Lesson 3:




In this lesson we will answer the following questions:

  • What is a watershed?
  • Why are watersheds important to water/wastewater treatment plant operators?
  • How is contamination in water affected by the distance from the contamination source?
  • How do we find the boundaries of a watershed on a topographical map? 
  • What problems in a watershed should a plant operator be aware of?

Reading Assignment

There is no set reading assignment for this lesson.  However, you should use your textbook to clarify your understanding of watersheds. 


The Importance of Watersheds

What is a Watershed? 


Imagine a watershed as an enormous bowl. As water falls onto the bowl's rim, it either flows down the inside of the bowl or down the outside of the bowl.


The rim of the bowl or the watershed boundary is sometimes referred to as the ridgeline or watershed divide. This ridge line separates one watershed from another. Topographic maps have a scale of 1:24,000 (which means that one inch measured on the map represents 24,000 inches (2000') on the ground).

By definition, a watershed is an area where all rainfall collects into a common location.  The common location could be a stream, a pond, a river, etc. A watershed is the area of land where all of the water that falls in it and drains off of it goes to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. Watersheds can be as small as a footprint or large enough to encompass all the land that drains water into rivers. The word watershed is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. The watershed consists of surface water - lakes, streams, reservoirs, and wetlands - and all the underlying ground water. Large watersheds contain many smaller watersheds. It all depends on the outflow point; all of the land that drains water to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area above the river-outflow point.   

A watershed is an area of land that feeds all the water running under it and draining off of it into a body of water. It combines with other watersheds to form a network of rivers and streams that progressively drain into larger water areas. Topography determines where and how water flows. Ridge tops surrounding a body of water determine the boundary of a watershed. Imagine turning an open umbrella upside down in the rain. Rain that hits anywhere within the umbrella's surface area would go to the bottom at the center of the umbrella. Any rain that didn't hit the umbrella would fall to the ground. The umbrella is like a watershed; it collects everything that falls into it. High points in elevation, ridges and hills, form watershed boundaries by separating the flow of water into two or more watersheds. for example, water that falls on top of a hill may flow down either side of the hill. Water that flows down one side of a hill will most likely enter into one watershed while water flowing down the opposite side of a hill will most likely enter into a different watershed. A watershed includes not only the water that flows over the surface of the land, bt also includes water that infiltrates the subsurface and flows underground into the river system.

Waterways within the watershed all feed into that main body of water, which could be a river, lake, or stream. The beginnings of a water source are called headwaters. The spot where headwaters progressively join other water sources is called the confluence, and the endpoint of the waterways that open into the main body of water is called the mouth. Water does not simply hit the land and roll off it into a stream. Rainwater is lost through absorption by plants, evaporation and consumption by humans. These factors also depend on the area and the type of soil the water comes into contact with. Each watershed has its own unique characteristics.  Size, permeability of the soil, vegetation type, soil type, slope, microorganisms, pH, DO (Dissolved Oxygen), and temperature all affect the water in a watershed.  As you will see, each watershed also presents its own problems for the water treatment technician. 

The land use activities in a watershed can affect the quality of the water. For example, if the water flowing into a river first flows across a road, the water will dissolve the salt on the road (used for melting snow) and carry the salt into the river. Other land use activities that can affect the quality of the water are applications of pesticides and fertilizers, large amounts of manurek and leaking underground storage tanks (mainly used for oil and gasoline). Watersheds include natural and unnatural features. Natural watersheds have mostly natural features, such as mountains, trees, shrubs, grasses, dirt, and free-flowing streams and rivers. On the other hand, urban watersheds have unnatural features created by people. For example, parking lots, city streets, schools, buildings and channelized rivers are part of the urban watershed.



