This section of the river is called the Ice Meadows. Excerpt from adirondackalmanack.com:

During [the winter], the magic happens that makes the Ice Meadows what they are. Within the Hudson River, a collection of loose ice crystals are forming that look like slush in the water, this is called frazil ice. Frazil ice forms in super cooled water. When frazil ice groups together, it forms pancake ice. Like the name suggests, it looks like yummy pancakes with upturned edges. These pancakes can be as large as 10 feet across. When the pancakes group together, they will cover the surface of the river with a skin of ice. This skin will grow and can reach a thickness of glacial proportions, as high as 15 feet.

The ice grows and pushes up on the shore and will scour the shoreline of the Ice Meadows depositing organic matter and removing trees. The thick ice can take into April to melt, which shortens the growing season, which maintains the cooler climate. This allows the unique alpine species to grow.

At least 5 endangered plant species and 5 threatened plant species are known to live in the Ice Meadows. These include: Ohio Goldenrod, Auricled Twayblade and the Dwarf Sand-cherry. A species of special concern is the dragonfly: the Extra-striped snaketail. There are other species found only in this unique habitat, they are common to alpine habitats and are generally dwarf species.

There are so few occurrences of the Ice Meadows in New York, that they need to be protected. Major threats include: invasive species, development and trampling by visitors. Little research has been conducted on this unique habitat that truly not much is known. There are only between 10-20 known Ice Meadows throughout New York State, they can be found along the upper portions of rivers near mountains.

https://guides.nynhp.org/riverside-ice-meadow/?fbclid=IwAR2NovU2T1iOAoOvmzeT_3MWhblSIgGyYh7T_5cKqGykjIHm06WM2EScRUM

https://www.protectadks.org/small-chunks-of-frazil-ice-make-massive-ice-sheets-over-the-ice-meadows-on-the-hudson-river/

https://www.adirondackalmanack.com/2020/03/the-hudson-rivers-mysterious-frazil-ice.html

https://www.adirondackalmanack.com/2011/01/frazil-ice-at-hudson-river-ice-meadows.html

https://www.adirondackalmanack.com/2019/06/rare-plants-inhabit-adirondack-ice-meadows.html