River water quality league tables
The Ministry has ranked sites in the National River Water Quality Network based on their water quality. The national network includes 77 sites located on 35 rivers. These are typically the larger rivers in New Zealand, which drain about half of New Zealand’s land area. These league tables do not include regional council water quality data and should not be used to interpret the state of water quality within any one region.
The sites are ranked in three league tables based on nutrient levels, water quality for recreational use and biological indicators. Many natural and land-use factors affect water quality and the health of rivers and streams (eg, nutrient inputs, flow levels and riparian and instream habitat). Therefore, the relative1 ranking of sites is not necessarily similar for all three league tables, ie, sites may rank high for nutrients but low for biological indicators.
Nutrients in 2007
This first table ranks the 77 National Network sites based on their median nutrient levels in 2007. A site’s overall rank is based on relative1 median levels of all four nutrients: nitrate (also known as oxidised nitrogen), total nitrogen, dissolved reactive phosphorus and total phosphorus.
Key findings:
- Where there is more than one site on a catchment, the upstream site usually has the better rank. There are a few exceptions to this, for example, the Mohaka (Hawke’s Bay) and Motu (Gisborne) Rivers.
- In most cases, each site is ranked similarly for all four nutrients. There are a few exceptions to this, for example, the Shotover River at Bowens Peak has relatively low levels of nitrate, total nitrogen and dissolved reactive phosphorus but high levels of total phosphorus. On the other hand, Oreti River at Lumsden has relatively high nitrogen levels but relatively low phosphorus levels.
- The state of nutrients in 2007 at these 77 sites is varied throughout the country, ie, most regions have sites that feature among both the worst and best sites.
Water quality for recreational use in 2007
This table ranks 76 National Network sites based on two variables that are often used to indicate whether water quality at a site is suitable for recreational use: water clarity and levels of the indicator bacteria, Escherichia coli (E. coli). A site’s overall rank is based on relative1 median water clarity and 95th percentile E. coli bacteria levels in 2007.
Note that the National Network was not primarily designed to check whether water quality is suitable for recreational use. Regional councils monitor water quality for recreational use at about 200 freshwater sites every summer.
Key findings:
- Where there is more than one site on a catchment, the upstream site usually has the better rank. There are a few exceptions to this, for example, the Opihi (Canterbury) and Rangitaiki (Bay of Plenty) Rivers.
- In many cases, rankings for clarity differ from rankings for E. coli, especially for sites with better water quality for recreational use.
- The state of water quality for recreational use in 2007 at these 76 sites is varied throughout the country, ie, most regions have sites that feature amongst the worst and best sites. However, there are some exceptions to this, for example, the Manawatu-Wanganui region has three rivers that feature in the worst 10 sites, the Waikato region has two, and Taranaki region has two. On the other hand, Tasman, Hawke’s Bay and Canterbury regions all have two rivers in the best 10 sites.
- Typically a site’s ranking for recreational use is similar to its ranking for nutrient levels. There a few exceptions to this, for example, Buller River at Te Kuha has an overall ranking of 22 for nutrient levels, but an overall ranking of 59 for water quality for recreational use.
Water quality for recreational use league table
Biological indicators
This table ranks 66 National Network sites based on four biological indicators for macroinvertebrates and periphyton (algal) cover. This includes average Macroinvertebrate Community Index (MCI) and percentage of Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies) taxa for 2005-2007 and the annual mean and maximum periphyton cover for 1990-2006. A site’s overall rank is based on relative1 scores for each of these four indicators.
Key findings:
- Where there is more than one site on a catchment, the upstream site usually has the better rank.
- In many cases, the rank based on the two macroinvertebrate indicators differs to the rank for the two periphyton indicators.
- The state of water quality based on these four biological indicators at these 66 sites is varied throughout the country ie, most regions have sites that feature amongst the worst and best sites.
- In many cases, a site’s ranking for biological indicators does not relate well to its ranking for nutrient levels and/or water quality for recreational use. For example, the Monowai River below gates site is ranked 2nd for nutrient levels and 5th for recreational use, but is ranked 65th for the biological indicators. This is likely to be a result of the site being downstream of a reservoir and highlights the importance of looking at water bodies in a holistic way.
Biological indicators league table
1. Each site has been ranked relative to other sites by calculating a standard score for each value. The standard score used is the value minus the median divided by the interquartile range.
Last updated: 20 October 2009
