Sediment plumes in Lake Superior

(photo by Peter Wolter, NRRI)

Sediment plumes in Lake Superior. Suspended sediments are carried to the lake from streams and rivers like Kingsbury Creek, pictured below.

Plot of Storm-Induced Turbidity Spike Turbidity in Kingsbury Creek

See how a rainstorm affected Tischer Creek.

Turbidity and TSS

Turbidity is a water quality parameter that refers to how clear the water is. The greater the amount of total suspended solids (TSS; also called total suspended sediment) in the water, the murkier it appears and the higher the measured turbidity. Clay, silt, and sand from soils, phytoplankton (suspended algae), bits of decaying vegetation, industrial wastes and sewage are common suspended solids. Turbidity is usually measured using an optical instrument called a turbidimeter. If the water is darkly stained from dissolved organic material (usually coming from bogs and other wetlands), this may also contribute to decreased clarity.

Why is it Important?

Measuring turbidity in streams is extremely important as an indicator of the concentration of suspended sediments in the water. Sediments are a natural part of streams and other water bodies and even the most pristine streams in undeveloped watersheds will run muddy during high flows. However, excessive sedimentation in streams and rivers is considered to be the major cause of surface water pollution in the U.S (38% of stream miles) followed by pathogens at 36%, and nutrients at 28%. Nutrients are the leading source of impairment to lakes, ponds, and reservoirs (US EPA 2000).

High turbidity and suspended solids in streams and lakes may be caused by many factors including:

  • soil erosion associated with agricultural practices, construction site runoff

    photos from Stream Corridor Restoration: Principles, Processes, and Practices (10/98).
    Interagency Stream Restoration Working Group (FISRWG)
  • domestic and industrial wastewater discharge
  • urban runoff from roads, parking lots and other impervious surfaces
  • flooding and chronically increased flow rates (see Flow)
  • algae growth from nutrient enrichment (called eutrophication)
  • dredging operations in the stream itself or in feeder tributaries or ditches

    photo from Visualizing the Great Lakes: Images of a Region
  • channelization

    photo from Stream Corridor Restoration: Principles, Processes, and Practices (10/98).
    Interagency Stream Restoration Working Group (FISRWG)
  • removal of riparian vegetation and other stream bank disturbances
  • too many bottom-feeding fish (such as carp) that stir up bottom sediments

Macrophytes provide:

  • food for invertebrates, amphibians and fish
  • habitat (cover)
  • substrate for eggs
  • anchors for shoreline sediments
  • oxygen production

Expected impacts of pollution and effects on organisms

— Light, food, mechanical,
oxygen and temperature —

Increased turbidity affects a stream and the organisms that live in it in many ways and if the water becomes too turbid, it loses the ability to support a wide variety of plants and other aquatic organisms. Suspended solids may cause the water color to darken thereby reducing the amount of light available for aquatic vegetation, algae and mosses to grow by photosynthesis. Reduced plant matter means less food and habitat for herbivorous organisms such as snails, insects and juvenile fish. As photosynthesis slows, less oxygen is released into the water during the daytime and the plants may even die. As they decompose, bacteria will use up even more oxygen from the water. Reduced clarity also interferes with the ability of visual insect and fish predators to find their prey and may also impair reproduction when visual cues are a part of courtship and mating.

Fine particulate sediment can also have purely mechanical effects by clogging sensitive fish and insect gills, abrading soft tissues, and scouring algal and microbial "mats" growing on rocks. Growth rates and resistance to disease may be reduced and proper egg and larval development may be prevented. As particles of silt, clay, and other organic materials settle to the bottom, they can suffocate newly hatched larvae and potentially interfere with particle feeding activities. Settling sediments can fill in spaces between rocks which could have been used by benthic aquatic organisms for homes. The organic matter fraction of these settling solids is a source of food but too much of it can lead to oxygen depletion (see the biological oxygen demand section for more details and also the dissolved oxygen pages on Water on the Web -- link1 and link2). This is especially important in cold-water systems, such as the 12 designated trout streams in Duluth, because their native fish and insect eggs require continuously high levels of oxygen to develop properly.

Another negative effect of increased TSS is that turbid waters usually become warmer because suspended solids darken the water and absorb more heat from sunlight. Warm water holds less oxygen than cold water, so O2 levels will decrease in addition to the direct effects associated with increased on cold- and cool-water adapted native species. This adds to the other increasing effects of urbanization such as heating by parking lots and roads and by the removal of shading by riparian vegetation. (see temperature)

— Toxic compound and nutrient associations —

Suspended solids also provide adsorption surfaces and a route of transmission for many organic contaminants, heavy metals, and some nutrients. Many of the most toxic industrial compounds such as dioxins and furans, PCB's (polychlorinated biphenyls), PAH's (polycyclic aromatic hydrocarbons), many pesticides and heavy metals such as mercury, cadmium, lead, zinc, and chromium are "sticky" molecules that adhere to both fine organic and clay particles. The particles may provide a route of accumulation into the food web via ingestion but they may also act to bind the pollutants in areas of deep water where particles can settle out. In deep lakes this may be essentially permanent , but in streams it is likely to be only temporary until the next strong flushing event from high flows.

