Streambed sedimentation

  • Image, Streambed sedimentation.

    Archived 27 April 2017

    Fine sediment is the collective term for inorganic particles smaller than 2mm that are deposited on the beds of rivers and streams. Urban development, agriculture, and plantation forestry around waterways can increase the amount of sediment entering river systems. Sediment can clog the spaces between pebbles used by aquatic insects and fish, and degrade food sources and sites used for egg laying. Excessive sedimentation can also affect the suitability of rivers and streams for recreation. 

    We classified Streambed sedimentation as a case study.

    Key findings

    At 10,025 sites measured between 1990 and 2011, sediment covered an average 28.6 percent of the riverbed.

    • Modelling estimates that the average proportion of sediment coverage would be about 7.7 percent without the effects of human land use.
    • Rivers and streams with higher proportions of their bed covered by fine sediment include those in lowland, urban, or pastoral areas, particularly in the northern half of the North Island.

    Streambed sedimentation, 1990-2011 – interactive map

    Definition and methodology

    Fine sediment is the collective term for inorganic particles smaller than 2mm deposited on the beds of rivers and streams. Using the Wentworth (1922) classification system, fine sediment is characterised by particle size as mud and silt (< 0.0625mm) and sand (0.0625mm–2mm). Some New Zealand streambeds are naturally dominated by fine sediment, although these waterways are usually very small, with low slopes, low rainfall, and sandy soils.

    Streambed sedimentation is typically measured by visually estimating the proportion of the bed covered by different-sized substrates. Information is collected during official fish surveys and stored in NIWA’s Freshwater Fish Database.

    We report on observed in-stream sediment values for 10,025 sites, dating from 1990 to 2011. However, the data for Fiordland was incomplete for this period, so we used all the available information for the region (1970–2011) to fill the gap.

    We used a regression model (which predicts one variable from other variables) to estimate the relative proportion of fine sediment cover in every New Zealand river or stream. This is inferred from the measured proportion of fine sediment cover and predictors such as the slope of the river, climate, and catchment land cover. The model had a cross-validation error of 0.67 and explained 45 percent of the variance in proportion of fine sediment cover. This suggested the model performed well (Clapcott et al, 2011) relative to the large-scale geomorphic models used elsewhere.

    Modelling of natural sediment cover was based on estimated pre-human natural land cover (up until 700–800 years ago). The model had a cross-validation error of 0.51 and explained 26 percent of the variance in fine sediment cover (Clapcott et al, 2011).

    Data quality

    We classified Streambed sedimentation as a case study.

    Relevance

     This case study is a direct measure of the ‘Condition and physical characteristics of freshwater habitats’ topic.

    Accuracy

     The accuracy of the data source is of medium quality.

    See Data quality information for more detail.

    References

    Clapcott, JE, Young, RG, Harding, JS, Matthaei, CD, Quinn, JM, & Death, RG (2011). Sediment assessment methods: Protocols and guidelines for assessing the effects of deposited fine sediment on in-stream values. Available from www.cawthron.org.nz.

    Wentworth, C (1922). A scale of grade and class terms for clastic sediments. Journal of Geology, 30(5), 377–392. Available from www.jstor.org.

     

    Published 21 October 2015
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