A Dynamic Seascape

Ōhiwa Harbour’s story began around 10,000 years ago. That’s when the ocean began to rise from about 150 metres below its present level. Over 4000 years, the water gradually infiltrated the landscape, drowning everything in its path until finally stabilising and creating the coastline more or less as we know it today.

 

Geologically, the area is predominantly greywacke of Late Jurassic to Late Cretaceous age, draped by loess (wind-blown silt) during the Pleistocene Ice Ages and more recently covered in volcanic ash and pumice from the Rotorua and Taupo volcanic centres. Most of the soils of the Ōhiwa Harbour catchment are derived from this air-fall rhyolitic volcanic ash.

 

At low tide 80 per cent of its almost 27 square kilometres water surface disappears. Then only the main Kutarere and Ōhope channels and some smaller ones stick out like arms from the vast harbour bed.

​

There are six main islands: Ohakana, Uretara, Hokianga, Whangakopikopiko, Pataua and Motuotu.

​

It is a dynamic place. Through the tides the harbour constantly changes its face. In the longer term sedimentation, accretion and erosion permanently shape and shift channels, sandbanks and the two spits that make this inlet so unique.

 

Ōhiwa Harbour is unusual in that it has a spit on either side of the entrance. These spits change regularly due to currents and tides. In the last 100 years a wharf and houses were lost to the encroaching sea on Ōhiwa Spit on two occasions.

 

Just like all natural harbours Ōhiwa is slowly filling in and will eventually disappear. However this process of sedimentation, has been accelerated by human activities in the catchment that began with the clearing of the original forest cover. The Ōhiwa Harbour’s catchment is comparatively small, 171 square kilometres. Yet what happens on the land - forestry, dry stock and dairy farming, horticulture and lifestyle blocks - has a huge effect on the harbour. The Nukuhou River Valley is the main tributary of the Ōhiwa catchment.