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If you live in northern areas of the U.S. where lakes and rivers freeze over in the winter, you may have seen frazil ice and didn’t even realize it.

Frazil ice is a "slushy ice" that forms on lakes or slow-moving rivers. Many times, it looks like needles, roughly 3 millimeters in diameter floating on the surface.

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However last week, the Aroostook River in Washburn, Maine, had frazil ice form on the river. Forecasters explained in a post on X that the river’s flow is running very low because of drought conditions, which allowed frazil ice to form.

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For this particular type of ice to form, cold temperatures and winds combine to make the water's surface become supercooled, or drop to temperatures below freezing. However, the movement of the water prevents the surface from completely freezing over, so these small plates (or needles, as mentioned above) form and float on the surface.

Just last year, park rangers at Yosemite National Park took to social media to warn hikers about frazil ice: "It’s not a solid surface despite its appearance, so when its slush hides the boundaries of sure footing, falling in and becoming trapped beneath is a serious hazard."

Many times, frazil ice can appear like a solid sheet of ice, so the main hazard is falling through to extremely cold water.

J​ennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.