Why are some lagoons brackish?

Lagoons are classified as brackish when they receive a significant amount of freshwater runoff from the land in relation to the volume of seawater entering through tidal inlets.1

The salinity of a lagoon is determined by the balance between freshwater input and tidal exchange. In brackish lagoons, the influx of fresh water—often transported by rivers—dilutes the seawater, resulting in lower salinity levels compared to the open ocean. Because these environments are sheltered and often shallow, the mixing of these water sources creates a variable salinity gradient, with levels typically increasing toward the tidal inlets where seawater enters.123

This dynamic is distinct from other lagoon types, such as hypersaline lagoons, which occur in hot, arid regions where evaporation rates exceed freshwater input, or marine lagoons, which maintain salinity levels similar to the open ocean.1