Transparency and Turbidity

Water Quality Measurement  | Physical Parameter

Lake Pend Oreille | Pend Oreille River

Turbidity and transparency (measured using Secchi Depth) are measures of how clear a water sample is.  When solids are suspended in the water, it can become murky.  The murkier the water appears from these solids, the higher the measured turbidity and the lower the transparency. A greater Secchi Depth equals a greater transparency, or clearer water. On our graph, the Y-axis is inverted so that the top of the graph represents the top of the water.

Water quality problems correlated to high turbidity and low transparency include increased water temperatures (darker water absorbs more sunlight), reduction in dissolved oxygen, and the many problems associated with erosion, stormwater/runoff pollution, and excessive algal growth.

Transparency is often measured in the field instead of turbidity. Transparency is “an integrated measure of light scattering and absorption” (EPA 2010). It is important to note that turbidity does not quantify the amount of total suspended solids (TSS). Tests for TSS are often time consuming and costly. Turbidity and transparency measurements are cost-effective ways to estimate TSS.

Secchi Depth (pronounced seckky)
Secchi Depth is one way to measure transparency. It measures how far light can penetrate the water column. The Secchi disk is weighted and split into alternating black and white quadrants. The disk is lowered into the lake until it can be no longer seen by the tester. The depth at which this occurs, called the Secchi Depth, is an estimate of water transparency. Generally, the more material present in the water (e.g., algae, silt) the shorter the Secchi Depth. Very clear water, such as in Crater Lake, Oregon, will have a greater Secchi Depth.

Water quality problems correlated to high turbidity/low transparency include:

  • Increased water temperature (darker water absorbs more sunlight)
  • Reduction in photosynthesis and plant growth (when sunlight cannot penetrate water)
  • Reduction in dissolved oxygen
    Damage to aquatic habitat
  • Increased fill-in rates to water bodies
  • Negative effects to fish and macroinvertebrates
  • Influx of metals and organic contaminants that attach to solids
    Some sources of material contributing to low transparency:

    • Erosion
    • Agricultural, construction and logging practices
    • Snowmelt, storm events, other hydrologic factors
    • Waste discharge
    • Urban runoff
    • Re-suspension of bottom sediments
    • Large numbers of bottom feeders (such as carp)
    • Excessive algal growth
    • Dredging or channelization

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