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The  health of Mangakotukutuku Stream is affected by a wide range of factors, both natural and caused by human activities, which interact and vary in importance in different parts of the stream. Extensive deposits of orange-coloured iron flocs on the streambed, obvious in the upper parts of the Rukuhia branch, can limit aquatic life but may be natural, although it is unclear whether this has been made worse by drainange activities in Rukuhia Swamp.

Human-induced factors that currently or potentially threaten the health of Mangakotukutuku Stream include:

Agricultural activities

The Peacockes branch has relatively good quality water and habitat, and supports a wide range of sensitive aquatic species. This branch currently drains a mainly agricultural catchment but is buffered for much of its length from the effects of farming by a deeply incised and well-vegetated gully. Urbanisation of this sub-catchment could have significant adverse ecological effects if stormwater runoff and roading are not managed appropriately.

The other two main branches of Mangakotukutuku Stream have agricultural land in their headwaters, which do not flow through incised gullies and therefore are more directly impacted by land use practices on adjacent farms and life-style blocks. Although some parts of the stream are fenced from stock and have trees that provide shade, other parts are open to access and damage from farm animals, as well as from machinery which may be used to improve drainage at the expense of ecological values. Stock access to streams can cause direct nutrient and bacterial inputs from faeces and also erode banks leading to sediment inputs. Runoff from fertilisers also increases nutrient levels, and agricultural pesticides and herbicides that enter streams can directly kill aquatic life.

Urbanisation

Once the Te Anau and Rukuhia branches enter Hamilton City they flow through gullies or parks for most of their length. These gullies buffer them to some extent from the  impacts associated with urban landuse, such as lack of shade. However, at least 15 stormwater outlets flow into Mangaotukutuku Stream, and several smaller tributaries which originate in the urban area recieve direct urban runoff from roads, driveways and other impervious surfaces. In addition to these inputs, alkathene pipes can commonly be seen delivering stormwater to the stream.

Stormwater can threaten stream health in two main ways: (1) by delivering pollutants such as copper, zinc, and a range of  more complex chemicals washed from rooves, roads and carparks, and (2) by increasing peak floodflows which can cause erosion of stream banks. High peak flows are caused by rainwater running off roads and rooves directly into pipes which discharge into the stream, rather than soaking through the ground as would occur naturally. Historically, the Mangakotukutuku Stream has cut down through unconsolidated sediments which erode and collapse at high flows if not bound by tree roots. Lack of bank protection, coupled with high peak flows from stormwater runoff and sediment inputs from upstream agricultural land, have contributed to the very turbid water and silty bed of Mangakotukutuku Stream.

Fish passage

Many native fish species need to move up and down streams to complete their life-cycles. This can involve moving down to the sea as small larvae, or in the case of eels as adults, and then swimming back upstream as whitebait or elvers to find suitable habitat. Species differ in their abilities to climb obstacles; inanga are poor climbers, shortfin eels and giant kokopu have moderate climbing abilities, and banded kokopu are excellent cimbers. However, even the most skilled climbers cannot get past culverts that are perched, and some of the weaker climbers may be stopped by culverts that are scoured at the downstream end or have the water flowing through too fast.

We know from fish sampling that some eels, banded kokopu and giant kokopu have penetrated up to the headwaters of the stream past Ohaupo Road, and occasionally inanga can get past the Peacockes Road culvert. A white-faced heron can often be seen there waiting for unsuspecting fish to swim through. In other places, perched culverts clearly prevent fish passage and limit the biological potential of some parts of the stream, for example off Malabar Rd in Ftizroy Park.

Nuisance species

Introduced species can adversely affect native species by competing for food etc, by eating them (predation), by  behaving aggressively towards them, and by damaging habitat. For example, mosquitofish have been found to attack some native fish and koi carp can stir up sediments and even increase bank erosion through their feeding activities. Fortunately, nuisance aquatic species such as these are not widespread in the Mangakotukutuku; koi carp have only been seen from beow the Peacockes Rd culvert and mosquitofish have only been found in the lower part of the stream. For koi carp, this may partly be because the road culvert plays an important role in limiting their access to upstream sections. No nuisance aquatic plant species have been found so far, and this may partly be due to the high turbidity of the water. Let's keep the upper Mangakotukutuku free of aquatic pests. Click here to find out more about  pest fish from the DoC website.

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Erosion on mainstem

Pipe discharging into mainstem
Rubbish in mainstem

Perched culvert
Grey water discharge



Pipe discharge