On most days, the tannin-filled water of a swamp appears dark and opaque. But across the South, colder weather brings a chance for this unique ecosystem to deck out in an otherworldly iridescence. When the conditions are just right, the surface of the water can take on a rainbow sheen that might look like pollution from an oil spill but actually is a product of completely natural forces.
So how do these rainbow pools happen?
Cypress swamps, which often play host to this phenomenon, are complex systems of decay and renewal. Bald cypress trees produce an oil that saturates their wood, bark, and needles, and when those needles fall in the winter, the oil seeps into the water. Meanwhile, as temperatures drop, the biochemical ballet happening beneath the water’s surface slows. Decaying plant matter—leaves, moss, and even submerged logs—release organic compounds as they gradually decompose. One of these compounds is a class of molecules called humic acids, which interact with the oil-slicked water.
Microbial life plays a role too; bacteria that digest organic matter release minuscule amounts of methane, which can rise to the surface and help form a thin, delicate film. Then the conditions are almost right for a rainbow pool to form. Add a few days of calm, still weather—windy conditions will disrupt the film—and you have the perfect canvas for refraction. When light hits the water at the right angle, it scatters, creating a sudden kaleidoscope of reds, oranges, yellows, greens, blues, and purples.
Rainbow pools also sometimes occur outside of swamps—in small lakes and ponds where the water is fairly stagnant, for example—and in warmer weather. But if you’re looking to catch one, they’ve been documented in wetlands all over Florida, at Great Dismal Swamp and at First Landing State Park in Virginia, and in South Carolina’s Congaree National Park. And though a swamp has its own kind of beauty any day, a rainbow swamp is a fleeting reminder of just how mysterious these often-forgotten places really are.
