Visual clarity in rivers – national network
Current situation
In 2007, the national median visual clarity for rivers in the national network was two metres. Water clarity varies widely between rivers around the country. Visual clarity of only 10 to 40 centimetres is common in rivers with very high levels of sediment. These rivers include the lower Manawatū, the Waitara in Taranaki and the Waipaoa in the Gisborne district.
Visual clarity (metres) for the 77 national network sites, 2007
| 5th percentile (rivers with murkier water) |
Median | 95th percentile (rivers with clearer water) |
|
|---|---|---|---|
| Visual clarity | 0.51 | 2.01 | 6.33 |
Notes:
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) Calculated from annual median data.
Data source: compiled by the Ministry for the Environment using data collected by the National Institute of Water and Atmospheric Research (NIWA).
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) Calculated from annual median data.
Data source: compiled by the Ministry for the Environment using data collected by the National Institute of Water and Atmospheric Research (NIWA).
Soil erosion is a common cause of low levels of clarity in New Zealand rivers and streams. This may be a consequence of poorly managed farmland (for example, the collapse of unprotected stream banks and sediment run-off from paddocks). Urban development and harvesting of plantation forestry can also produce high volumes of sediment run-off.
Natural factors can also determine clarity. For example, the low level of clarity in the Waipaoa River is caused by the geology of the catchment. Sandstones, mudstones and gravels are easily eroded, which leads to high-suspended sediment loads.
Visibility of more than 10 metres is common in the country’s clearest rivers (the upper Motueka, Clutha, and Monowai Rivers). The upper catchment of the Motueka River is almost entirely native bush or bare mountain rock and as a result, the amount of sediment that enters the upper reaches of the river after rainfall is minimal.
The sites in the national network have been ranked based on nutrient levels, water clarity, bacterial levels, macroinvertebrates and periphyton. The results are presented in river water quality league tables.
Long-term trends
Between 1989 and 2007, the median visual clarity improved in rivers within the national network by 0.6 per cent per year. Since 2002, the median visual clarity for monitored New Zealand rivers ranged between 1.3 and 2.1 metres.
This meets the requirements for ecosystem protection in the Australia and New Zealand Environment and Conservation Council (ANZECC) guidelines (see note below) but has at times been below the recommended minimum for human recreation (1.6 metres).
Clarity trends in rivers in the national network, 1989-2007
Notes:
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) The ANZECC guidelines state that clarity of less than 0.7 metres, averaged for upland and lowland rivers, is unacceptable. These guidelines are ‘trigger values’ for the protection of ecosystems and the recreational and aesthetic values of waterways. If a trigger value is reached, it does not necessarily mean that ecosystem damage is occurring or that recreation is no longer possible, but it provides advance warning that a problem may be emerging.
Data source: compiled by the Ministry for the Environment using data collected by NIWA.
(1) Hazen method used to calculate 5th and 95th percentiles.
(2) The ANZECC guidelines state that clarity of less than 0.7 metres, averaged for upland and lowland rivers, is unacceptable. These guidelines are ‘trigger values’ for the protection of ecosystems and the recreational and aesthetic values of waterways. If a trigger value is reached, it does not necessarily mean that ecosystem damage is occurring or that recreation is no longer possible, but it provides advance warning that a problem may be emerging.
Data source: compiled by the Ministry for the Environment using data collected by NIWA.
Read table of data of this graph
It is not yet possible to identify why the water clarity in rivers has improved. However, the improvement may be related to a reduction in sediment in the water as a result of better forestry and farm management (for example, fencing to prevent stock trampling river and stream banks).
Of the 77 network sites, 27 had significant increasing (improving) trends in visual clarity between 1989 and 2007, of which 25 were environmentally meaningful1. Ten sites did show environmentally meaningful decreases in visual clarity in the same period, which is seen as a decline in water quality.
What can you do?
- Fence off rivers, lakes and wetlands to help protect our waterways. Planting native vegetation or other plants enhances water quality by filtering sediment, faecal bacteria and nutrients from surface water run-off.
- Planting trees also prevents soil erosion. Minimise water use by using plants that are suitable for local conditions and use mulch around them to retain moisture.
- Farmer? Prevent stock from entering waterways.
- For more advice on stream and wetland protection contact your local or regional council or the New Zealand Landcare Trust.
- Follow urban design and industry best practice guidelines to reduce contaminants getting into waterways.
This information has come from Environment New Zealand 2007 and the technical report: Water quality trends at National River Water Quality Network sites 1989-2007.
This data comes from the National River Water Quality Network which is operated by NIWA with funding from the Foundation for Research, Science and Technology.
You can access the national network data for 2007, as well as more site information, in this spreadsheet.
Return to the main river water quality page
Back to footnote reference 1 A 'meaningful' trend is both statistically significant and environmentally meaningful, while a 'significant' trend is statistically significant but not necessarily environmentally meaningful (eg, the scale of the change could be very minor).
Last updated: 3 June 2011
