River & stream health 2002/03

Key points:

Water quality and stream health

The Greater Wellington Regional Council monitors water quality and stream health at 51 sites in streams and rivers throughout the region. As in previous years, the best water quality and stream health during 2002-03 was in streams and rivers flowing out of the Tararua and Rimutaka Ranges. These rivers - the Waiohine and Tauherenikau, and the upper reaches of the Otaki, Hutt, Waikanae, Wainuiomata and Ruamahanga - have little or no human influence except in their lower reaches.

There was mild to moderate pollution in the Mangaroa, Waitohu, lower Ruamahanga, and other rivers flowing through agricultural land. This is mainly caused by high levels of nutrients, sediment and bacteria in farm runoff. Stock access to streams and their banks also increases pollution and degrades stream habitat.

The poorest water quality and stream health was in streams flowing through urban areas. Among the most polluted were the Karori, Porirua, Ngauranga, Owhiro and Waiwhetu streams. Urban streams are generally contaminated by nutrients, sediment, faecal bacteria and heavy metals. These contaminants are carried to streams in stormwater from roofs, roads and industrial areas, although some may come from chemical spills and sewage-stormwater cross connections.

The continued poor condition of our urban streams prompted us to commission a study into the ecological condition of ten streams in Wellington and Porirua. We found that, as the amount of catchment covered by hard surfaces (roads and roofs) increases, the ecological condition of the stream decreases. Our results are similar to those from studies overseas and elsewhere in New Zealand. However, the ecological decline in our streams is much less marked than in Auckland streams, possibly because the land is steeper, with faster runoff and less opportunity for contaminants to accumulate, and many stream channels are not greatly modified. The study will be extended in 2003-04.

Discharges of sewage from Paraparaumu on the Kapiti Coast and from five Wairarapa towns continue to degrade streams in the area. Water quality in the Ngarara Stream and Wainuiomata River is still improving since the discharges of sewage into them stopped in 2001.

Stream health as indicated by the types of macroinvertebrate species collected from the stream. Macroinvertebrates are tiny animals without backbones, like insects, worms and snails. Streams with clean water have high numbers of pollution sensitive species like stoneflies and mayflies. These insects are rarely present in severely polluted streams like the Owhiro, Ngauranga and Porirua streams. Instead there are high numbers of pollution tolerant species like midge larvae and snails. Photos: Stephen Moore.

What a difference the shade makes!

In streams flowing through open country without bushy vegetation, water temperature, oxygen levels, acidity, and conductivity can fluctuate significantly over the course of a day, causing harm to stream life. Many macroinvertebrates and fish get stressed when water temperatures get above 20 degrees and some can't survive water temperatures over 25 degrees. The graph below shows temperature changes in a well shaded river compared with a river that flows through open country.

Changes in water temperature in the Wainuiomata River and Horokiri Stream over two days from 5/02/03 to 7/02/03. The well-shaded Wainuiomata River site (shown in blue) had lower, more stable temperatures than the Horokiri Stream site (shown in red) which has little upstream shade.

Suitability for swimming

Water quality at 20 popular swimming spots is monitored weekly between November and March and assessed using national water quality guidelines for swimming. During 2002-03, eight sites were always within guideline limits for safe recreational use, while the remaining 12 sites did not exceed the guideline more than three times.

In the western part of the region, all breaches of the guideline occurred during or immediately after rainfall events. This was not always the case in the Wairarapa where there were three breaches not associated with rainfall - twice on the Ruamahanga River upstream of Masterton, and once at Bentley's Beach in South Wairarapa. Follow up samples taken within two days at those sites did not identify the source of pollution but showed that bacteria numbers had returned to safe levels.

See our bathing water quality pages to find out more about where water quality is suitable for swimming.

Suitability of 20 river sites for swimming, kayaking and other recreation during the 2002-03 summer. We recommend that people do not use rivers for recreation during or shortly after heavy rainfall because that's usually when they are affected by high bacteria levels in urban and rural runoff.

What is Greater Wellington doing?

Monitoring and investigation: In 2002-03, we reviewed our rivers monitoring programme to improve coverage of different stream types. We also investigated:

• changes in the health of three streams that were fenced and planted in 2001
• the ecology of streams in the Wellington and Porirua urban areas
• the level of contaminants in stormwater
• barriers to fish passage.

Greater Wellington runs three environmental education programmes:

• Take Action for Water - working with 8-12 year olds in schools,
• Take Charge - working with industry, and
• Take Care - working with volunteer community groups.

Riparian management: from 2004, we will provide financial assistance to landowners to plant streams in 12 selected high priority catchments. This assistance will only be provided where streams have already been fenced.

Improving water quality: six polluted rivers and streams have been identified in our ten year plan to have their water quality improved. Work is underway on the Waiwhetu, Waitohu, and Kaiwharawhara streams.

Policy and regulation: we control, through the Regional Freshwater Plan, discharges of pollutants into water and other activities that affect rivers.

What can you do?

• Keep animals, especially cattle and deer, out of rivers and streams.
• Don't put chemicals or any other waste in stormwater drains, rivers or streams.