Learn about Watersheds
The Maine Watershed Web is developing curricular materials aimed primarily at undergraduate and high-school audiences.
By topic
- What is a watershed?
- Stream flow
- Runoff processes
- Water-Quality Indicators
- Physical
- Chemical
- Biological
By resource type:
- powerpoints
- primers
- self-quizzes
What is a Watershed?
A watershed is an area where water drains into a common body such as a stream, river, or ocean. The water can come from rain and melting snow or it can come from places like a neighbor watering their lawn or a running faucet in the kitchen. Even if you don't live within sight of a stream or river, the rain that falls on your roof and the water you use in your bathroom eventually runs back to the ocean. Because of the fact that all water is working its way back to the ocean, it means that wherever there is water, there is a watershed. The watershed acts as a basin to catch precipitation which then runs back to the ocean through a drainage network made up of streams and rivers. Watershed boundaries are defined by the hills and valleys, or topography, around a stream or river.
Stream Flow
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Stream flow is a fundamental property that affects channel shape and the diversity of stream habitats, the flux of nutrients and pollutants, and the frequency and severity of floods. Stream flow is controlled by such watershed variables as size, topography, geology, land use and presence of wetlands, as well as climatic variables such as rainfall, snow cover and temeprature. Two important concepts in the analysis of stream flow are:
As explained below, stage is relatively quick and easy to measure, whereas discharge measurements are more time consuming and may involve specialized equipment. Because of this, a rating curve--a graphical or mathematical relationship between stage and discharge--is commonly constructed in order to produce discharge estimates from stage readings. |
Nequasset Stream at low flow
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How is Stream Flow Measured?
Dilution Gaging
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Dilution gaging involves adding a non-reactive, dissolved chemical tracer to a flowing stream, and monitoring how this tracer becomes diluted after it has completely mixed throughout the water as it travels downstream. The greater the flow, the greater will be the level of dilution of the tracer. The technique may be used in streams that are difficult to gage with current meters. Such streams include (a) those which are too shallow and/or slow, and (b) those with an uneven, rocky bottom and/or an irregular wavy surface, as is common for high-gradient mountain streams. A convenient tracer is common salt, which is cheap, easily available, and readily measured in the stream with an electrical conductivity meter. At the concentrations and measurement durations typically used for dilution gaging, salt is also not harmful to the stream ecosystem. Other tracers include fluorescent dyes, such as rhodamine, which can be measured at very low concentrations and are thus useful for high stream flows. Detecting the concentrations of such dyes quantitatively, however, requires more-expensive instrumentation (field or laboratory fluorometer). |
 Rhodamine dye plume in stream: image from Wayne Wurtsbaugh (http://www.aslo.org/photopost/ showphoto.php/photo/938/ sort/1/cat/all/page/14 |
 Schematic chart of dilution gaging by constant-rate injection Salt-wave graph for slug-dilution gaging at Mt. Ararat Stream, Maine |
Dilution gaging can be accomplished by one of two basic approaches:
The constant-rate method generally gives somewhat more-precise results, especially for low flows, but (a) requires additional equipment to control tracer injection, and (b) is impractical for high-volume flows. |
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The details of of dilution gaging are presented at length in the U.S. Geological Survey manual on the Measurement and Computation of Streamflow (Water Supply Paper 2175). R.D. (Dan) Moore at the University of British Columbia has written very useful summaries of the salt-dilution approach as shorter articles in the Streamline Watershed Management Bulletin, including introductions to Readers are advised to consult these sources for details, but an overview is provided below. |
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Constant Rate of Injection Method The diagram to the right illustrates how stream discharge Q is calculated via the constant-rate-of-injection method. A solution with tracer concentration Cis is injected at a rate q into a stream flowing at discharge Q with background tracer concentration Cbg. After mixing downstream, the tracer reaches a plateau of constant diluted concentration Cs. By conservation of mass, the discharge-concentration products of the stream flow (QCbg) and the injection solution (qCis) must add to be the same as the product of the cumulative discharge and concentration downstream ([Q+q]Cs). The equation may be rearranged to solve for stream discharge Q: Q = [(Cis - Cs)/(Cs - Cbg)] q
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 Schematic diagram of constant-rate injection gaging |
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Instantaneous Slug-Injection Method The basis of the slug-injection method is also conservation of mass, specifically accounting for all of the injected tracer as it passes the downstream sampling point. After being dumped into the stream, the tracer slug will move downstream via advection with the flowing water, but will also undergo longitudinal dispersion, or downstream stretching, because some parts of the streamflow are faster than others. |
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Hillslope flow processes
| Hillslope flow processes describe the movement of water within a catchment before it arrives at the stream channel. Understanding these processes are important for predicting how streams respond to rain events, as well as some of the water-quality characterisitcs of the water reaching the channel. | |
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Rain that falls onto the ground during a storm can take one of three basic paths:
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Velocity-Area Method: Acoustic Doppler Current Profiler
Acoustic Doppler Current Profiler on tethered boat:: Cathance River, Maine; April 30, 2008: Cathance River, Maine; April 30, 2008
An Acoustic Doppler Current Profiler, or ADCP, is an instrument which measures the Doppler shift of echoes returning from moving water parcels and converts the data into 3-D flow information.
