The Nature of the Wairau Wetlands

Forest and Bird, Issue 314, 1 November 2004, Page 31

The Nature of the Wairau Wetlands

Then the shallow inshore waters began to fill as gravel dumped by the Awatere River, and the detritus eroding off the White Bluffs, were picked up by the Canterbury current on its way north and left as a narrow finger pointing across the bay. The boulder bank grew until it was big enough to thwart the Wairau River and trap the sediment and gravels it carried. Gradually, the bay filled in, leaving lagoons behind the boulder bank and new beach ridges out from the northern cliffs. Today, the lagoons, 10.5 kilometres east-southeast of Blenheim, cover nearly 2300 hectares, of which 1800 hectares are in Crown ownership and held as a Conservation Area. Some 40 percent of that is designated as Wildlife Refuge. The whole area is nationally significant as one of the best examples in New Zealand of a river-mouth lagoon; a bird’s-foot delta, with a narrow boulder-barrier. Twenty per cent of the lagoons is swept by tidal flows. The main channel, Te Aropipi, drains the three lagoons — Big, Chandlers and Upper — which are fed freshwater by the Opawa and Wairau Rivers and various small streams, springs and kidney ponds. The waters are brackish — between 17 and 35 parts per thousand of salt. The lagoons slumped after earthquakes in the 1840s and 50s, but still only average half a metre deep, with fine sediment bottoms. Broad flats of glasswort and the coastal grass Hordeum marinum surround the lagoons, peppered with flax, sea primrose and bachelor’s buttons in the west. The southern margins are mostly grazed sea rush and flax, with shore ribbonwood on higher ground. The lagoons are a nationally important birdlife habitat, with 94 bird species on record. Some 60 pairs of royal spoonbills breed on lagoon islands, and the boulder bank hosts a modest colony of Caspian terns. Other breeders include pied shag, little shag, red-billed gull, black-backed gull, pied stilt, variable oystercatcher, spur-winged plover, banded dotterel, grey duck, grey teal and paradise duck. Large flocks of waders — bar-tailed godwit, pied stilt, pied oystercatcher and lesser knot — and many species of migrant and vagrant waders have been sighted here. Bittern, crakes and banded rail may still haunt the freshwater raupo margins of the Opawa River. Yellow-bellied flounder and sand flounder spawn in the lagoons. Ss: thousand years ago, Cloudy Bay reached much further inland into Marlborough.