When most people talk about the greening of urban spaces, they probably aren’t referring to dyeing rivers green for St. Patrick’s Day.

Yet various cities in the United States (not to mention Ireland) have decided the wearing of the green should include their waterways, starting with the Chicago River in 1962.

Tampa, Florida, has been transforming the Hillsborough River for several hours each March 17, starting in 2012. Brad Baird, the administrator of public works and utilities services for the city, said it’s become a tradition in several other cities as well, including Indianapolis (the White River), Jamestown, New York (the Chadakoin River) and San Antonio, Texas (the San Antonio River). Charlotte, North Carolina, “greens” the channel at the U.S. National Whitewater Kayaking Center, Baird says.

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The rivers might be green, but can the same be said of artificially coloring an ecosystem as though it were kids’ cereal?

Chicago won’t divulge the source of its powder, which is sprinkled in along a stretch of river and churned by motorboats to spread the Kelly-green color. Organizers of the annual coloring o’ the river say the powder is based on a vegetable dye and is perfectly safe, according to Margaret Frisbie, executive director of Friends of the Chicago River. She agrees the dye is indeed probably safe.

Yet Frisbie, whose organization advocates for restoring the river ecosystem, isn’t a fan of the tradition. She says the Chicago River, no longer largely devoid of wildlife as it was four decades ago, provides habitat for fish, beavers and other wildlife. Treating it as an urban decoration encourages people to continue throwing in trash or leftover drinks, she says, because they don’t think of it as a wild environment. Probably no one would think of dyeing the Merced River in Yosemite National Park. And why this one color for this one holiday, Frisbie wonders? Could rivers one day be dyed pink for Breast Cancer Awareness Month, and so on?

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But her organization doesn’t make an issue of it. The city has been working closely with Friends of the Chicago River to accomplish far more important environmental objectives, such as reducing sewer overflows into the river. “You don’t bite the hand that does so much to clean up the river,” she says.

The secrecy around the dye Chicago uses doesn’t help allay concerns among the suspicious. Both Chicago and Tampa use a powder that’s orange when dry and turns yellow when it comes in contact with the rivers’ waters. Asked if Chicago’s dye might be the same substance, Tampa city spokeswoman Ashley Bauman replied, “I’m sure it is.”

And Tampa’s dye is no secret: It’s a commercially sold colorant called Bright Dyes, and it’s EPA-approved for plenty of uses related to public waterways. Baird said Bright Dyes is used to detect leaks in municipal water pipes, track effluent from power plants, hunt down illegal sewer connections and help detect dangerous pollutants in waterways. It remains in the top couple of feet of the river water, then dissipates along with the water flow and degrades within 12 hours.

Which leaves only the question: Why bother turning a river shamrock-green? “Our mayor, Bob Buckhorn, is super-Irish,” says Tampa's Bauman. “On St. Patrick’s Day, he wears bright green with clovers embroidered on it.”

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Karin Klein is a California-based journalist covering health, science, education and food policy. For 26 years she worked as an assigning editor and editorial writer for The Los Angeles Times.