What is an estuary?
An estuary is a partly enclosed body of water where fresh water coming down the rivers mixes with salt water from the sea. A range of coastal landforms fit this description, including bays, lagoons, harbours, inlets, sounds, fiords and swamps.
Flush with nutrients and inhabited by resilient organisms, estuaries are among the most productive ecosystems on earth. They provide rich feeding grounds for coastal fish and migratory birds, and spawning areas for fish and shellfish. They are also important in maintaining the quality of coastal waters.
New Zealand’s estuaries
Long favoured as settlement sites by Māori and European alike, New Zealand’s estuaries are intricately bound up with people’s activities on the surrounding land.
There are about 300 estuaries distributed fairly evenly around New Zealand’s coast. Most are small – 200 hectares or less; 30 are over 1,700 hectares, and the largest is Kaipara Harbour at 15,000 hectares. All are recent in origin, having formed within the last 6,500 years, since the last ice age ended. They were created as the rising seas flooded and drowned low-lying coastal river valleys and bays.
Life and death of estuaries
Estuaries are temporary landforms. Rivers carry sediment eroded from the land into them, and tides bring in sand from the sea floor. If more sediment comes in than flushes out, an estuary will age and eventually die: it fills up with so much sediment that it turns into dry land.
Around New Zealand, infilling of estuaries progresses at different rates according to their shape, and the volume of sediment arriving. The fiords of the lower South Island have hardly changed in the last 6,500 years. In contrast, Waitematā Harbour has accumulated metres of sediment and is at an advanced stage of infilling. If sediments continue to accumulate on its vast intertidal mudflats, some areas will rise above the high-tide level and become permanently dry.
Sometimes a dramatic event can lead to the death of an estuary, or a section of it. During the 1931 Hawke’s Bay earthquake part of the Ahuriri Lagoon was raised by over 1 metre, well above the high-tide level.
Human activity has increased erosion and changed the flow of rivers into and through estuaries, speeding up the infilling of many. Scientists analysing estuary sediments have discovered that before humans arrived in New Zealand, estuaries filled slowly – less than 1 millimetre of sediment accumulated on the estuary floor each year. In some North Island estuaries, sediments are now accumulating at an annual rate of 20 millimetres.