Why moving water is slow to freeze
Still water in a pond freezes from the top down once the surface cools to 32 degrees F. A waterfall is different. The water is moving fast and mixing constantly, so heat from deeper water keeps reaching the surface and the flow resists forming a solid lid. The ice that builds on a waterfall grows mostly from the outside in: from spray, from slower water along the edges, and from water seeping out of the rock beside the falls.
In fast, turbulent water the temperature can drop a tiny fraction of a degree below freezing without turning to solid ice. U.S. Geological Survey (USGS) research describes this supercooled water as the place where two kinds of underwater ice form. Frazil ice is a cloud of small, loose ice crystals that grow in the turbulent current. Anchor ice forms when those crystals stick to rocks on the streambed and keep growing there. Both can build up below a waterfall and in the rapids above it.
Spray, mist and the ice cone
Most of the visible ice on a frozen waterfall comes from spray. Fine droplets thrown off the falling water land on cold rock, plants and older ice and freeze on contact. Layer by layer this builds icicles, curtains and columns around the stream. In a hard cold spell the ice can grow until it nearly encloses the falling water, which is why a waterfall can look frozen solid while water still runs behind the ice.
At the base of a tall waterfall the same process can build a large mound. At Yosemite National Park, the National Park Service (NPS) states that a cone of ice builds up at the base of Upper Yosemite Fall in winter, sometimes taller than 300 feet, and usually melts by mid-April. Yosemite also produces frazil ice that people can see from the trails: NPS explains that it forms when waterfall mist freezes and then floats down the creek, most often in April and usually before 9 a.m., and most famously in Yosemite Creek just below Lower Yosemite Fall.
Seeps and ice curtains
Some of the best-known frozen falls in the country are not rivers at all. Along Lake Superior in Michigan, NPS describes water seeping out of the porous sandstone cliffs of Pictured Rocks National Lakeshore and freezing into curtains and columns of white ice. The park states that these formations reach 20 to 50 feet high, generally begin to form by mid-December and remain until early April. In northern Illinois, the Illinois Department of Natural Resources states that icefalls form in the sandstone canyons of Starved Rock State Park in January, where small seasonal streams pour over the canyon walls.
U.S. waterfalls documented to freeze
The table lists waterfalls where the managing agency itself describes winter ice. Conditions change every year, and none of these falls freezes on a fixed date.
| Waterfall | State | Managing agency | What the agency documents |
|---|---|---|---|
| Niagara Falls (American side) | NY | New York State Parks (Niagara Falls State Park) | Heavy winter ice; the falls kept flowing during the 2014 Polar Vortex |
| Upper Yosemite Fall | CA | National Park Service (Yosemite National Park) | Ice cone at the base, sometimes over 300 ft tall |
| Munising Falls | MI | National Park Service (Pictured Rocks National Lakeshore) | Freezes in winter; 800-ft trail from a plowed lot |
| Sand Point ice curtains | MI | National Park Service (Pictured Rocks National Lakeshore) | Ice curtains and columns 20 to 50 ft high, mid-December to early April |
| Brandywine Falls (60 ft) | OH | National Park Service (Cuyahoga Valley National Park) | Ice becomes the main winter feature; boardwalk may close in icy conditions |
| Fort Falls | NE | U.S. Fish and Wildlife Service (Fort Niobrara National Wildlife Refuge), on the Niobrara National Scenic River | Ice forms along the sides and changes through the winter |
| Starved Rock canyon icefalls | IL | Illinois Department of Natural Resources | Icefalls in the sandstone canyons in January |
| Gooseberry Falls, Upper Falls | MN | Minnesota Department of Natural Resources (Gooseberry Falls State Park) | Upper Falls freezes during extended cold spells |
Rules and access in winter
Agencies often change access when the ice is in. At Cuyahoga Valley, NPS states that the Brandywine Falls boardwalk may be closed during icy conditions. At Pictured Rocks, the park notes that the Munising Falls trail may be ice covered and that visitors are not to leave the trail or cross railings. At Starved Rock, Illinois DNR lists French Canyon as not suitable in winter without traction devices and lists Tonti Canyon as closed to hikers. At Niagara, New York State Parks states that walking out onto the ice bridges below the falls has been forbidden since February 1914.
Formal ice climbing is a separate activity with its own rules. Pictured Rocks, for example, states that ice climbing is the only kind of cliff climbing allowed in the park. That activity is outside the scope of this page.
When to expect ice
The timing tracks the coldest weeks of each region. At Pictured Rocks, NPS gives mid-December to early April. Illinois DNR points to January at Starved Rock. In Yosemite, the ice cone at Upper Yosemite Fall builds through winter and is usually gone by mid-April, while frazil ice is most common in April. Around Niagara, NOAA normals for nearby Buffalo show January as the coldest month, with a normal mean of 25.5 degrees F for 1991 to 2020.