New Zealand’s dynamic weather and geology combine to form some remarkable crossing points between land and sea, through which currents and tides flow.
Fiordland – ocean stratification
An extreme example of ocean stratification is found in Fiordland. A remarkably high rainfall creates runoff from the steep sloping sides so that a brackish layer floats over the saltier and denser ocean water of the fiords. Fresh surface waters in Doubtful Sound can have as little as 20% of the salinity of oceanic waters. The top layer is fed by highly variable rainfall and moved around by tides and wind. Organisms living in this environment need to be well adapted to the varying salinity.
Swimmers and divers at New Zealand beaches notice layers of cold and warm water. Warm water is lighter than cold water and floats to the surface. The salt content also affects density. The combined result is a stratified ocean with distinct layers sometimes only a few metres thick, which mix only during strong storms.
In New Zealand huge quantities of sediment are carried downriver and form buffer zones between fresh and sea water in the form of muddy tidal estuaries. These estuaries, like Manukau Harbour with its 4-metre tidal range, change with the tide every six hours from coastal lakes to expansive mudflats crossed with river channels. This variability makes it very difficult to model and study their behaviour.
Tidal filling and emptying of the estuary basins can generate complex, dangerous wave patterns and strong currents that travel through relatively narrow passages. For example, Manukau Harbour entrance sustains currents travelling at speeds of around 1 metre per second, which swirl around the headlands and make boating difficult.
Heavy rainfall can bring about plumes of sediment at river mouths. Their evolution directly affects life in the nearby sea and shoreline. Plumes are a mixture of fresh water and the river’s sediment load, with some dilution caused by currents. The plume emerging from Pelorus River, in the Marlborough Sounds, moves out into Pelorus Sound at almost 1 metre per second.
On the open coast, plumes are pushed along by the wind and tides, and slowly drop suspended sediment. The sediment accumulates depending on the local waves which, if strong enough, will re-suspend the sediment and keeping it moving. Plumes are easily visible from the air anywhere along the New Zealand coast where rivers meet the sea.