Sediments can also be a major source of the plant nutrients phosphorus, nitrogen (in its ammonium form) and iron. In sensitive receiving waters, particularly northern Minnesota lakes, excess nutrients can over stimulate algal and higher plant growth leading to a host of water quality problems. See the WOW, LakeAccess, and Shoreland Management websites for further information.

— Drinking water, water quality standards and aesthetic effects —

Suspended sediments also interfere with the recreational use and aesthetic enjoyment of water but there are because of the wide variation in TSS and turbidity there are generally no numerical criteria for TSS or turbidity that apply to all streams and lakes. The Water Quality Rules for the Waters of Minnesota are developed by the Minnesota Pollution Control Agency. General provisions are included to prevent the degradation of any water, irrespective of a specific standard and a general anti-degradation statement. The GENERAL STANDARDS FOR DISCHARGERS TO WATERS OF THE STATE (Chapter 7050.0210 Subparagraph 2) further states that Nuisance conditions are prohibited and that:

"No sewage, industrial waste, or other wastes shall be discharged from either point or nonpoint sources into any waters of the state so as to cause any nuisance conditions, such as the presence of significant amounts of floating solids, scum, visible oil film, excessive suspended solids, material discoloration, obnoxious odors, gas ebullition, deleterious sludge deposits, undesirable slimes or fungus growths, aquatic habitat degradation, excessive growths of aquatic plants, or other offensive or harmful effects."

In Class 2A (coldwater fishery) and Class 2B (Cool or warmwater fishery) Waters the State has set chronic turbidity standards of 10 NTU and 25 NTU, respectively where "chronic standard" means the highest water concentration of a toxicant to which organisms can be exposed indefinitely without causing chronic toxicity (7050.0220 SPECIFIC STANDARDS OF QUALITY AND PURITY BY ASSOCIATED USE CLASSES). This definition allows for periodically high fluctuations that are associated with storm events. State Water Quality Rules also set specific limits for TSS for the discharges from industrial and municipal wastewater treatment plants, usually in the range of 30-60 mg TSS/L.

Turbidity also adds real costs to the treatment of surface water supplies used for drinking water since the Soil erosion particulate material causing it must be virtually eliminated for effective disinfection to occur. Adding chlorine in a variety of forms is the typical process used to disinfect domestic water, but the source water must be clarified by filtration to an extremely high degree prior to chlorination (see LINK to Duluth Water Treatment Plant, MN Dept of Health and EPA DRINKING WATER sites for further information). This is because many disease-causing microorganisms adhere to particlulates and as a result receive less exposure to disinfection processes.

Lakes and Ponds

The major source of turbidity in the open water zone of most lakes in northern Minnesota is usually phytoplankton, but closer to shore, particulates may also be clays and silts from shoreline erosion or resuspended bottom sediments. Both of these turn the western arm of Lake Superior near Duluth brown on a windy day as in the image at the top of this section. Organic detritus from stream and/or wastewater discharges may also contribute turbidity to lakes as can soil erosion from streams draining agricultural land (see image to right).

Dose effects - How much and for how long?

Very high levels of turbidity for a short period of time may not be significant and may even be less of a problem than a lower level that persists longer. The figure below shows how aquatic organisms are generally affected.

Relational Trends of Freshwater Fish Activity to Turbidity Values and Time
Schematic adapted from "Turbidty: A Water Quality Measure", Water Action Volunteers, Monitoring Factsheet Series, UW-Extension, Environmental Resources Center. It is a generic, un-calibrated impact assessment model based on Newcombe, C. P., and J. O. T. Jensen. 1996. Channel suspended sediment and fisheries: a synthesis for quantitative assessment of risk and impact. North American Journal of Fisheries Management. 16: 693-727.

DVTool animation

Summer Storm at Tischer Creek

Ever noticed the flow of your local stream changing from a clear trickle to a mud flow after a storm? See what happened in Tischer Creek from a couple of inches of rain the night of July 7, 2003.

Use our Data Visualization Tools to find your own examples.

Reasons for Natural Variation

Turbidity varies seasonally, and in larger bodies of water with depth, in response to natural and human-caused physical, chemical and biological changes in streams and lakes. Mineral and organic particles washed in from the watershed vary largely in response to hydrological events such as storms and snowmelt that are seasonal and vary widely in intensity, timing and duration from year to year.

In Duluth, stream flow regimes throughout the year are typically characterized by low, or base-flow conditions that most commonly occur in summer and winter, the spring snowmelt runoff (high flow) period, and sporadic periods of storm runoff (high flow).

Duluthians know better than most people about how variable these periods are and how different years can be. We must be careful to interpret water quality data in light of how the streams are flowing. This is the major reason for the need for consistent long-term monitoring programs. Without decades-long strings of data acquired using comparable and quality assured methodology, it is impossible to tease out the cause and effect relationships between human activities and suspected environmental problems (see our QA/QC page for details).

