1 September 2011
Christchurch waterway ecology and health investigated following earthquake
Two reports commissioned by Environment Canterbury and Christchurch City Council that summarise how the February
earthquake affected the ecology and health of Christchurch’s two main rivers and estuary have been released.
In all, eight studies were undertaken by NIWA, EOS Ecology, University of Canterbury and Aquatic Ecology to help
determine the ecological effects of wastewater discharges and physical changes to waterways. The reports also provide
advice on how to manage the effects and impacts of the earthquakes.
The studies focused on toxicity levels, aquatic invertebrate and fish communities, inanga (whitebait) spawning habitats
as well as identifying ecologically important areas in the Avon/Ōtākaro and Heathcote/Ōpawaho rivers. They also looked
at morphological effects, estuarine habitat and food safety of cockles in the Avon-Heathcote Estuary/Ihutai.
Ken Taylor, Environment Canterbury’s Director of Investigations and Monitoring, said the earthquake had both direct and
indirect impacts on Christchurch’s waterways.
“The physical characteristics of some of our waterways have been altered following February’s earthquake. River bed
levels have changed and banks have collapsed, altering the alignment of channels. There have also been significant
changes to the bed of the Avon-Heathcote Estuary/Ihutai.
“The discharge of sewage, trade waste and sediment-laden stormwater into waterways has also resulted in physical changes
to water and altered the water chemistry. This has impacted on the local ecology including vegetation, fish,
invertebrates and micro-organisms,” he said.
Immediately following the earthquake large volumes of untreated wastewater were discharged into both rivers. It is
estimated around 35,000 cubic metres per day of wastewater was discharged into the Avon/ Ōtākaro in March, with the
volume decreasing significantly in the following months.
In late April to early May modelled and observed dissolved oxygen (DO) concentrations in some parts of the rivers were
below guidelines for aquatic species. With higher discharge levels in February and March the DO concentrations were
likely to be very low in some areas meaning fish kills were possible in both rivers. A study looking at survival of
common invertebrate species found that the changes in water quality resulting from the wastewater discharges increased
the mortality of sensitive species such as freshwater hoppers.
Studies investigating fish and invertebrate populations in the upper reaches of the Avon/ Ōtākaro, above the influence
of most wastewater discharges, found there had not been major changes in the abundance or types of species present.
Site-specific changes were found though, particularly where habitat changes due to sediment inputs had reduced the
occurrence of sensitive species, such as caddisflies and bluegill bullies.
A survey of inanga spawning sites found that damage to sites on both rivers was highly variable, ranging from minor to
severe. Inanga eggs were observed at only the Avondale site but this may not reflect earthquake effects alone,
The input of wastewater into the two rivers and the estuary has also been found to have had a significant impact on the
Avon-Heathcote Estuary/Ihutai Bacteria levels increased dramatically after the February earthquake and the levels in
cockles at the two river mouth sites were almost twice as high as any previous record when treated wastewater was
discharged into the estuary, and more than four times as high as levels measured since the treated wastewater discharge
was removed in March 2010. There has been consistent advice since the February 22 earthquake not to gather or eat
shellfish from the estuary.
There have also been changes in bed height and estuary depth. The northern part of the estuary subsided between 0.2 and
0.5m and the southern part of the estuary rose by between 0.3 and 0.5m. The area affected by liquefaction mounds was
found to be extensive, which affects estuarine biota by smothering and changing of dominant sediment particle sizes. The
liquefaction sediments contained lower concentrations of heavy metals and organic matter than the pre-existing sediment,
which may aid in estuary rehabilitation.
“The ecological and microbiological studies of our waterways following February’s earthquake will enable informed
decisions to be made on a number of important issues. These issues include recreational water use, as well as works in
and around waterways to stabilise banks and restore flow capacity.
“Environment Canterbury and Christchurch City Council will continue to work with Community Public Health and all of the
agencies involved in the research to review urban water issues,” said Ken Taylor.
The study suggests a number of actions to mitigate the effects of the earthquake on waterways. For example it recommends
measures to improve the spawning attractiveness of areas on the two rivers. This includes sediment trapping fences, weed
control measures and sediment removal.
The study also identifies five ecological values that need to be considered should any major in-stream or bank-side
remedial works be taken. These include trout and inanga/whitebait spawning areas, native fish population and areas
supporting significant invertebrate and macrophyte communities.
For summary reports of the post-earthquake studies undertaken by NIWA, EOS Ecology, AEL and the University of
Canterbury, with support from Environment Canterbury and CCC visit http://ecan.govt.nz/services/online-services/monitoring/pages/ecological-effects-christchurch-waterways.aspx .
Up-to-date information on public waterways affected by sewage discharges can be found at www.ecan.govt.nz/services/online-services/monitoring/ .
ENDS