River water quality: 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 water clarity. Some water bodies naturally have low water clarity. Poor clarity affects the habitat and food supply of aquatic life, such as fish and aquatic birds, and the growth of aquatic plants. It can also impact on the aesthetic values and recreational use of rivers and streams. We report on the state and trends in river water clarity.

    We classified River water quality trends: clarity as a case study.

    Key findings

    Median river water clarity was greatest at sites in the native land-cover class compared with sites in the exotic forest, urban, or pastoral land-cover classes from 2009 to 2013.

    • Model-based estimations of river water clarity showed 5.7 percent of river length did not meet the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) 2000 trigger values for slightly disturbed ecosystems for the five-year period 2009–13 (upland and lowland).
    • For sites in the pastoral land-cover class, river water clarity worsened at 46.2 percent of sites and improved at 33.1 percent of sites over the 20-year period 1994–2013.
    • For sites in the pastoral land-cover class, river water quality improved at 31.1 percent of sites and worsened at 17.4 percent of sites over the 10-year period 2004–13.

    Figure 1

    Note: Sites are classified by dominant land cover in the upstream catchment. Higher values for water clarity are better than lower values, so data below the dashed line do not meet the trigger value. The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent) of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The dashed line (at 0.8 metres) represents the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) recommended trigger value for water clarity for slightly disturbed lowland streams. The value for lowland streams was chosen as most of the sites are in lowland areas (the trigger value for upland streams is 0.6 metres).

    Figure 2

    Note: Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) provide default trigger values for water clarity indicative of unmodified or slightly disturbed ecosystems in New Zealand.  Trigger values are essentially ‘warning’ values where water clarity has reached a threshold that may require an investigation or a management action. For total water clarity, the number of exceedances for upland and lowland segments were calculated separately and then added together.

    Figure 3

    Note: Sites are classified by land cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Figure 4

    Note: Sites are classified by land cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Figure 5

    18 April 2019: The interactive map will be available again when it has been updated.

    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). Higher values represent water of better clarity, and lower values of worse clarity.

    Many regional councils monitor river water quality at sites regularly to help manage environmental impacts. These sites tend to be in catchments dominated by agricultural land use. Rivers in low-lying and hilly areas in the North and South islands are well represented, while mountainous areas in the South Island and parts of the central North Island are not.

    For the analysis presented here, NIWA used water clarity data from up to 454 sites monitored by them and regional councils with consistent time periods and comparable methods (Larned et al, 2015).

    State of river water clarity was calculated at 454 sites using a five-year median for each site over the period  2009–13. Ten-year trends (2004–13) were calculated at 386 sites; 20-year trends (1994 – 2013) at 200 sites. Trend data are adjusted for the influence of variation in river flow, given that values are strongly correlated with flow. Increasing and decreasing trends at sites are inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator. Indeterminate trends mean data were insufficient to determine trend direction.

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

    If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

    This is because although our data are sourced from regional councils, we adjust some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those produced by regional councils.

    NIWA has also modelled current state in river water quality clarity using data monitored by them and regional councils (Larned et al, 2016). Median river water clarity for 2009-13 was predicted for all river segments using Random Forest modelling and predictors (explanatory variables) from the Freshwater Ecosystem of New Zealand database. The model performed well with low bias (R2 of 0.59).

    Data quality 

    Topic Classification   Relevance Accuracy 
     Freshwater quality, quantity and flows  Case study

     relevance-direct
    Direct

     accuracy-high
    High

     

    See Data quality information for more detail.

    References

    Davies-Colley, RJ (1988). Measuring water clarity with a black disc. Limnology and Oceanography, 33(4), 616–623. Retrieved from http://onlinelibrary.wiley.com.

    Larned, S, Snelder, T, Unwin, & M (2017). Water quality in New Zealand rivers: Modelled water quality state. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2016-070. Wellington: NIWA.

    Larned, S, Snelder, T, Unwin, M, McBride, G, Verburg, P, & McMillan, H (2015). Analysis of water quality in New Zealand lakes and rivers: Data sources, data sets, assumptions, limitations, methods and results. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2015-033. Retrieved from https://data.mfe.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.

    Archived pages

    See River water quality trends: clarity and Geographic pattern of river water clarity (archived April 2017).

    Updated 18 April 2019

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