Why Watersheds Matter

Now that you know what watersheds are, why should their health matter to you? Watersheds directly affect water quality, whether it's for drinking or recreation. For example, algae blooms from fertilizer runoff draining into water harm watershed health, as do mercury and lead seeping into the water supply due to pollution. Unhealthy watersheds affect wildlife. The polluted water supply that results can become harmful to humans. The threat of erosion also exists. Water flowing to a stream picks up dirt along the way. If the water picks up enough soil over time, the land along that stream will become unstable and eventually erode away. If you live along a river bank, this could mean losing your backyard. For wildlife that lives in this area, it means a loss of their habitat. The sharp increase in development around the world may contribute to some of the problems affecting watersheds today. Urban development often involves removing plants, artificially changing the surface topography and altering naturally formed drainage networks. All of these factors affect an area's watershed. In addition, manmade land covers, such as asphalt roads or buildings, act as a "fast lane" for rainfall. Rainwater that would have been absorbed by soil and plants instead is sent directly into streams. These fast lanes increase the chances for flooding because more water pools in that area than a stream can hold. Watersheds matter to water treatment because you will need to know the flow of water that is included in your water sources. If pollution is dumped upstream from the treatment plant, you will need to know if the affected water way will be part of your water source or not.   Watersheds will also inform the operator what type of pollutants may be in the water source, such as agricultural pesticides or herbicides, industrial wastes orconstruction sites.



Protecting Your Watersheds

Now that you know how important watersheds are, how can we help protect them? Several laws exist to protect watersheds. The first was the Watershed Protection and Flood Prevention Act in 1954, which helped coordinate federal and state flood prevention efforts. The Act was amended in 1972 to add conservation efforts. The EPA also developed a program to help watersheds in 1996. The watershed approach is an environmental management program designed to address the declining watershed health by combining public and private efforts to address the worst contamination issues.

Watersheds supply our drinking water, water for agriculture and manufacturing, offer opportunities for recreation and provide habitat to numerous plants and animals. Unfortunately various forms of pollution, including runoff and erosion, can interfere with the health of the watershed. Therefore, it is important to protect the quality of our watershed. Earth is covered in 70% water and unfortunately 40-50% of our nation's waters are impaired or threatened. The leading causes of pollution in our waterways are sediments, bacteria (such as E. coli) amd excess nutrients (such as nitrogen and phosphorus). Although nutrients sould like things that belong in a healthy environment, they can cause big problems in a poorly managed watershed. For instance, sediment can suffocate fish by clogging their gills and the presence of bacteria alone can indicate that other viruses and germs can be found in the water as well.

Sources of Water Pollution

Point Sources

A point source is a single, identifiable source of pollution, such as a pipe or a drain. Industrial wastes are commonly discharged into rivers and the sea in this way. High risk point source waste discharges are regulated by the EPA. The main point source dischargers are factories and sewage treatment plants, which release treated wastewater.



Nonpoint Sources

Nonpoint source polllution (NPS) is a combination of pollutants from a large area rather than from specific identifiable souraces such as discharge pipes. Runoff is generally associated with nonpoint source pollution, as water is emptied into streams or rivers after accumulating contaminants from sources like gardens, parking lots or construction sites. Nonpoint source pollution is a term used to describe pollution resulting from many diffuse sources, in direct contrast to point source pollution which results from a single source. It generally results from land runoff, precipitation, atmospheric deposition, drainage, seepage, or hydrological modification (rainfall and snowmelt) where tracing pollution back to a single source is difficult. This type of source affects a water body from sources such as polluted runoff from agricultural areas draining into a river, or wind-borne debris blowing out to sea. NPS may derive from many different sources with no specific solution may change to rectify the problem, making it difficult to regulate. This type of pollution source is difficult to control because it comes from the every day activities of many different people, such as lawn fertilization, applying pesticides, road construction or building construction.

Contaminated stormwater washed off parking lots, roads and highways, and lawns is called urban runoff. This runoff is often classified as a type of NPS pollution. Some people may also consider it a point source because many times it is channeled into municipal storm drain systems and discharged through pipes to nearby surface waters. Unfortunately, not all urban runoff flows through storm drain systems before entering water bodies. some may flow directly into water bodies, especially in developing and suburban areas. Also, unlike other types of point sources, such as industrial discharges, sewage treatment plants and other operations, pollution in urban runoff cannot be attributed so one activity or even group of activities. Therefore, because it is not caused by an easily identified and regulated activity, urban runoff pollution sources are also often treated as true non-point sources as municipalities work to abate them.

Nonpoint source pollution is responsible for sediment that fills in lakes and streams, covers fish habitat, and reduces visibility in the water. It is also responsible for destroyed fish and wildlife habitat, unsafe drinking water, fish kills, and reduced aesthetic and recreational value of waterbodies. Bacteria from manure or human sewage can cause health problems. Excessive nutrients in water from either chemical fertilizer or organic matter can cause algae blooms in lakes, sometimes making lakes smelly and boating difficult. High levels of nutrients in waterbodies can also cause fish kills, by decreasing dissolved oxygen in lakes and streams. Excessive agricultural or lawn chemicals can also cause the water to be unfit for drinking, or require additional and costly treatment.

