A macrotidal mixing machine
The Han River Estuary is a place of immense physical forces, defined by one of the largest tidal ranges on the planet. Near the mouth at Incheon, the difference between high and low tide can exceed 9 meters, officially classifying it as a macrotidal system. This massive influx of seawater from the Yellow Sea pushes deep inland, overpowering the river's natural flow. The tidal influence extends up to 130 kilometers upstream, causing the river's water level and flow direction to fluctuate twice daily. This interaction between the powerful tide and the river's freshwater discharge, which averages 613 cubic meters per second, creates a dynamic zone of brackish water important to the region's character.
This constant battle between river and sea governs the estuary's form and function. The system is described as ebb-dominant, meaning the currents flowing out to sea are generally stronger and last longer than the flood currents coming in. This powerful outflow is responsible for transporting enormous quantities of sediment. The Han River annually empties about 2.5 x 10^10 cubic meters of water and up to 1.3 x 10^6 tons of sediment into Gyeonggi Bay. This sediment load is not flushed out to sea; the tidal forces sort and deposit it, building and sustaining the vast tidal flats, or 'getbol', that fringe the estuary. The largest of these, the Ganghwa tidal flats, are a direct result of sediment supplied primarily by the Yeomha Channel, one of the estuary's main conduits. These sedimentary processes have created a complex underwater terrain of channels and large, mud-rich sandbars that are up to 27 meters thick and can extend for over 50 kilometers.