Water surfaces tell radically different stories. One lake can hold a person up like a cork, while another nearby can fill a bottle for drinking. Both often start the same way: a tectonic basin opens, subsidence creates a cradle for water, and rivers and groundwater begin to collect in the low spot.
From there, hydrology takes over. In a closed basin with intense evaporation and little outflow, dissolved ions from rock weathering accumulate, driving salinity and water density sharply upward. High density boosts buoyant force, so a human body displaces less volume and floats higher. In open basins, steady inflow and outflow flush those ions away, maintaining low salinity and a gentle density gradient that keeps the water fresh enough to drink.
The contrast turns on a few mechanistic levers: mass balance of inflow versus evaporation, rate of ion dissolution, and the geometry of the basin that controls mixing and stratification. Geological forces may write the initial template, but fluid dynamics and chemical equilibrium silently edit it into either a buoyant, briny laboratory or an ordinary, potable lake.