Velocity-Area Method: Section by Section
Stage/Gage Height
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Stage, also known as gage height or water level, refers to the elevation of the water surface relative to some reference point, or datum. The zero point for the datum is usually chosen to lie below the stream bed, but any vertical reference can work. Stage can be negative if the datum is selected above the stream surface. A common way to measure stage is through use of a staff gage, essentially a vertically oriented ruler that extends throught the stream surface.
Note: gage and gauge are both considered acceptable spellings. |
 Staff gage showing stage reading of ~3.96 feet: image from www.sudburyriver.org/id16.html |
 Measuring stage with a water-level meter |
Stage is relatively quick and easy to measure by using a staff gage. Another approach is to use a water-level meter to determine the distance below the top of a stilling well. The meter a measuring tape with a open circuit at the end is lowered until reaching the water, triggering a beep and lighting an indicator. |
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Stage can also be recorded "continuously" using:
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Water-Quality Indicators
Biological Indicators
Bacteria
| Bacteria are... |
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Escherichia coli, more commonly abbreviated as E. coli, is a bacterium found in the gut of warm-blooded animals (mammals--including humans--and birds). Although a few strains of E. coli can cause disease, most are harmless. E. coli is used primarily as an indicator organism for fecal input into water bodies, which may be associated with other pathogens. E. coli is one species of a broader group of fecal coliform bacteria. |
 E. coli (scanning electron microscope image) |
How are Bacteria Measured?
Chemical Indicators
Alkalinity and Acid-Neutralizing Capacity
Soon.
Dissolved Oxygen
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Dissolved oxygen, or DO, consists of widely dispersed molecules of oxygen gas (O2) dissolved in water.
Dissolved oxygen is also used as a source of energy for some bacteria as they decompose dissolved and particulate organic materials in waters. |
Dissolved oxygen is a fundamental water-quality variable because it is essential for fish, insect larvae and other aquatic animals: it's what they breathe!
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| Dissolved oxygen also impacts chemical reactions in water. For example, in the presence of dissolved oxygen ("oxidizing conditions") elements such as iron and phosphorus react to form insoluble compounds. In the absence of DO ("reducing conditions"), iron and phosphorus, among others, remain dissolved within the water. |  Iron-oxide "rust" formed where low-oxygen groundwater discharges to surface and comes in contact with air: Image from Christine Smith, Maine DEP |
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How much DO can water hold? Just as only so much sugar can be dissolved in a cup of coffee, only so much oxygen can be dissolved in natural waters. The amount of DO that can be held by water .... |
Links to other web sites with information on dissolved oxygen:
- Dissolved Oxygen curriculum module from The Partnership for Environmental Education and Rural Health (PEER), Texas A&M University: http://peer.tamu.edu/curriculum_modules/Water_Quality/Module_3/index.htm
- Water on the Web: http://waterontheweb.org/under/waterquality/oxygen.html
Dissolved-Oxygen Saturation and Its Controls
| Saturation is the maximum amount of a solute (the stuff being dissolved, in this case oxygen gas) that a solvent (the "host liquid," in this case water) can hold at equilibrium (that is, without the solute [DO] escaping over time). Water at a specified temperature, salinity and pressure can hold only so much dissolved oxygen, and any extra DO will eventually escape back to the atmosphere. |  The cap on a root-beer bottle prevents dissolved gases from reaching equilibrium with the atmosphere, so the drink retains "extra" dissolved gas. |
 Once the root beer is poured into a glass, the "extra" dissolved gas (carbonation) escapes. Equilibrium is achieved when the drink goes completely flat. |
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Dissolved Oxygen Percent Saturation, or DO % Saturation is the actual concentration of DO in the water divided by the equlibrium saturation value times 100%: DO % Saturation = 100% x (DO concentration in mg/l)/(DO equilibrium saturation in mg/l) As discussed below, the DO equilibrium saturation value can be determined if temperature, salinity and atmospheric pressure are known. |
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DO Saturation and Temperature Our common experience with dissolving solids leads us to believe that hot liquids can hold more dissolved material than cold liquids can: You can dissolved more sugar in hot tea than in iced tea: conversion of solids to liquids is enhanced as more heat energy is supplied |
