Geographic pattern of natural river flows

  • Image, Geographic pattern natural river flows.

    River flow is the quantity of water passing a point over a certain time. Each river or stream has its own natural flow characteristics, such as peak flows following rain or high spring flows from snow melt. Overall, this affects how much water is available for irrigation, drinking water, hydroelectricity generation, and recreational activities. River flows also influence a waterway’s physical form, habitat, and ecological processes like migration, spawning, and food supply for aquatic life.

    We classified Geographic pattern of natural river flows as a national indicator.

    Key findings

     Trend not assessed

    Natural river flows are lower on the country’s eastern side compared with the western side (after standardising for different catchment sizes), except for the large east coast rivers of the South Island that originate in the Southern Alps.

    • Most rivers have lower flows in February compared with their long-term average flow.
    • Rivers with higher-than-average February flows include West Coast rivers that are fed by snow and ice melt from the Southern Alps.
    • The lower reaches of West Coast rivers are flushed by increased flows from rain and snow melt more than 25 times a year.
    • On the South Island’s east coast, and in parts of Hawke’s Bay and Bay of Plenty, flows are steadier and tend to be lower than average, with less variation between seasons and less frequent high (peak) flows.

    Figure 1

    Geographic pattern of natural river flows – interactive map

    Figure 2

    Mean annual low flow (MALF) – interactive map

    Figure 3

    February flow divided by mean flow – interactive map

    Figure 4

    Frequency of high flows – interactive map

    Definition and methodology

    Estimating river flows provides information about the availability of water for people and the environment. Statistics capture a range of characteristics, such as the frequency of peak and low flows, and seasonal patterns.

    NIWA and regional councils have measured flows at 485 river sites for five or more years. The data from these sites enable flows to be modelled on every river in New Zealand.

    Our flow statistics are estimates of flows that have not been modified by water extraction for use, or by dams. They were derived from models using flow measurements at the 485 sites. Other information, such as a catchment’s dominant land cover and the surrounding landscape characteristics (eg climate, elevation, and geology), is used to estimate flows across the whole landscape.

    Some rivers have larger flows simply because their catchments are larger. To compare rivers in different-sized catchments, we also report specific mean flows, which is the flow generated per unit of catchment area; it removes the effect of catchment size.

    The statistics provide a baseline against which observed flow changes may be compared. Actual flows vary in response to shorter-term weather patterns and/or climate trends, the effect of water extraction for use, and changes to channels.

    The statistics do not represent observed flow regimes for rivers downstream from large engineering schemes and dams, such as those found on the Waikato, Waitaki, Clutha, and Waiau rivers.

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

    About 48.4 percent of New Zealand’s river length is fed by catchments that are mainly influenced by indigenous land cover, while 45.7 percent are influenced mainly by pasture, 5.1 percent by exotic forest, and 0.8 percent by urban land cover.

    Data quality

    Topic Classification   Relevance Accuracy 
     Freshwater quality, quantity and flows  National indicator

     
    Direct

    accuracy-high
    High

     See Data quality information for more detail.

    References

    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 Geographic pattern of natural river flows (archived April 2017).

    Updated 27 April 2017

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