Some examples from Duluth's streams

Short-term events: Data "snapshots" from short-term high runoff events for the monitored Duluth streams may be found in the STREAMS section of this website (look in the "Storm Data" column of the data availability table).

What does the graph mean?

Look at one of the rain events (blue bars). After each rain event:

Kingsbury Creek - Summer Storm
  • Stream depth increases, due to runoff.
  • Turbidity increases.
    Sediments are washed from the watershed into the stream. Loose sediments from construction projects, roads, parking lots, and un-vegetated soils are carried into the streams.
  • Conductivity (salt concentration) decreases due to dilution from the large volume of water entering the stream during the storm

Seasonal and annual variations: This data graph (coming soon!) shows the variability we've seen so far at Kingsbury Creek.

Turbidity measurement

There are actually several ways to estimate turbidity and these are all being used in DuluthStreams. Field and lab turbidimeters measures the scattering effect suspended particles have on light.

Field turbidity measurements at Chester, Kingsbury and Tischer Creeks are made with submersible turbidity sensors that are described in the Quality Assurance section.

"Bench" turbidimeters are used for manually collected water samples. We compare stream water to calibration standards in the NRRI-UMD Water Quality Lab.

Transparency tubes or Secchi tubes (also called turbidity tubes) are being used by volunteer efforts at local schools (see Riverwatch). The method follows that used by volunteers throughout Minnesota in the Citizen Stream Monitoring Program (coordinated by the Minnesota Pollution Control Agency). The "S-Tube" or "T-Tube" gives us a measure of clarity similar to what limnologists and lake volunteers measure when they use a Secchi Disk in deep water (WOW for details). It involves looking down a tube at a black and white disk and recording how much stream water is needed to make the disk disappear. In 2012, the MPCA replaced all transparency tubes used in the Citizen Stream Monitoring Program (CSMP) with 100 cm Secchi tubes. Learn more about the reasons for the switch on their website. Additional details may be found at Water-On-the-Web.

What in the world are Nephelometric Turbidity Units (NTU's)?

They are the units we use when we measure Turbidity. The term Nephelometric refers to the way the instrument estimates how light is scattered by suspended particulate material in the water. The Nephelometer, also called a turbidimeter, has a photocell (similar to the one on your camera or your bathroom nightlight) set at 90 degrees to the direction of the light beam to estimate scattered rather than absorbed light. This measurement generally provides a very good correlation with the concentration of particles in the water that affect clarity.

Formazin Turbidity Standards

Turbidity of Suspended Clay

TSS - Methodology

Calculate TSS as:

     TSS(mg/L) = ([A-B]*1000)/C


  1. Final dried weight of the filter
    (in milligrams)
  2. Initial weight of the filter
    (in milligrams)
  3. Volume of water filtered
    (in Liters)

Filter Samples

How do we measure TSS directly?

Usually, we measure turbidity to provide a cheap estimate of the total suspended solids or sediments (TSS) concentration (in milligrams dry weight/L). Basically we pre-weigh a disc of filter paper made from tiny glass fibers and then suck a measured volume of water through it. The filter is then dried and re-weighed to calculate the weight of particulate material that was in he water sample. It's pretty simple but tedious which is why a turbidimeter is useful. We can also burn the filter at 475 °C in a lab furnace to vaporize the organic matter, which we can then calculate as the difference between the total solids and the mineral residue remaining on the combusted filter. This is tedious and difficult to do accurately for low turbidity water - the reason why a turbidimeter is often used. The transparency tube is even cheaper but not easily automated and not as sensitive as a turbidimeter.

How do TSS and turbidity compare?

The figure below shows how the two measurements compare for an erodible shoreline area of Lake Independence (a Water-on-the-Web lake in the Minneapolis-St. Paul metro area). The relationship can vary considerably between different aquatic systems and even at different times for the same stream or lake so there's no simple conversion factor to use.

TSS vs turbidity

A general rule of thumb:

1 mgTSS/L ~ 1.0 to 1.5 NTU's of turbidity

BUT - turbidity scattering depends on particle size so this only an approximation.


Allan, J.D. 1995. Stream Ecology: Structure and Function of Running Waters. Chapman and Hall. London.

Hauer, F.R. and Lamberti, G.A. (ed.'s) 1996. Methods in Stream Ecology. Academic Press Inc. San Diego, CA.

Merritt. R.W. and Cummins, K.W. (ed.'s) 1984. An Introduction to the Aquatic Insects (2nd edition). Kendall/Hunt Publishing Company. Dubuque, IA.

Michaud, J.P. 1991. A citizen's guide to understanding and monitoring lakes and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications Office, Olympia, WA, USA (360) 407-7472.

Tester, J.R. 1995. Minnesota's Natural Heritage: An Ecological Perspective. University of Minnesota Press. Minneapolis, MN.

US EPA 2000 National Water Quality Inventory: 1998 Report to Congress. Last updated October 5, 2000.