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 an excellent presentation by Christchurch City Council freshwater ecologist, Belinda Margetts, presented at the Natural Environment Recovery Programme March 2015 (2:55-12:46). 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 a presentation by Belinda Margetts, presented at the Caring for Urban Waterways Forum 2016.
Characteristics of an urban river syndrome include
The Ōpāwaho-Heathcote aquatic plant life has changed significantly. The plants have changed from mainly filamentous green algae that floated in or on the water, to being mainly larger, leafy, introduced plants like the curly leaved pondweed, or Potamogeton crispus. 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 is also a problem.
The invertebrate fauna of the Ōpāwaho-Heathcote river are typical to that found in other urban streams. It is dominated by worms, two species of snails, two kinds of crustaceans, and midges. Different species of fish including bullies, trout, eels, flounder and whitebait have all been observed in the Heathcote/Ōpāwaho. Freshwater mussels (Kakihi) and freshwater crayfish (Koura) 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.
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 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:
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.
Haytons Stream is currently a pollution ‘hot-spot’ in the Ōpāwaho-Heathcote catchment.
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 has grown, 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 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.
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.