How Estuaries are Formed

Every river that meanders through the countryside will eventually reach the sea. At the river’s mouth, this partially enclosed frontier of fresh river water and briny seawater essentially defines an estuary, which is one of the most productive ecosystems on Earth.

The majority of estuaries around the world today were formed during the Holocene period (that is, approximately 12,000 years ago) as rising sea levels flooded river valleys. However, estuaries can come about by other means too, such as glaciation or oceanographic and tectonic processes.

The brown, boggy expanse of mud that is so typical of these areas is the product of sand and silt washed down and deposited by the river. Decaying matter is washed into the estuary too, making it rich in nutrients and also lending it that distinctive low-tide odour. In the case of much larger rivers, this deposition of sediment will form a delta.

It’s the transport of nutrients and biological matter washed from land to sea and back that makes an estuary so productive. But this isn’t just for flora and fauna, as estuaries also provide sheltered natural harbours that buzz with human life too.

EstuariesEstuaries are at the mercy of the tides, which flush the sandy, muddy expanse with saltwater twice a day. The extent of this mixing is defined by the cycle of the tides and directly affects an estuary’s unique characteristics.

These areas can range from well-mixed environments to a heavily stratified basin of contrasting chemical properties. Regardless of type, however, every estuary is teeming with life, offering food and shelter to organisms ranging from microbes through to top predators.

Life in the mud

Life in the mudWith nutrients readily available in the sediment and water, all kinds of life are drawn to these productive biological melting pots.

The microbial community thrives on the silt and sediment washed down from the riverbed and the mud flats are packed with invertebrates, providing food for hundreds of bird species. Algal growth blooms and filter feeders, such as mussels, are attracted to live in the oxygenated flats. Estuaries are also home to plenty of fish and crustaceans such as mullet, bass and spider crabs. These in turn are the perfect meal for predators like seals and herons.

Estuaries serve as vital nursery grounds. Many young species exist here, such as salmon smolts (a stage in between fry and adult), which feed to fatten up before venturing into the open ocean.


Many think the St Lawrence River Estuary in Canada is Earth’s largest. 12 million litres (3.2 million gallons) of water pour into the Gulf of St Lawrence per second!


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