Frazil is a type of ice that forms as tiny, loose crystals suspended in turbulent water rather than as a solid sheet on the surface. You’ve likely seen it without knowing its name: a slushy, soupy layer of ice needles drifting in a fast-moving river or churning ocean. These crystals are the first stage of ice formation in any body of water with enough motion to prevent a smooth frozen surface, and they play a surprisingly large role in everything from global ocean currents to hydroelectric power outages.
How Frazil Ice Forms
Frazil needs two ingredients: water that has cooled just below its freezing point (a state called supercooling) and enough turbulence to keep the crystals from settling into a solid layer. In a still lake, cooling water simply freezes into a flat sheet across the top. But in a fast river, ocean waves, or rapids, the churning motion mixes the slightly supercooled water throughout the column, and ice crystals nucleate as tiny suspended particles instead.
The supercooling involved is remarkably small, often just a fraction of a degree below 0°C. At supercooling of 0.1°C, frazil crystals will stick to metal surfaces like steel but not to plastic. Push that down to 0.2 or 0.3°C, and they become sticky enough to cling to plastic too. This adhesive behavior is a key reason frazil causes so many problems for infrastructure.
Once a few initial crystals appear, a process called secondary nucleation takes over. Existing crystals collide with each other and break apart in the turbulence, seeding massive numbers of new crystals in a sudden burst. This explains why frazil production can seem to explode: a river goes from clear water to a thick slurry in a short time as each crystal spawns more crystals.
What the Crystals Look Like
Individual frazil crystals are needle-like or disk-shaped and extremely small. In their initial growth stage, most have a radius of about 0.3 millimeters, roughly the size of a grain of fine sand. At low concentrations, the crystals are nearly invisible in the water. As they multiply, they give the water a greasy, opaque appearance, which is why accumulated frazil is sometimes called “grease ice.”
From Slush to Solid Ice
Frazil is the starting point for most sea ice on Earth. What happens next depends on weather conditions.
In calm seas, frazil crystals collect at the surface and merge into grease ice, a smooth, oily-looking film. That film thickens into a continuous thin sheet called nilas, which can slide over itself when light winds push it around. Over time, nilas grows into solid sheet ice with a smooth bottom.
In rough seas, wind and waves push frazil crystals together into slushy, circular clumps called pancake ice. These rounded discs, which can range from dinner-plate size to several meters across, bump against each other and develop raised edges. If thick enough, pancake ice can pile up into ridges. Both pathways start with the same tiny frazil crystals, but the end result looks completely different.
Hanging Dams in Rivers
One of frazil’s most dramatic effects in freshwater is the formation of hanging dams. These are thick accumulations of frazil that collect on the underside of an existing ice cover, growing downward into the river like an inverted dam.
They form where fast-flowing river sections meet slower water. The turbulent upstream stretch produces enormous quantities of frazil, which flows downstream and gets swept beneath the ice cover where the river widens, deepens, or flattens out. If the current beneath the ice isn’t strong enough to push the crystals farther downstream, they settle against the underside of the ice and pile up.
These underwater ice masses can reach staggering sizes. Field studies on the Smoky River in Alberta, Canada, documented hanging dams between 300 and 700 meters long, with frazil accumulations reaching 11 to 16 meters thick below the water surface. A study on the Ottawa River found frazil depositing in a 75-meter-deep trench that stretched over 1.2 kilometers. Hanging dams restrict water flow and can cause severe flooding when they partially block a river channel or release suddenly during a thaw.
The Role of Frazil in Ocean Circulation
Frazil ice plays a critical part in one of the planet’s most important climate processes: the formation of Antarctic Bottom Water, the cold, dense water mass that fills the deepest layer of the world’s oceans and drives global ocean circulation.
In coastal areas called polynyas, where strong winds blow surface ice away from shore and expose open water to frigid air, frazil forms rapidly throughout the water column. Research at Antarctica’s Cape Darnley polynya found that underwater frazil dominates ice production there, occasionally penetrating to depths of at least 80 meters. Because frazil stays suspended rather than forming a solid lid, it prevents an insulating ice cover from developing on the surface. This keeps the water exposed to the cold air, which drives even more ice production.
When saltwater freezes into ice, the salt gets left behind in the surrounding water, a process called brine rejection. The intense frazil production in polynyas concentrates salt in the water over two to three months, making it progressively denser. This high-salinity water eventually sinks off the continental shelf and mixes with deeper ocean layers, forming Antarctic Bottom Water. Studies using underwater acoustic measurements and satellite data found that ice production in frazil-dominated areas was nearly twice as large as in areas covered by thin solid ice, making frazil the primary engine behind this deep-water formation.
Problems for Infrastructure
Frazil’s stickiness makes it a persistent headache for any facility that draws water from a river or lake during winter. Hydroelectric plants, municipal water intakes, and industrial cooling systems all use submerged screens or trash racks to filter debris. When frazil crystals flow into these structures, they adhere to the metal bars and build up rapidly, choking off water flow. A blocked intake can force a power plant offline or cut water supply to a city.
Engineers have developed several countermeasures. The most common is heating the trash rack bars just enough to keep them above freezing, since frazil will not stick to a surface that is even slightly warmer than 0°C. Heating methods include electric resistance heating, circulating warm fluids through hollow bars, or using infrared heaters aimed at the racks. Other approaches include removing fine screens during winter and replacing them with coarser ones, vibrating the racks to shake ice loose, regulating river flow upstream to promote a stable surface ice cover (which stops frazil production), and designing intake structures that let frazil pass through rather than accumulate.
Effects on Fish and River Life
Frazil doesn’t just affect infrastructure. It also disrupts aquatic habitats. Research on rainbow trout in Wyoming’s Big Horn River found that frazil episodes in January and February triggered significant changes in fish behavior. When frazil appeared, juvenile trout abandoned their normal activity areas and moved to refuges at the bottom of deep pools or tucked under shelf ice in shallow water near shore. These frazil-driven relocations often initiated longer-term movements, meaning the fish didn’t simply return to their original spots once conditions improved.
Overwinter declines in the abundance of small rainbow trout were observed in river sections downstream of reservoirs, where warm water releases prevent surface ice from forming. Without a stable ice cover, those stretches remain turbulent and exposed to cold air, creating ideal conditions for ongoing frazil production throughout the winter.
How Scientists Monitor Frazil
Because frazil forms underwater and is nearly transparent at low concentrations, detecting it isn’t straightforward. Researchers use upward-looking sonar systems mounted on the riverbed or seafloor that send acoustic pulses toward the surface and measure the signals that bounce back off suspended ice crystals. Higher-frequency sonar (around 420 to 546 kHz) is more sensitive to the tiny crystals than lower-frequency units (around 235 kHz), which produce weaker returns from frazil particles. These instruments were originally designed to measure ice thickness from below but have been adapted to estimate frazil concentration in real time, giving operators at water intakes and hydroelectric plants early warning before blockages develop.