Estuaries have a life cycle. They form, they age as they infill with sediment and they transform to other environments, such as freshwater marshes. They are fragile habitats vulnerable to time, tide, erosion, pollution and other effects from the land around them.
Forming of estuaries
Initially, estuaries were formed by rising sea levels. The sea level has slowly risen over the last 12,000 years – since the end of the last ice age – but has remained relatively stable during the last 6,000 years. As the sea rose, it drowned river valleys and filled glacial troughs, forming estuaries. Once formed, estuaries become traps for sediments – mud, sand and gravel carried in by rivers, streams, rain and run-off and sand from the ocean floor carried in by tides. Tidal flats build along the shore as these sediments accumulate.
During middle age, these flats become intertidal and more extensive, and are colonised by new communities of plants and animals that can tolerate being exposed to the air at low tide. Sandflats are replaced by mudflats as more fine sediment is delivered to the estuaries following catchment deforestation, as has occurred in New Zealand over the last several hundred years since the arrival of people.
The end of an estuary
How quickly estuaries age depends on how quickly they fill up with sediment. The tidal channel of the estuary is flanked by extensive intertidal flats, which link the land to the sea. These tidal flats are colonised by plants, with freshwater marshes eventually replacing saltmarshes in the upper reaches of the estuary.
If more sediment comes in than goes out, the estuary will fill up and eventually become dry land.
The speed at which an estuary comes to an end varies. Sedimentation is usually a slow process, but tectonic activity (earthquakes) can bring estuaries to a sudden end. For example, the 1931 Napier earthquake raised part of the Ahuriri Lagoon by over 1 metre, well above tide level, and it became dry land. Some estuaries change little over thousands of years. The fiords of the lower South Island, for example, have had little change in over 6,500 years.
Types of estuaries
Estuaries have many shapes and sizes. In New Zealand, there are around 300 estuaries. They can range from a few hectares to 15,000 hectares. Most are small – about 200 hectares or less. Thirty are over 1,700 hectares.
There are four main types of estuaries, based on how they were formed.
There are 164 bar-built estuaries. These form when a shallow lagoon or bay is protected from the ocean by a sand bar, delta or island. An example of a barrier-enclosed lagoon is Tairua on the east side of the Coromandel and the Avon Heathcote Estuary in Christchurch. A semi-enclosed bay is the Firth of Thames.
Coastal plains estuaries
There are 56 of these drowned river valleys. Coastal plains estuaries are formed when the rising sea fills existing river valleys. An example is the Ōkura Estuary, North Auckland, or Raglan on the west coast of the North Island. South Island examples are Queen Charlotte Sound and Otago Harbour.
Tectonic estuaries are caused by the folding of land surfaces due to volcanic activity. They are found along faultlines, such as Manukau Harbour in Auckland.
Fiords and rias
Fiords and rias are U-shaped valleys formed by glacial action. They are found where there is history of glacial activity such as the South Island’s West Coast.
Glaciers cut deep valleys out of the rock as they flowed to the sea. All these coastal valleys were then flooded by the sea as it rose to its present level about 6,500 years ago.
The difference between a fiord and a ria is that a fiord has a sill or rise at its mouth caused by the previous glacier. A ria does not have this accumulation of soil and rock at its mouth.
Human impact shortens life span
Although estuaries have a natural life span, human impact can shorten their life. For example, sedimentation happens naturally but can be sped up by tree or bush removal for urban or industrial development on estuarine coastlines. Usually more sediment comes in than can be removed by outgoing tides and so the life span of the estuary may be considerably shortened.
Learn more about the 4 main types of estuaries in New Zealand and others here.