Factsheet: Q&A: River Water Quality Trends Release Sept 2019

What do the national trends tell us about river water quality changes from 2009–2018?

The LAWA national trends are displayed in five trend categories (very likely improving, likely improving, indeterminate, likely degrading and very likely degrading) across nine widely recognised water quality indicators. The trends present a high-level national picture of river water quality changes over the past decade, and the results are a mix of improving and degrading sites for all water quality indicators.

The results show a range of both improving sites and degrading sites for all indicators over the last 10 years. Amongst the most certain water quality trends, degradations were more common than improvements for turbidity, total nitrogen (TN), total oxidised nitrogen (TON), faecal indicator bacteria (E. coli) and the Macroinvertebrate Invertebrate Community Index (MCI), whereas improvements were more common than degradations for clarity, ammoniacal nitrogen (NH4), total phosphorus (TP), and dissolved reactive phosphorus (DRP). Given New Zealand’s NPS-FM requirement to maintain or improve river health, the results indicate that more effort is required to achieve this goal.

Site by site trends and state are useful for community groups, landowners, and other parties interested in how their local waterways are tracking. Interested communities can use the interactive river water quality map to filter sites by trend and/or state, providing greater understanding of what’s happening at a catchment level over time. This is available on LAWA's River Quality Topic.

What do the water quality indicator trends by state quartile graphs tell us?

By comparing the national trend results against the current state of these sites, we can explore questions like “Are the worst sites getting better, or are they degrading further?” and “How does this compare with the best sites?”. 

When viewing trends by the current state (quartiles), there are no overall patterns common to all indicators, but for NH4 and DRP there is some indication that a higher proportion of the best sites (i.e., 1st quartile) are getting better. For TP there is some indication of a pattern of increasing proportions of degrading trends as you move from the best sites (1st quartile) to the worst sites (4th quartile).

The quartile groups are judged only against each other so the worst quartile may not necessarily be ecologically unhealthy, nor may the best quartile be ecologically healthy. It is all relative. See the ‘Calculating water state’ factsheet for context on river water quality state in New Zealand.

How were the trends evaluated?

Cawthron Institute led the analysis of trends. They applied the trend evaluation method developed by Graham McBride (NIWA) and coded by Dr Ton Snelder and Caroline Fraser (LandWaterPeople) to New Zealand’s most comprehensive river water quality dataset comprising monitoring data collected by all 16 regional and unitary councils and NIWA. Please see the ‘Calculating water quality trends’ factsheet for more.

What do the different coloured bars in the national trend summary graph mean?

The grey bars represent indeterminate trends: at some sites there is insufficient evidence to clearly say if water quality has improved or degraded over the last 10 years. It’s important to note that indeterminate does not mean the same as no change; it simply means the evidence isn’t strong enough to make a call.

A green bar represents the sites that have improved for that indicator. An improvement is generally a reduction in a water quality indicator score, for example phosphorus concentrations, except for visual clarity where an improvement is an increase in clarity or for MCI where a higher score means better ecological health at a site. 

A red/orange bar represents sites with a degrading trend in water quality over time. A degrading trend is generally an increase in a water quality indicator, but a decreasing score for clarity and MCI.

Please see the trends table for another view of the national trends with the number of sites given.

Why has LAWA added 15-year river water quality trends for individual sites?

This is the first year that we have shown 15-year river water quality trends, which provides a longer-term overview of changes to local water quality. In the future, more and more sites will have a sufficient data record to provide this longer-term view.

What causes river health to degrade?

Polluted run-off is the most significant contributor to water quality decline in New Zealand. Run-off is water running from our land to rivers, lakes, and beaches; along the way this water picks up pollutants such as chemicals and fine sediment from sources such as agriculture, horticulture, forestry, transport, urban streets, and homes. Some other factors that also impact water quality and river health include changes in climate, flow regime, shade, channelization, pest invasions and blockages to fish passage.

How can people find out more about river water quality near them?

New Zealanders interested in river water quality can explore river trends and/or state across all nine water quality indicators using the interactive river water quality map. Local sites can also be found by navigating to the region, then catchment, and then site of interest.

Can the trends in this summary be compared with any previous trend summaries?

Each year we implement the most appropriate methodology available, to generate our best estimate of nationwide trends in water quality.  This means that year to year the details of that methodology might be refined, leading to some difference at a site level.  While it is informative to look at changes in the proportion of different trend types at a national level, some caution should be exercised due to the differences in the method applied.

What is Land, Air, Water Aotearoa (LAWA)?

LAWA aims to connect New Zealanders with their environment through sharing scientific data online. It’s world-leading in its approach to making data collected by authorities, freely available to the public. This is made possible by a collaboration of NZ’s 16 regional and unitary councils, Ministry for the Environment, and Cawthron Institute. LAWA’s committed to sharing the best available information, so New Zealanders can make informed decisions.