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.
Estuaries 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.