Freshwater feature: the Bullock Creek polje
The Bullock Creek polje
Location of the Bullock Creek polje
A drain cuts through native sedge (Carex sinclairii) vegetation at Bullock Creek.
The limestone country of the Buller District offers some of New Zealand’s most spectacular scenery, from the famous blowhole and pancake rocks at Punakaiki to the bushclad gorges and towering bluffs of Paparoa National Park.
Every year, hundreds of trampers enjoy the challenge of the steep terrain and river crossings of the Inland Pack Track, unaware that one of New Zealand’s most unique wetlands, the Bullock Creek polje, is hidden a short distance away. A polje (pronounced 'poll-yer') is a large, flat-floored depression in a karst landscape, and Bullock Creek is our only example. Surrounded by tall cliffs and dense beech and kahikatea forest, it appears at first sight to be well protected and unlikely to be damaged by human activity. But the almost pristine bush and rivers belie almost a century of disturbance that has destroyed natural character in much of the wetland and left just a tantalising remnant of what once was there.
The pioneering attitude to wetlands was that they were unproductive wastelands, fit only for drainage and conversion to productive farmland. Despite the rough terrain and heavy rainfall, intrepid would-be graziers beat their way into the polje and began felling forest as early as 1874, and replacing the native wetland vegetation with introduced pasture grasses. By the turn of the twentieth century, most of the wetland had become farmland, with just a 100 ha remnant of wetland vegetation left at the northern end. And it’s still there today, owing its survival to the local geography and legendary West Coast rain. Within hours of heavy rainfall in the surrounding hills, vast quantities of water sweep down and inundate the area to a depth of a metre or more, swirling back into the wetland from subterranean passages unable to cope with such large volumes. Incredibly, even this extreme water regime didn’t deter development, and a network of drains was dug from the mid 1900s onwards, criss-crossing the wetland area. However, the area remained very wet, and the entire site finally passed into public ownership as conservation land in 1986 when farming became uneconomic.
Today, what remains in the surviving wetland is a mosaic of different vegetation types, separated by the network of drains of different ages. A mixture of exotic pasture grasses has invaded the driest areas and banks of the drains, encouraged by the disturbance and drier soils, but the native wetland plants still dominate. The catchment is pristine, without agricultural runoff or other human pollution, giving good reason to hope that Bullock Creek’s last bit of natural wetland is safe for the future.
NIWA, in collaboration with Landcare Research and the Department of Conservation, is carrying out restoration experiments at the site, by closing some drains and investigating how the hydrology, nutrient cycling, and vegetation respond once a more pre-European water regime is established. We are learning lessons at Bullock Creek that we can apply to restoration projects elsewhere, helping to reverse the long history of drainage, burning, and grazing of wetlands in New Zealand.
Karst geology
The word karst refers to terrain with well-developed subterranean rivers, lakes, and caves that form in highly soluble rock types like limestone, marble, and gypsum. The words karst and polje were both derived from Slovenia, where these types of landform are very common and were first described. Karst landscapes are found in a number of regions of New Zealand, and are popular for underground recreational activities such as caving and black-water rafting.