But dissolved gases behave differently than dissolved solids. Qualitatively, think of dissolved gas molecules as being "trapped" within surrounding molecules of liquid water. Water molecules move more sluggishly when cold, giving the dissolved gas fewer opportunities to slip though gaps between them. As the water molecules jostle more and faster with increasing temperature, it is more difficult to keep the dissolved gas "trapped" in between. Our experience with carbonated beverages supports this idea: unsealing a warm can or bottle of soda will produce more vigorous bubbling than unsealing a cold can. Dissolved gas is held less readily in warm soda, so it escapes more rapidly. Root beer fountain: A short metal rod is heated and then dropped into a freshly opened bottle of root beer. A fountain of foam shoots out since carbon dioxide is less soluble in warm root beer than cold root beer. From http://genchem.chem.wisc.edu/demonstrations/
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| Dissolved oxygen saturation decreases by about 2% for each 1ºC increase in temperature. For example, water at 20ºC (68ºF) can only hold ~60% of the dissolved oxygen at equilibrium that water at 0ºC (32ºF) can hold. Temperature is usually the most important determinant of oxygen-holding capacity. | |
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DO Saturation and Salinity The solubility of substances is not only affected by temperature, but also by the presence of other dissolved substances. The so-called common-ion effect in chemistry... |
How is Dissolved Oxygen Measured?
Dissolved oxygen is commonly reported as a concentration, in either:
For fresh water without excessive turbidity, 1 liter has a mass very close to 1 kilogram, or 1 million milligrams. Under these conditions, 1 mg/l is essentially the same as 1 ppm. |
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Dissolved oxygen is typically measured in one of three ways:
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| Colorimetric measurement of dissolved oxygen involves adding chemicals that react with dissolved oxygen to create new, colored compounds. The most common approach is the modified Winkler method, in which DO is converted to manganic sulfate (Mn2(SO4)3), which in turn reacts with potassium iodide (KI) to release free iodine (I2) into solution. The free iodine is responsible for the yellow-brown to orange color. The amount of free iodine, which is equivalent to the amount of original dissolved oxygen, is measured by titrating with sodium thiosulfate solution, which converts the free iodine to colorless sodium iodide (NaI). |  Colorimetric Analysis of Dissolved Oxygen: DO ranges from high (left) to low (right) |
 Polarographic dissolved oxygen sensor: www.hydrolab.com |
Electrochemical (polarographic) measurement of dissolved oxygen requires a more-expensive meter attached to a DO probe. The DO probe is immersed in water, and dissolved oxygen diffuses through a plastic or teflon membrane to encounter two electrodes separated by potassium chloride solution. DO reacts with water at the gold cathode, a reaction which is balanced at the silver anode by the formation of silver chloride and attendant release of electrons. The resulting current flow between the electrodes is measured by the electronics of the meter and is proportional to the DO present. Because DO is consumed by the reaction, there must be sufficient flow of water across the membrane surface to maintain equilibrium. The instrument must also be calibrated frequently. |
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Luminescent measurement of dissolved oxygen is the latest (and most expensive) technology, which requires less-frequent calibration than the polarographic method. A thin film of luminescent material (luminophor) is stimulated by a blue light-emitting diode (LED). As the luminophor returns to its non-stimulated state, it emits red light which is measured by the photo diode (the red LED is used for internal reference/standardization). Dissolved oxygen quenches the luminophor response: the more DO, the faster the return to the non-stimulated state and the shorter the delay before red light is emitted back. The meter electronics measure the amount of delay, and convert that value into the concentration of dissolved oxygen. |
 Luminescent DO sensor: www.hydrolab.com Schematic operation of luminescent DO sensor: www.hydrolab.com |
Where Does Dissolved Oxygen Come From?
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Dissolved oxygen has two main sources:
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| Reaeration coefficient... |
Where Does Dissolved Oxygen Go?
| Dissolved oxygen is lost from water primarily through respiration. | |
| The main contributors to respiration in most waters are bacteria, which break down organic material... | |
| If water is supersaturated with dissolved oxygen, the excess oxygen can diffuse back to the atmosphere. |
Physical Indicators
Physical indicators are such things as: temperature, electrical conductivity, turbidity, and color.