In our urban areas rainfall runoff as stormwater is one of the major nonpoint sources of pollution impacting the water quality of our waterways and bays. Stormwater from street surfaces is often contaminated with car oil, dust and the feces of animals and soil and sediment runoff from construction sites, and in industrial areas often contains more toxicants and chemicals. In farming areas NPS include pesticides, fertilizers, animal manure and soil washed into streams in rainfall runoff. Where stock is given access to stream banks they may foul the water and accelerate erosion. Nonpoint source pollution is often more difficult to control than point source pollution. In urban areas the provision of sewer systems and adequate street cleaning are important measures, while in farming and forestry areas, soil conservation practices and the controlled application of pesticides and fertilizers are necessary if pollution of waterways is to be avoided. While EPA does not directly license or approve many of these nonpoint source activities through regulation, we often work with other partners from government, industry and community across different aspects to help tackle these issues.

Wastewater Treatment 

Why are we interested in the sewage treatment end of a watershed?  Communities must release treated sewage into streams and rivers.  In addition to meeting standards set by the Environmental Protection Agency (EPA), wastewater treatment facilities must also consider where the released water is going.  There are often endangered species, such as mussels, and other organisms of concern in the streams below waste water treatment facilities.  We have to be sensitive to life downstream as we release treated sewage water into our watershed.  The wastewater treatment facility strives to maintain the health of the downstream ecosystem.  Our watershed also becomes the watershed of another water treatment facility somewhere downstream.  That facility will be concerned about what we're putting into their watershed for treatment purposes just as we're concerned about the waste water treatment facilities upstream.  So watershed is an important concept within both water treatment and sewage treatment.    


Watershed Boundaries and Contamination

Watershed boundaries define the aerial extent of surface water drainage to a point. The boundaries always follow the highest ridgeline around the stream channels and meet at the bottom or lowest point of the land where water flows out of the watershed. Other terms for watershed are drainage basin, river basin, or catchment.

How do we know the boundaries of the watershed?  Edges of a watershed are usually found in the highest areas around.  There, water falling as rain on one side of the mountain or hill runs down into one watershed while water falling on the other side of the mountain or hill runs down into another watershed.    Flow lines run perpendicular to contour lines.


The further we go from the source of contamination, the less concentrated the contamination will become until it is an insignificant component of the water running into the Gulf of Mexico.   

The total amount of contamination in the water is about the same when it reaches the Gulf of Mexico as it was when it left the source stream.  But there is so much water in the Gulf of Mexico that the contamination becomes very diluted.  You can see the same effect if you put a few drops of red dye in a small cup of water.  The water is quite red.  But if you pour the cup of water and dye into a gallon container of water and mix it up, the water only shows a faint hint of red color. 

Another example of the effect of distance and greater volume on dispersing contamination is a sneeze.  When you sneeze, germ and water particles are initially very concentrated just outside your nose.  But as they are blown further from your face, they disperse into the surrounding air and become much less concentrated.  There is so much air around you for the germ and water particles to disperse in, that the contamination becomes insignificant. 

We're interested in defining an area's watershed so that we can find potential sources of contamination to our facilities.  We also need to be sensitive to downstream life when releasing pollutants into the water.  As we consider sources of contamination in the watershed, we have to keep in mind the effect of distance on the concentration of the contaminant in the water. 





Defining Watershed Boundaries on a Topographical Map 


Let's define watershed on a topographical map (also known as a topo map).  First, you need to know how to read the map.   

The scale is usually found at the bottom of a paper map, but on the website mentioned later, the scale is found at the top of the map.  A typical scale is 1:24,000 meaning that every inch on the map is equivalent to 24,000 inches on the ground.  An arrow at the bottom of the map points to magnetic north.   

A topographical map shows differences in elevation by a series of contour lines.  You can think of a contour line as a trail for a lazy hiker who never wants to climb up or down.  Instead, he just walks around the side of a hill at the same elevation.   

On topo maps, contour lines usually show a difference in elevation of forty feet.  So, the lazy hiker on one contour line is forty feet higher or lower in elevation than the lazy hiker on the neighboring contour line.   

