Tides

Tides are an oceanographic agent that strongly governs the hydrodynamic regime, estuarine morphology, and the freshwater-saltwater balance of the Mekong Delta region. With its flat terrain, an average elevation of only about 1 meter above sea level, and a dense network of rivers and canals, the Mekong Delta is one of the deltas most strongly influenced by tides. Here, tides are not merely a simple natural phenomenon but also an agent that brings both essential benefits and poses challenges to production and livelihoods.

Characteristics

Semi-diurnal tidal regime and the difference between the two sea areas

The hydrological regime of the Mekong Delta is simultaneously governed by tides from two sea areas, namely the East Sea and the West Sea (Gulf of Thailand), with very distinct characteristics:

Area influenced by the East Sea (irregular semi-diurnal tide): This is the dominant tidal regime, strongly affecting most of the delta's area, especially the coastal zone from Long An to Ca Mau Cape. The East Sea tide has a semi-diurnal regime, meaning a tidal day typically has two high waters and two low waters. The tidal amplitude in this area is very large, averaging about 2.5 – 3.0 m, and can reach a maximum of 3.2 – 3.8 m, with even recorded levels up to 4.1 m. Thanks to this large amplitude, tides from the East Sea can propagate deep inland, affecting river flows at distances up to more than 200 km from the river mouths.

Area influenced by the West Sea (irregular diurnal tide): Meanwhile, the tide in the West Sea (Gulf of Thailand) is of a mixed type, predominantly diurnal (a day has only one high water and one low water). The tidal amplitude in this area is much smaller, averaging only about 0.4 – 1.2 m.

It is the difference in tidal regime and amplitude between the two sea areas that creates an extremely complex and diverse hydrological regime for the entire delta.

The of tide

Benefits

For generations, the people of the Mekong Delta have known how to "live with the tides" and utilize the benefits they bring:

Aquatic resources and alluvium: Tides act as an "ecological flow," carrying abundant aquatic resources (shrimp, fish, crab juveniles) deep into the interior fields, creating livelihoods for millions of people. The tidal flow also contributes to transporting alluvium and accreting coastal areas.

Serving agricultural production: The large tidal amplitude facilitates gravity-fed irrigation. Farmers utilize the tides to channel fresh water into rice paddies and orchards, while simultaneously draining acidic alum water, naturally improving the soil, and saving costs. This is one of the key factors contributing to making the Mekong Delta the largest rice granary in the country.

Challenges and hazards

Flooding due to high tides: High tide events with record peak water levels are occurring with increasing frequency. If historically a record high water level occurred only once every 10-20 years, in just the last 7 years, Can Tho City has witnessed 4 record-breaking events, with the peak tide level in 2025 reaching 2.35 m, causing widespread flooding. Many provinces such as Ca Mau, Dong Thap frequently record tide levels exceeding Alarm Level 3 by 20-40 cm, causing dyke breaches, deep inundation of houses, fields, gardens, and transportation systems.

Figure: Tidal-induced urban inundation in Can Tho

Saltwater intrusion: During the dry season, when upstream water flow significantly decreases, the large-amplitude tide from the East Sea acts as a "conduit" driving saltwater deep into the interior fields through the system of rivers and canals. This causes severe drought and salinity events, seriously affecting agricultural production and the domestic water supply of millions of people.

Figure: Saltwater intrusion impacts on agricultural production

Amplifying the impacts of sea-level rise and subsidence: Recent studies indicate that the tidal amplitude in the delta region is trending towards a significant increase. This is not only due to sea-level rise but is also amplified by local factors such as land subsidence and excessive sand mining, which deepens the riverbeds. This combination makes flooding due to high tides and saltwater intrusion increasingly severe and unpredictable.

In summary

Tides are an inseparable natural agent from the existence and development of the Mekong Delta. Deeply understanding their laws, proactively adapting to the increasingly growing negative impacts, and sustainably exploiting the benefits they bring is the key to protecting the Mekong Delta from the unpredictable fluctuations of nature in the future.