Color
Electrical conductivity
| Electrical conductivity, or simply conductivity, is a measure of water's ability to conduct an electric current. The measurement is important for what it indicates about the concentration of dissolved ions in the water, which in turn reflects groundwater input, catchment geology, or diverse human impacts. As the number of charged ions in the water increase, so does the electical conductivity. When compounds such as salt break down, they disssolve into both positively and negatively charged ions which are then attracted to their oppositely charged electrical current. Compounds such as sugar dissolves into neutral ions which do not conduct electricity and does not effect the electrical conductivity in the water. The glowing pickle is based on this idea. When high voltage is run through the pickle, the pickle can carry the current through it. This is the same principle as ionized water. When a current is run through the water, the ions are attracted to the electric charge, and these ions in turn carry the current through the water. |  Pickle Conducts Electricity: Image from: http://www.phys.unt.edu /~klittler/demo_room/e&m_photos /PickleGlow.jpg |
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Electrical Conductivity is primarily a measure of... 1. Ammount of dissolved ions 2. tpye of ions The more ions that are in the water, the higher the Electrical Conductivity. These ions must be charged however, as a neutral ion will not be attracted to an electric charge. |
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| Electrical Conductivity is not directly a measure of stream health, but rather indicates how the watershed works. (There is generally no standards) | |
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Electrical Conductivity vs. Specific Conductance -Conductivity varies with temperature. Heat and low humidity result in evaporation of the water, but leaves the ions behind giving the water a higher concentration of salt and other dissolved compounds. -usefullness of Specific Conductance |
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Why Electrical Conductivity? -Surface water has a much lower Electrical Conductivity than groundwater because the groundwater has been able to react with the minerals in the soil and rocks in the ground. This knowledge will help to identify an imput as surface water or groundwater by measuring Electrical Conductivity. -identify pullition sources (especially non-point-sources). If one area of a stream has a much higher Electrical Conductivity than the average for the steam, there is a good chance that there is groundwater flowing into the stream which has a high concentration of ions. -Electical Conductivity is also quick to measure. All it takes is setting a meter into the water and allowing it to take a measurement. |
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How is conductivity measured?
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Conductivity meters operate by immersing two electrodes in water and imposing positive and negative voltages on them. Electrically neutral molecules (e.g., the water molecules themselves) are not strongly attracted to either electrode and thus do not contribute much to an electric current. In contrast, positively and negatively charged ions are attracted to the electrode of opposite sign and move toward them, generating an electric current within the water that is measured by the meter. The more dissolved ions in the water, the greater will be the current generated. Conductivity (G) is calculated from the measured current (I) and voltage (E) using Ohm's Law: G = I/E. |
 Schematic diagram of conductivity cell: Image modified from Randy Holmes-Farley http://www.reefkeeping.com/issues /2004-04/rhf/feature/index.php |
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Units of measurement Conductivity also must take into account the distance over which the current travels (consider the electrode spacing in the operation of the meter). It is therefore reported and Siemens per m (S/m) in standard SI notation. In detail, conductivity in most natural waters are quite low, so is typically recorded in milliSiemens per centimeter (mS/cm) or microSiemens per centimeter (mS/cm), respectively a thousandth and a millionth Siemens per cm. For reference, normal sea water has a conductivity of about 42 mS/cm (~42,000 mS/cm). |
 Werner von Siemens: image from http://chem.ch.huji.ac.il /history/siemens.html |
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"Full-featured" conductivity meters, which typically cost hundreds of dollars, usually allow the measurement of conductivity over a range of concentrations, and allow the user to select units of measure conductivity, specific conductance, salinity and/or total dissolved solids. "Conductivity pens" are cheaper, typically $50-$100, but only measure over a fixed range and in one unit. |
Human Impacts on Electrical Conductivity
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Pollutants -road salt -landfill/septic leachate -impervious surface runoff -agrictultural runoff -first flush (graph) |
 Salt runoff from roads can raise stream conductivity: Image from http://www.ec.gc.ca/EnviroZine /images/Issue49 /salt_truck_l.jpg |
 Salt from various waterbodies: Image from: http://lakeaccess.org/russ /conductivity.htm |
During rain and snowmelt the initial flash of water into the watershed carries pollutants such as road salt and chemicals from automobiles into the watershed. This is followed by relatively low Electrical Conductivity, but when averaged the first flush and ensueing dip(see graph) even each other out to some extent. Impermeable surfaces such as asphalt may hold large amounts of Conductivity increasing contaminants which are carried into the watershed during the first flush. |
 First Flow Concentration: This graph shows the concentration of pollutants that flush into the watershed during a rain event. The initial spike is followed by a precipitous drop as all of the pollutants are now flushed into the watershed and impermeable surfaces as well as soils are 'clean'. |
