In many fjords, freshwater from rivers floats directly on top of salty ocean water without mixing.

This phenomenon is primarily driven by density differences caused by salinity, a concept known as the halocline. Freshwater has a density of approximately 1,000 kilograms per cubic meter, while seawater averages about 1,025 kilograms per cubic meter due to dissolved salts. In deep, narrow inlets like the Sognefjord in Norway or the Milford Sound in New Zealand, large volumes of glacial meltwater and river runoff flow into the sea.The unique geography of a fjord plays a critical role in maintaining this separation. High mountain walls act as a windbreak, reducing the surface turbulence that would normally stir the water. Without significant wind energy or tidal mixing, the lighter freshwater forms a distinct upper layer that can be several meters thick. This stratification is so stable that it can create a 'dead water' effect, where ships experience mysterious resistance as they generate internal waves between the two layers.Oceanographers from institutions like the Norwegian Institute for Water Research (NIVA) study these layers to understand nutrient cycling and marine life. The top freshwater layer often remains much warmer in the summer but can freeze quickly in the winter, while the saltier water below stays at a constant temperature. This layering also affects light penetration and oxygen levels, creating unique micro-ecosystems where species must adapt to rapid changes in salinity within just a few meters of depth.