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Ecological Status of the Ōpāwaho-

Heathcote River 

Prior to European settlement, Māori would have viewed  the river and estuary as a cornucopia or kete of riches, filled with birds, fish and other useful resources for their daily and seasonal use.  However, until relatively recently, European settlement meant disaster for health of the Ōpāwaho Heathcote River. Although initially used for shipping purposes, its status was relegated to that of drain or sewer conduit by a growing industrial and residential population. Until the aesthetics of amenity value overtook convenience in the mid-20th century, all manner of toxic chemicals, rubbish and sewerage were dumped in the river.

Current monitoring by the Christchurch City Council indicates that the Ōpāwaho-Heathcote River has characteristics typical of an ‘urban river syndrome’. It has the worst water quality of any river in Christchurch.  

For specific information about the ecological health of the Opawaho-Heathcote river, watch this presentation 'Recovery of the Heathcote River: Water Quality and Aquatic Ecology' by Christchurch City Council freshwater ecologist, Dr Belinda Margetts, presented at the Natural Environment Recovery Programme March 2015 (approx 10mins, Dr Margetts begins at 2:55min). Belinda’s presentation also includes comments on the impact of the 2011 earthquakes and the aquatic ecology of the Ōpāwaho-Heathcote River.

For an overview of the ecological health of waterways in Christchurch in general, watch this  'Caring for our Urban Waterways' presentation also by Dr Belinda Margetts, presented at the Caring for Urban Waterways Forum 2016.

Characteristics of an urban river syndrome include:

  1. Water, unable to soak through the ground and recharge aquifers or rivers, instead runs along gutters and into stormwater drains, collecting pollutants and litter along the way.
  2. Riparian (river bank) vegetation is removed for building or other developments, or so that people can see the waterways. However riverbank vegetation is important to help keep river banks stable, and provide habitat for fish, birds and insects. (See more details below about plants and animals).
  3. Erosion of unstable stream banks.
  4. Large amounts of sediment can be washed into the waterway as a result clearing vegetation, new subdivisions, or the building and extension of houses. This is especially relevant for the Ōpāwaho-Heathcote River.
  5. A greater variation in river flows – much lower flows when there is little rain and much shorter,  sharper floods in times of heavy rain because the water runs off quickly rather than soaking into the ground and recharging aquifers. (See more details about flooding below)
  6. Straightening of stream channels, culverting and piping of waterways decreases the amount of habitat available for invertebrates and fish.
  7. Pollution (see more details below)
  8. Excess nutrients. Nutrients are substances that provides nourishment essential for life and growth.Nutrients in waterways include nitrogen and phosphorus - e.g. from garden fertiliser dissolved in rainwater, or from farming activities. Too much of these nutrients can upset the balance of life in the waterway.
  9. Proliferation of invasive fresh water weeds. 

 

Flora and Fauna
 

Aquatic Plants

The Ōpāwaho Heathcote River aquatic plant life has changed significantly. The plants have changed from mainly filamentous green algae that floated in or on the water, to larger, leafy, introduced plants such as Curly-leafed pondweed (Potomogeton crispus)Oxygen weed (Egeria densa)Canadian pondweed (Elodea canadensis) . These are attached to the bottom and tend to clog up the river, slowing down the water flow and holding sediment in place so that it is no longer flushed out when there is a flood.  Yellow Flag Iris (Iris pseudacorus)  is also becoming a problem in Christchurch waterways, particularly along the Ōtākaro Avon River. It has the potential to enter the Ōpāwaho-Heathcote River on the tide from the estuary, so vigilance is required to stop its spread. All of these introduced plants are pest species which have major impacts on indigenous plant biodiversity, hydrogeneration, irrigation, flood protection and recreation.

Riparian Plants

Stream restoration groups are playing a vital part in restoring the riparian vegetation which would have been present in various gradients along the river banks from source to sea. 

King George V Reserve in St Martins/Beckenham is an excellent living example of a restored riparian ecosystem showcasing the plant species which would have been present on these stretches of the Ōpāwaho Heathcote River banks prior to human habitation. The area is planted in zones of terrace slopes of tōtara (Podocarpus totara) and matai (Prumnopitys taxifolia), woodlands of kānuka (Kunzea spp.), river levees of kōwhai (Saphora spp.) and houhere (Hoheria populnea), flood plain forest with kahikatea (Dacrycarpus dacrydioides) and pōkākā (Elaeocarpus hookerianus) swampland  planted with pūrei (Carex secta), harakeke (Phormium tenax) and ti kōuka (Cordyline australis)

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Fauna

The invertebrate fauna of the Ōpāwaho-Heathcote River are typical to that found in other urban streams. The macroinvertebrate community is dominated by worms - primarily oligochaete (segmented) worms,  two species of snails - New Zealand mud snail (Potamopyrgus antipodarium) and the introduced Physella acuta two kinds of crustaceans - seed shrimp (ostracods) and the freshwater amphipod (Paracalliope fluviatilis) - and midges.

Different species of fish including bullies, introduced trout, tūna (eels). pātiki (flounder) and inaka (whitebait) have all been observed in the Heathcote/Ōpāwaho. Kākahi (freshwater mussels) and kōura (freshwater crayfish) are living in Cashmere Stream, although it appears that there are very few young mussels.

