How does river runoff affect water movement in a coastal lagoon?

River runoff significantly influences water movement and circulation within a coastal lagoon. The amount of runoff from adjacent land areas is a primary factor in determining the degree of water circulation, alongside tidal range and the width of tidal inlets.1

When a major river flows into a lagoon, it creates an estuarine lagoon, which functions similarly to an estuary. During periods of heavy river flooding, the volume of water escaping to the ocean can dramatically increase current velocities within the lagoon's channels. For example, in the Gippsland lagoons, while standard tidal currents may reach 5.6 km per hour, river floods can accelerate these currents to 13 km per hour.1

Additionally, runoff affects the salinity gradient of the lagoon. In brackish lagoons, high levels of runoff cause salinity to decrease, with values typically increasing toward the tidal inlets where seawater enters. Conversely, in arid regions where evaporation exceeds land drainage, the circulation pattern may reverse, with denser, hypersaline water sinking and drawing in surface seawater to replace the volume lost to evaporation.2