.

The ice floes and jams of January 1978 on the Potomac River were bad, but not nearly as bad as what occurred during mid- to late February 1918, one of the most catastrophic ice dams in D.C. history. News clippings from the time tell of 10-foot walls of ice plowing into the Georgetown Waterfront after the winter’s first thaw.

On Valentine’s Day that year, the ice dam stretched from the Highway Bridge at Potomac Park to High Island above Chain Bridge. It was 15 feet high in places, according to a Washington Post article dated Feb. 15, 1918.

The flood associated with the 1918 ice dam pushed water 16 feet above normal in Georgetown, which was the highest water level since the flood of 1889. Ice blocks crushed buildings along the Georgetown Waterfront.

U.S. Army engineers ultimately used dynamite to break up the ice jam of 1918, according to the same Washington Post article. After the explosion, “water rushed down the river at approximately 20 miles an hour, carrying ice, logs, trees and debris,” The Post wrote.

In 1948, a particularly large ice dam formed near Washington, threatening to flood the city.  The Coast Guard dispatched the  cutter Mohican to break through the dam to prevent flooding, according to a Washington Post article dated Feb. 17, 1948.

The plan wasn’t exactly successful. When the ice dam of 1948 burst, “giant slabs of ice went rumbling down the river. Trees along the banks were snapped like toothpicks,” the story recalls. And bridge pilings and wood fenders of Highway Bridge and Memorial Bridge were damaged by the pressure of the ice.

Soon after the Mohican broke up the large ice dam, a second ice dam formed above Chain Bridge, but Army engineers stated that dam was “somewhat less serious” than the first dam downriver.