On Chicago’s riverfront, 6-year-old Elly Weber sat on her father’s shoulders wearing some bobbly, antennae-like shamrock headgear and marveled at how the river first turned yellow and then, almost immediately, an eye-popping green.

Her 3-year-old brother, Sean, was equally stupefied.

“It’s getting all green,’’ he cried out. “Will everything turn green? Why?’’

A few kayakers couldn’t resist the chance to paddle through the unnatural-looking water. Crowds watching from a bridge roared their approval when one of the paddlers purposely half-capsized his kayak, dunking himself in the green water.

In New York, a sea of green, kilts and bagpipes flowed along 5th Avenue as large crowds gathered for the city’s 251st annual Saint Patrick’s Day Parade.

Cardinal Timothy Dolan, leader of the city’s Roman Catholics, announced before the parade stepped off that iconic St. Patrick’s Cathedral would undergo a $175 million renovation. He said the first phase will involve cleaning the cathedral’s soot-damaged exterior and replacing its windows.

Even President Barack Obama paid tribute to his Irish heritage, as one of his great-great-great grandfathers on his Kansas mother’s side emigrated from Ireland in 1850. Last year, Obama visited his ancestral home of Moneygall and drank a Guinness at the local pub.

This year, the president stopped into the Dubliner Restaurant and Pub near Washington’s landmark Union Station for a pint, accompanied by Moneygall bar owner Ollie Hayes and Henry Healy, an eighth cousin to Obama and the closest relative still living in Ireland.

Across the ocean in Dublin, an estimated half a million people turned out for the parade that serves as a focal point for worldwide celebrations. It brought some cheer to a nation that has been grappling with 14 percent unemployment, a massive debt burden and a resumption of emigration levels last seen in the 1980s.