Bison create waterproof micro-wetlands by compacting clay with their own body weight

European bison act as hydrologic sculptors by creating wallows that function as waterproof micro-wetlands. When these 1,000-kilogram herbivores roll in the dirt, their massive body weight compacts the clay substrate into a dense liner that is 95% impermeable to water percolation. These basins, averaging two meters in diameter, retain rainwater long after surrounding areas have dried.