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 Dilution of Concentrated Pollutants: This graph shows how the concentration of pollutants that cause conductivity to rise is quickly exhausted during a rain event or rapid snow melting. As the water level lowers, the concentration begins to increase until it is flushed by the next rain event. |
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Natural controls on Electrical Conductivity
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Precipitation-> distilled water->low Electrical Conductivity -When water evaporates it leaves the dissolved ions behind and therefore has a low conductivity as rainfall. |
 Conductivity Sources: Shows dissolved ions in the ocean, rainfall, streams and groundwater |
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Groundwater-> reactions with minerals->higher Electrical Conductivity -Soil contains minerals which react with the groundwater resulting in a higher conductivity. The more turbid the water is, conductivity is typically higher. (salt water vs. soil water experiment) |
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Type of bedrock -Water flowing over limestone which dissolves easily, will have a higher conductivity than water flowing over quartz which does not dissolve easily nor does it put alot of ions into the water when it does. Rocks containing calcite will also dissolve ions into the water resulting in a high conductivity. |
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Temperature
| Of the many water-quality variables, temperature is perhaps the most widely measured and reported. Much of this popularity arises because temperature is relatively easy and inexpensive to measure, as well as fairly straightforward to interpret at a basic level. Yet this familiarity may also lead us to undervalue the usefulness of temperature data, and not to think deeply about the processes that drive thermal changes in waters. |  Model for stream temperature showing complexities: image from www.tag.washington.edu/research/ dv24/dv24.htm |
| Temperature is a fundamental control on what kinds of aquatic life can live in a stream. Organisms ... |
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| Temperature also controls the rate at which many chemical processes occur in streams, and can determine the direction of chemical reactions as well. Temperature is critical to the abundance of important dissolved gases, such as oxygen. |  Channel catfish prefer water temperatures of ~80 to 85 ºF: image from www.fish.state.pa.us/pafish/chancatm.jpg |
Controls:
Controls on temperature include:
- weather and climate
- relative proportions of groundwater and surface water
- shading
- ponds and wetlands
How is temperature measured?
| Temperature must be measured on site with a thermometer or electronic thermistor. The most-useful thermometer models are armored in plastic or metal, and contain non-toxic liquids (in particular, mercury thermometers should not be used in field settings) | |
| Most thermometers used for water-temperature measurements are so-called total-immersion models, which require the entire thermometer to be placed below the water surface. The thermometer is removed and read quickly, being careful not to hold it by the bulb end (where heat from your hands will cause temperature to rise). | |
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Thermistors are "electronic thermometers" that measure the electrical resistance of an imposed current and use the relationship between resistance and temperature to convert that measurement to a temperature value. Many thermistors have the advantage of providing digital read-outs, reducing estimation error and, when combined with some kind of datalogger, allowing temperatures to be recorded at specified intervals over time.
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Natural controls on temperature
| The primary control on water temperature is air temperature. |  Temperature variations at Nequasset Stream, Maine: Note general correspondence between records |
| In detail, water is harder to warm up as heat is added and slower to cool as heat is lost. | |
Turbidity
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Turbidity is an optical property of water. It measures the amount of light scattered by suspended particles, and can be considered as the "cloudiness" of a water sample. Turbidity is contributed mainly by suspended sediment and/or plankton, that is, solid particles of inorganic or biological origin.
High turbidity in streams may indicate an abundance of sedimentary particles that block out light, limiting primary productivity (for example, algae or submerged aquatic vegetation) in the water. High sediment concentrations may also interefere with organisms directly (for example, by clogging their gills) or plug gravelly stream deposits so that oxygenated water cannot easily pass through.
Turbidity contributed by suspended sediment is usually highly dependent on stream flow. During storm events, higher flows pick up sediment from the stream bed and banks and keep it suspended in the turbulent flow. When flows subside, the sediment settles out, leaving the water more clear. |
 Stream with high turbidity (from high sediment load): image from townhall.townofchapelhill.org
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How is turbidity measured?
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Turbidity is measured in one of two fundamental ways:
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| Electronic turbidity meters include those that measure small water samples removed from the stream, as well as those which measure turbidity in place within the stream. |  Portable electronic turbidity meter that measures water samples in vials: image from www.hach.com |
 Digital turbidity meter for use with datalogger: image from www.ftsinc.com |
 Example of continuous turbidity measurements from Nequasset Stream, Maine: Note that turbidity climbs quickly and peaks during rising portion of storm-flow events |
Human Impacts on Turbidity
 Sediment runoff from construction sites can be a major source of turbidity in streams: image from http://www.cityofcartersville.org/images/pages /N174/construction%20erosion.jpg |
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Natural Controls on Turbidity