River and stream water quality and ecology - Greater Wellington ...

River and stream water quality and ecology in the Wellington region: State and trends
community composition at RSoE sites showed that the occurrence of soft
sedimentary substrate was highly correlated with the occurrence of pollutiontolerant
macroinvertebrate taxa and all but one of the soft-bottomed sites were
assigned to a macroinvertebrate class of ‘fair’ or ‘poor’. Unsurprisingly these
sites also had poor water clarity.
In addition to being poor at soft-bottomed sites, water clarity was also poor at a
number of hard-bottomed sites with highly erodible and/or soft sedimentary
geology catchments, such as Awhea River at Tora Road and Mataikona
tributary at Sugar Loaf Road. Greater Wellington (and its predecessor the
Wairarapa Catchment Board) has been working with landowners in the eastern
Wairarapa hill country for a number of decades to develop farm or
sustainability plans and retire and re-vegetate erosion-prone land. In 2009 the
Wellington Regional Erosion Control Initiative (WRECI) was launched,
leading to the development of more comprehensive farm management plans to
address soil erosion in targeted catchments. While this has resulted in
significant soil conservation gains for landowners, visual clarity and total
suspended sediment monitoring results indicate that stream sediment loads
remain high (as do sedimentation rates in the Whareama Estuary – see Oliver
and Milne (2012)), suggesting that additional monitoring and management
approaches are needed to identify and protect erosion ‘hot spots’ within the
catchment. Although no work has been done to date to quantify such hot spots,
it is likely that stream bank collapse and erosion are key sources of instream
sediment (A. Stewart 33 , pers. comm. 2012). Addressing this will require
extending beyond planting on farms to the retiring, fencing and re-vegetating
of riparian margins of affected rivers.
The Horokiri Stream catchment, which drains to the Pauatahanui Arm of
Porirua Harbour, is also characterised by steep and unstable soils. Soil/stream
bank erosion and sediment runoff from forestry tracking activities may explain
the significant decline in visual clarity (0.27 m/year and 0.17 m/year in raw and
flow-adjusted data, respectively) recorded at Horokiri Stream at Snodgrass
over the period July 2006 to June 2011. According to Oliver and Milne (2012),
in the order of 200 ha of forestry clearance is expected in this catchment over
the next five years, highlighting the need for effective sediment control
measures to reduce sediment runoff to the Horokiri Stream.
In stable, hard sedimentary catchments poor water clarity or high TSS
concentrations recorded on some occasions during dry weather were associated
with stock crossings (eg, Parkvale Stream at Weir), instream flood protection
or gravel extraction works (eg, Hutt River at Boulcott and Ruamahanga River
at Te Ore Ore), and specific pollution incidents. As noted in Section 3.4.1,
there has been significant urban development in parts of the Wellington region
over the last decade, particularly in the northern suburbs of Wellington city.
Greater Wellington pollution incident records, along with compliance
assessments of consented earthworks sites, confirm a number of instances
where sediment control mechanisms (eg, silt ponds) have failed (Figure 8.2),
resulting in large volumes of sediment entering nearby streams. A number of
the incidents have been reported in the Porirua Harbour catchment, where there
33 Andrew Stewart, Land Management Project Co-ordinator, Greater Wellington.
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