Britannica Sources used in this answer

Why does ice float on lakes and rivers?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

water rapids, Niagara Falls, Canada — water Water is the most plentiful compound on Earth and is essential to life. Although water molecules are simple in structure (H₂O), the physical and chemical properties of water are extraordinarily complicated.

Ice floats on lakes and rivers because it is less dense than liquid water. While most substances become denser as they cool, water exhibits an anomalous behavior due to its molecular structure.

As water cools toward its freezing point, hydrogen bonds cause the molecules to arrange themselves into a rigid, open hexagonal lattice. This structure increases the space between molecules, resulting in a lower density for ice (approximately 917 kg/m³) compared to liquid water (about 1,000 kg/m³).12

According to Archimedes’ principle, this lower density allows ice to remain buoyant on the surface. This process is vital for the environment, as the floating ice forms an insulating layer that protects aquatic life in lakes and rivers from freezing temperatures, preventing bodies of water from freezing solid from the bottom up.13