What is an estuary?
An estuary is an area of water on the coast, where fresh water and sea water mix. It often forms at the mouth of a river, with large mud flats where the tides wash in and out. It is a unique home for many creatures, and a few plants.
New Zealand’s estuaries
There are about 300 estuaries around New Zealand’s coastline, including the South Island fiords. The biggest is Kaipara Harbour. The estuaries formed about 6,500 years ago when the sea began to flood river valleys and bays.
How estuaries die
Estuaries are always changing. Rivers bring sediment (soil and other matter) into them, and the ocean brings in sand. This is taken out again on the tides, but sometimes the sediment builds up. Over time, this can turn the estuary into dry land, and the estuary dies.
Plants of the estuary
These special plants have to survive salt water, storm waves and other tough conditions. Two common New Zealand estuary plants are seagrass and mangroves.
Animals of the estuary
The creatures of the estuary range from tiny organisms to eels and godwits. Burrowing mudworms are eaten by birds and fish. Crabs make tunnels or hide under rocks. Cockles help filter the water and are food for birds and humans. Many native fish, including flounder and kahawai, use the estuary. Flocks of wading birds such as herons feed on the mudflats, and thousands of godwits and other migrating birds arrive each spring. Swamp birds such as pūkeko and bitterns breed among the mangroves and rushes.
Why are estuaries important?
- Birds and fish breed and eat there.
- They help keep coastal waters healthy.
- People use them for sports, fishing, and other recreation.
For Māori, estuaries were valuable food-gathering places. But as European settlement grew, farms, towns and cities began to pollute the estuaries. The land was also filled in for projects such as sewerage works and rubbish dumps. Māori protested, and there are now rules about protecting estuaries. Today, people are aware of how important it is to keep estuaries healthy.