There are now more birds and more kinds of birds than there were in the mid-1980s, largely due to the habitat restoration work that was undertaken in the early 1990’s in the lower Ōpāwaho-Heathcote. Scaup, pukeko, native ducks, cormorants and gulls are all present.

 

Sediment

After a heavy rainfall, the Ōpāwaho-Heathcote river very often looks muddy or brown. This is due to sediment being deposited into the river, which is tiny particles of rock or soil suspended in water. Excess sediment in the Ōpāwaho-Heathcote river is a problem because it alters the habitat that many freshwater insects, fish and a number of birds species require to thrive.

Prior to European colonisation, the wetlands and vegetation along the side of the Ōpāwaho-Heathcote and the forest growing on the hillsides would have ensured that this sediment loading was minimal. Removal of hill vegetation, farming, and urban development on the hills significantly increased the sediment load to the Heathcote/Ōpāwaho and its major tributary, Cashmere Stream. Other tributaries with sediment loading include Couling Creek and Cashmere Brook.


Pollution

Pollution is caused when substances (usually made or concentrated by human activity) contaminate the natural elements, such as air, land and water. Water is polluted when chemical, biological or physical material gets into fresh or ocean waters in amounts that do not occur naturally. Raised levels of contaminants such as heavy metals affect the quality of the water and the health of the plants and animals that live there. In cities such as Christchurch, these pollution problems are made worse by many hard, surfaces that does not absorb water (e.g. roads, car parks). When it rains, the water runs quickly across these surfaces, washing contaminants into our waterways.

Contaminants can include:

1. Brake pad dust and rubber from vehicle tyres.

2. Metals such as lead, copper and zinc, some of which come from roofing materials

3. Fine particles of chemicals from industrial processes  or from vehicle exhausts, which drop out of polluted air or are washed out of polluted air when it rains.

4. Chemicals that collect on impervious, hard surfaces. Examples include oil that leaks from cars or trucks onto  the roads, polycyclic aromatic hydrocarbons (PAHs) from the use of tar seal, detergents used when  people wash their cars on their driveways, or paint chemicals that get into the stormwater system when  people wash their brushes outside.

5.Faeces from mammals (such as dogs, hedgehogs,  possums) and birds. Too much faecal matter in the water means that there are likely to be pathogens  present in the water. The amount of faecal matter in the water is  measured by the number of E.coli  bacteria (short for Escherichia coli) per 100 ml. This measurement is used to decide when it is safe to drink the water, to eat  shellfish that have been in the water, or to swim. Faeces can also enter Christchurch waterways in times of high rainfall events, where water in storm water drains overflow and mix with the sewerage network.

6. Chemical spills. Nowadays,  chemicals spills are usually accidents. An example of a chemical spill was in 2008 when blue ink was  spilled into the Heathcote River.  Some  chemical spills can remain invisible and only become obvious when the river smells (as happened when  diesel was spilled into the headwaters of the river in 2005) or when plants and animals  start to die. All of these contaminants can affect the environment for a very long   time. In Christchurch, these contaminants get into our rivers and may  eventually end up in the sediments of  the Avon-Heathcote Estuary/Ihutai.

Most of the time, these contaminants are below guideline values in the city’s rivers. However, in places that have a relatively dry climate as Christchurch does, pollutants can build up over long, dry periods and run into the waterways in high concentrations the next time it rains. This is usually known as the 'first flush'.

Haytons Stream is currently a pollution ‘hot-spot’ in the Ōpāwaho-Heathcote catchment.


Flooding and salination

Much of Christchurch used to be wetlands, which provided a buffer against floods and droughts by acting a bit like a sponge, which soaks up water at times of high rainfall and releases it slowly after the rain has stopped. Since 1850, most of the wetland “sponges” had been drained and cleared as Christchurch city grew, but the water that they held still needs to go somewhere if it can’t get down the streams. Water runs very quickly off urban areas with their many impermeable (hard) surfaces such as roofs, driveways and roads.  

As Christchurch City grew over time and surface water volumes increased during heavy rain events, flooding became a significant issue in the lower Ōpāwaho-Heathcote in the 1970’s, particularly in Clarendon, Richardson, Aynsley Terraces. In response to this, the City Council constructed the Woolston Cut, which cut a 450 meter straight line from one end of a loop in a lower part of the river to the other, with the aim to drain as much water away as quickly as possible. Opened in 1986, it became evident  by 1988 that the Woolston Cut was having a significant negative environmental impact on the river. Salinated water was intruding further up the river, changing the soil structure of the river banks, which in turn weakened and collapsed, killing trees and changing habitat. Burrowing crabs also contributed to damage to the banks.  

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A tidal barrage was built by the City Council in 1993 at a cost of $1.5 million. It was designed to direct the river through the loop when the river is at a normal volume of flow. When the river floods, the barrage opens temporarily and allows the excess flood water through the cut as well as the loop. This has helped to reduce the flow/ of salinated water up the river.  The Council also established Lower Heathcote Environmental Enhancement Programme, planted native species up and down the river banks to help stabilise the banks.

More recent flood management techniques include the construction of flood retention basins by the Christchurch City Council in the upper reaches of the river (e.g. Wigram and Hendersons basin). Instead of immediately flowing into the river, water is instead held in ponds. As a result, floods in the Ōpāwaho-Heathcote now have lower peaks.

 


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