Hilltops are easy to find on topographic maps because they are shown as closed circles.  The lazy hiker can walk around and around the top of the hill on the same elevation.  In contrast, streams are usually shown as Vs with the point of the V being the upstream end.  



When examining a topo map, you will need to orient yourself by finding known objects.  Look for peaks (which are often labelled by name), buildings (shown as small squares), churches, graveyards, etc.  Labelled roads can also help you get your bearings.   

Once you are oriented, find the point you are interested in.  Continuing with the sample map shown above, I have highlighted Creek X.  Now you can begin to define the area's watershed boundaries.  



Starting at the creek, go uphill in every direction (except directly downstream) until you reach the highest points.  Sometimes the highest point above the creek may be a peak, shown as a closed circle.  Other times, the highest point is a ridge, shown as an elongated U- shape.  In any case, on the other side of these highest points, a new watershed begins.  Connect the highest points around the creek with a line and you have drawn in the watershed boundary.  This line also marks the headwaters of the watershed.   


Watershed Monitoring 

Aquatic life in our lakes and rivers is a good indicator of how successful our efforts are at keeping the water clean.  Many organisms are unable to survive in contaminated water. 

We can also monitor the cleanliness of water directly by testing the concentrations of various pollutants in the water.  The Department of Environmental Quality (DEQ) is the agency that monitors watersheds.  They analyze data at stations set up in different areas.   

As a plant operator, you need to have an understanding of what potential sources of contamination are contained within your watershed.  Without this knowledge, you won't be able to respond adequately to problems as they arise.   

The watershed that feeds Greenville, Tennessee, goes through an industrial area that had an acid spill.  A hose carrying sulfuric acid burst and emptied the contents of the tank car into the river.  The water operator that was on duty noticed that the pH of the water coming into the plant was dropping.    The first thing the operator did was to shut the plant down.  He was able to guess that the acid came from a spill in the industrial area even though no one at the company there had noticed the problem.  So the operator called the other water treatment plants downstream of the spill area to let them know about the contamination.  

You never know when a tank car will overturn and spill a contaminant into your watershed.  Usually the amount of time that you will have to respond to such a situation is not what you would like it to be.  What's a reasonable amount of time to be able to respond to a problem?  Time enough to prevent irreversible damage or expensive correction.  The people who are responsible for water treatment facilities want an operator who is on his toes to catch problems as they come down the stream, an operator who will take appropriate action or shut the plant down, if necessary.  This prevents cleanup cost and problems with treated water.  

Knowing your watershed can also help you prevent problems before they occur.  What would do if you found many diseased or dead animals in your watershed?  You wouldn't want dead animals in the water that you were treating for consumption.  Without knowing the boundaries of your watershed, you would unable to stay on top of these problems.   

Other Watershed Problems 

You may wonder, what's the likelihood of anything in the watershed ever creating a problem anyway?  Don't we floc, filter, and chlorinate the input water, thus eliminating almost any problems in the output water?  

Despite all of that filtration, certain organisms are able to pass through a filter.  Cryptosporidium is one example.  Cryptosporidium is a parasite often found in the intestines of livestock.  The parasite contaminates water when the feces from the animal interact with a water source.  In a healthy population, human infection results in acute diarrhea, which lasts for 2-3 weeks.  But the parasite can be life threatening for people who suffer from suppressed immune systems such as AIDS patients, children, and the elderly.   



Since filtration and chlorine are ineffective in treating water contaminated with Cryptosporidium, the Environmental Protection Agency (EPA) requires that all public water supply systems serving more than 100,000 connections must monitor for Cryptosporidium.  Although we are unlikely to have Cryptosporidium in our watershed, some of the larger cities with larger watersheds might.  Plant operators have to test the water flowing into their plants to determine which contaminants might be a problem.   

When monitoring a watershed, we take into account the accessibility of the area.  People can enter the watershed at Appalachia along only one road, so sources of contamination there are relatively easy to monitor.  In contrast, the Big Stone Gap watershed is accessible by two roads and fishing is allowed in the reservoir by permit only.  So monitoring the Big Stone Gap watershed is a little bit more complicated.  The more public access a watershed has, the more potential there is for hazard.   

One last concern of watershed maintenance is diversion of the water for other uses.  As water gets diverted from a watershed, there is less water for contamination to be diluted in.  There is also less water available to be used by the communities downstream.    



