River water quality trends: clarity

  • Image, River water quality trends: clarity.

    Water clarity is a measure of underwater visibility in rivers and streams. Fine particles like silt, mud, and organic material can reduce the clarity of water. Poor clarity affects the habitat and food supply of aquatic life such as fish and aquatic birds. It can also impact on the aesthetic values and recreational use of rivers and streams.

    We classified River water quality trends: clarity as a national indicator.

    Key findings

     Increasing trend (improving state)

    Water clarity at 77 monitored sites on 35 large rivers improved 0.6 percent a year between 1989 and 2013 (the median trend after adjusting for the influence of river flow).

    • The trend for water clarity significantly increased at 49 of the 77 sites between 1989 and 2013, but significantly decreased at seven sites. A trend could not be determined for the remaining 21 sites.
    • During the 10-year period 2004–13, water clarity increased by a median of 0.8 percent a year.

    Figure 1

    Note: Data from the National River Water Quality Network (77 sites on 35 large rivers).

    Figure 2

    Water clarity median and trends – interactive map

    Definition and methodology

    Water clarity is measured by placing a black disc in the water. The disc is viewed horizontally through an underwater periscope at increasing distances, until it disappears from sight. This method provides a consistent measure of the greatest distance an object is visible through the water (Davies-Colley, 1988).

    NIWA has measured water clarity at 77 sites along 35 major rivers each month since 1989. The rivers drain about 50 percent of New Zealand’s land area. Of the 77 sites, 40 monitor catchments dominated by pastoral land cover, three sites are in catchments of mainly exotic forest land cover, and 34 are in catchments with mainly indigenous land cover. The long-term data from these sites are particularly useful for tracking trends in water clarity over time (Ballantine & Davies-Colley, 2014).

    Regional council data can be used to assess trends over shorter periods (10 and 20 years). However, regional councils focus their monitoring more on pastoral catchments than on indigenous landscapes (and therefore their data do not represent water quality uniformly across the country).

    The influence of variations in river flow was removed from the data before the trend was analysed.

    Sites were classified by land cover using the River Environment Classification (Snelder & Biggs, 2002).

    Data quality

    We classified River water quality trends: clarity as a national indicator.


    relevance-direct This national indicator is a direct measure of the ‘Condition and physical characteristics of freshwater habitats’ topic.


    accuracy-high The accuracy of the data source is of high quality.

    See Data quality information for more detail.


    Ballantine, DJ & Davies-Colley, RJ (2014). Water quality trends in New Zealand rivers: 1989–2009. Environmental Monitoring and Assessment, 186(3), 1939–1950.

    Davies-Colley, RJ (1988). Measuring water clarity with a black disc. Limnology and Oceanography, 33(4), 616–623. Accessed 18 August 2015 from www.horizons.govt.nz.

    Snelder, T & Biggs, B (2002). Multiscale river environment classification for water resources management. Journal of the American Water Resources Association, 38(5), 1225–1239.


    Published 21 October 2015

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