How a buried stream became a well-formed sieve

The southern catchment spans around a third of our farm’s total area and discharges into an extensive gully wetland, eventually flowing into the Whangaipeteki Stream. The water then goes on a 2.6 km serpentine journey to the Waikato River. As on many farms in New Zealand, for a few decades the water had been channelled into a drain system, to make the most of pastureland and to reduce ‘bogging’, or ‘pugging’ by stock.

However, the early weeks of the New Year saw us carrying out earthworks to create two wetland treatment weirs. Weirs are like sieves, helping to reduce the amount of impurities that leave the farm. By raising the water levels in the stream we have increased the amount of time the water is on the farm. Over time, this will have several benefits: it will improve the efficiency of the removal of suspended sediment; help the denitrification process; and scale back the presence of nitrogen, phosphorus, copper and zinc by perhaps 75 per cent. Faecal pathogen levels, too, are expected to decrease naturally by 80–90 per cent through increased exposure to the sunlight and microbial plant communities.

Fish survey records show that kōura, bully species, and shortfin and longfin eels are found in streams near the farm. By making sure there are enough rocks for them to pass over, we are hopefully providing habitat for all these species. In the eight months since the weirs have been in commission, we’ve observed water boatmen, frogs, birds, ducks and stalks.

As well as raising the water levels, like our other wetland regeneration work, we have given the area a ‘wide berth’. As the weirs were created in the warmer, drier months, we initially fenced the area off with electric fence standards. This allowed us to gauge how high the water would likely rise to and therefore where we should place the fencing. By designing a generous ‘buffer zone’, we have allowed for many species to be planted on the riparian areas and gully slopes. So far, we have planted raupō and large-stature rautahi, as well as native rushes and grasses on the weirs’ shallow sides. We have also planted tōtara, as these would have stood lofty on the gully slopes more than a century ago.

We’ve been able to see that water leaving the weirs is improved and that, in times of heavy rainfall, sediment leaving the farm has been reduced. In the coming weeks, moreover, we are going to extract annual samples of the water going into the weir system, and further samples as it leaves the farm and flows into the Whangaipeteki. Over time, our monitoring should demonstrate lower nitrogen and phosphate levels in the water leaving the farm. Because three tributaries spring out of the ground on our farm, we have a rare opportunity to make sure the water starts off life as it should go on.