Finding a Watershed on Topographic Maps


In order to sucessfully delineate a watershed boundary, the evaluator will need to visualize the landscape as represented by a topographic map. This is not difficult once the following basic concepts are understood.



Basic Concepts

When viewing a topo map you will need to understand the features you are seeing in order to help determine the watershed characteristics. Below are an explanation of the basic elements of concern on a topo map.


Contour Interval

The contour interval is the difference in elevation between two adjacent contours. Typically, the contour interval is given in the map legend. Think of it as the vertical distance you would need to climb or descend from one contour to the next.



Contour Lines

Contour lines represent a ground elevation or vertical distance above a reference point such as sea level. All points along a contour line are the same elevation.




A hill is an area of high ground. From a hilltop, the ground slopes down in all directions. A hill is shown on a map by contour lines forming concentric (having a common center) circles. The inside of the smallest closed circle is the hilltop.




A saddle is a dip or low point between two areas of higher ground. A saddle is not necessarily the lower ground between two hilltops; it may be simply a dip or break along a level ridge crest. If you are in a saddle, there represented as an hourglass, such as the representation below.




A valley is a stretched-out groove in the land, usually formed by streams or rivers. A valley begins with high ground on three sides, and usually has a course of running water through it. The contour lines representing a valley are either V-shaped or U-shaped. The shape enables you to determine the direction water is flowing. Think of the U or V shape as an arrow that points downstream and toward lower ground.




A ridge is a sloping line of high ground. If you are standing on the centerline of a ridge, you will normally have low ground in three directions and high ground in one direction with varying degrees of slope. If you were to cross a ridge at a right angle, you would climb steeply to its crest and then descend steeply to its base. Contour lines forming a ridge tend to be U-shaped or V-shaped. Again, the closed end of the contour line points downward away from high ground.



How do contours relate to water flow? A general rule of thumb is that water flow is perpendicular to contour lines. In the case of the isolated hill, water flows down on all sides of the hill. Water flows from the top of the saddle or ridge, down each side in the same way water flows down each side of a garden wall. As the water continues downhill it flows into progressively larger watercourses and ultimately into the ocean. Each tributary has tributaries, and each one of these tributaries has a watershed. Watershed boundaries always follow the highest ridgeline around the stream channels and meet at the bottom or lowest point of the land where water flows out of the watershed.


Watershed is a very important concept for water and wastewater technicians.  The watershed in which a plant is located determines the purity of the water flowing into a plant.  Plant workers must also be aware of the downstream ecology and human communities which are influenced by the water they release.   

The technician needs to know the watershed's boundaries in order to determine potential sources of contamination.  He should also know how contamination flows through a watershed, influencing all downstream watersheds but becoming more dilute as the distance from the contamination source grows.   

The technician who understands his watershed will be able to respond to problems quickly and efficiently.  He will know when to shut the plant down, when to warn other treatment plants, and when to merely adjust the operation of his plant.




Send the following assignment to the instructor:

To print the assignment and have at hand when exploring the StreamStats page go here: Assignment 3 Print


You will inspect your waterway by exploring the USGS StremStats page at https://streamstats.usgs.gov/ss/

Click on the + button at the top of the map:

You will need to drag the map as you are zooming in to make sure the location you wish to see is within the viewing window. You will need to zoom in until you reach level 9 to view the blue triangles, which represent stream gauges that gather data. You will need to zoom into level 15 before you can delineate the watershed in question.

Once you are at level 15 you can delineate the watershed on the map. You will see these options on the left hand of the map:


For this particular example, I am choosing Virginia. One you choose the state (if a choice is available) the Delineate button will appear:


It will ask you to click on a blue line (stream) in the area you wish to determine the watershed boundary. It will begin calculating and a report will be generated showing the watershed boundary, as below:


Once you have chosen the location and determined the watershed boundary, please click on the Continue button:


Once your report is calculated and a separate page pops up I want you to save it as a .pdf by click on the Print button at the bottom of the page:

Once the print page dialogue comes up you will need to change the print options by selecting the Change button:


Once the options pop up, you will need to choose Save as pdf:


Then click save.

For this assignment you will need to send me the .pdf file you created showing the delineated watershed. You can choose any location on the map as long as the watershed is clearly defined.





Answer the questions in the Lesson 3 quiz When you have gotten all the answers correct, print the page and either mail or fax it to the instructor.