It was all quite counterintuitive.
With the landscape looking greener than it had since the deforestation of the 1800s, how could the Mahurangi Harbour be
choking to death?
Auckland Regional Council scientists had not anticipated the disturbing picture that they had built up, over 10 years. Their sampling programme had been designed to benchmark the Mahurangi and other estuaries,
before significant development occurred in the respective catchments.
The regional council’s previous work on sediment had been concentrated on urban catchments, where the often-catastrophic impact of earthmoving on stream and estuary fauna was expected and well understood. Largely undeveloped, the Mahurangi’s good luck was to have been included in the benchmarking programme.
The Mahurangi studies indicated that
sediment-sensitive benthic communities of the harbour were becoming increasingly stressed—cockles and horse mussels in the areas sampled had ceased to breed. The proximate likely cause was an elevated sediment accumulation rate. But in the absence of any obvious increase in incoming soil, it was suspected that the harbour’s ability to shuck off its sediment had somehow reduced.
It is instructive to observe the plumes of mud-coloured water that run well out from the mouths of the neighbouring Pühoi and Waiwera rivers after heavy rain events, in comparison to the relatively clear water off the Mahurangi Heads. The
harbour’s tidal prism (flushing volume), in relation to its catchment area is much larger than is the case with the Waiwera and Pühoi estuaries, which are regularly flushed free. The Mahurangi, to its detriment, is all too efficient at retaining
its sediment.
Regardless of the ultimate cause, the scientists determined that it was time to act. To ‘kick start’ the response, a five-year action plan with a regional council budget of nearly $3 million was announced. While it was portrayed as a joint initiative with Rodney District Council, there was to be no additional budget contribution from that quarter.
The funding was targeted primarily at assisting pastoral property holders retire and restore riparian margins. Unsurprisingly, this was considered the obvious and immediate priority.
However, with the restoration of riparian margins alone calling for some 20–40 million plants, and even larger area of the catchment in need of retirement, it was clear to observers that a long-term strategy was called for. If the most erodible of southern-facing slopes were to be retired—say 10% of the third of the catchment that is of steep terrain—another 200 million plants were going to be needed.
Meantime, with a view to assisting volunteer groups to plant areas of public land, the regional council offered to help establish community nurseries. When Friends of the Mahurangi sought professional advice, it was less than encouraging. If the objective was to reduce costs, experienced nurserymen advised, having plants grown under contract by production nurseries was likely to be more effective.
And while the Rodney district boasts a
superb community nursery exemplum, at Täwharanui Regional Park, it was thought unlikely that the same model would attract comparable dedication or make a quantifiable impression on the Mahurangi catchment, which is more than 20 times the land area.
The hunt was on to find an economically sustainable means of reviving the harbour.
Coming section The Search for Silver Bullets
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Historical footnote Large-scale deforestation of the Mahurangi catchment occurred in the mid to late 1800s, but the process spanned a century dating from the establishment of Gordon Browne’s spar station in 1832. Browne’s business received a setback when HMS
Buffalo called in 1834—spoiling things, in his opinion, by paying local Mäori too much for their labours extracting kauri spars. Things only got worse. When operating from Mercury Bay, two shiploads of his spars were
condemned by the admiralty—Browne suffered a mental breakdown.
The Open-Ground and Container-Raised Indigenous Plants Comparison is a Sustainable Farming Fund project.
Beyond the Regatta: Mahurangi photographs from the early 1900s depict a largely denuded landscape. Photographer Henry Winkelmann 1901 Publication Jade River : A History of the Mahurangi
Mountains of Mud: ‘Warkworth in winter was an island in a sea of mud.’