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Watersheds: Introduction

A watershed, or drainage basin, is a land area that slopes downward and drains water into one primary waterway or reservoir. Watersheds consist of a series of brooks, streams, creeks, and rivers. The boundaries of a watershed can be identified by locating the highest elevations of land around the primary waterway or reservoir. The colorful map below, from the United States Geological Survey (USGS), highlights the major watersheds in North America. Many of these combine to form the massive Mississippi watershed.

The Mississippi River watershed is the largest watershed in the United States. It is fed by tributaries that extend from the Appalachian Mountains in the east to the Rocky Mountains in the west.

While a large portion of the United States surface area drains into the Mississippi River and enters the Gulf of Mexico at New Orleans, look at the Great Lakes basin (dark green area in the map above). The Great Lakes basin contains all five of the Great Lakes (Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior: HOMES). Thus, it contains some of the largest bodies of fresh water in the world. Lake Superior alone is large enough to fit the entire country of Switzerland!

Rather than draining into the Mississippi River and then into the Gulf of Mexico and the Atlantic Ocean, the runoff from the Great Lakes basin flows through Lake Erie and spills over into Lake Ontario, and from there down the Saint Lawerence River and into Saint Lawerence Bay and the North Atlantic Ocean. In passing from Lake Erie to Lake Ontario, all of the water drained from the Great Lakes basin drops over a 51 m (167 ft) cliff called the Niagara Falls (below).

A visit to Niagara Falls accentuates the massive amount of water that flows continuously from the Great Lakes basin.  At peak times of the year, 168,000 cubic meters of water flow over the Falls per minute. That is well over 60 Olympic-size swimming pools… every minute.

The rock-weathering effect of so much water removes over a meter of the Niagara Falls’ cliff-face every year. As a result, over the past 12,500 years (the approximate age of the Falls), the Falls itself has moved upstream over 11 kilometers (about 7 miles)! This suggests the tremendous power that the Falls emptying the Grate Lakes watershed has. In fact, the power generated by hydroelectric plants at the Falls is used to meet the electrical demands of over 3.8 million homes!


Abiotic Factors

Several abiotic (non-biological) factors affect the structure and function of watersheds. These include water velocity, water turbidity, sedimentation, dissolved oxygen, and salinity. All of these factors are important in determining what organisms can live in a given area of a watershed. You will experiment with each of these abiotic factors in the lab during the course of this CELL.

The speed and direction of water flow in a stream channel is water velocity. Water velocity is defined as the time it takes for a given particle of water to travel a given distance, and it is measured in units such as meters per second (m/s) or feet per second (ft/s). Water velocity is affected by the position of water in a stream channel, the type of flow, the amount of water in a stream channel, and the area of a stream channel.


Water Velocity

Water velocity varies depending on the position of the water in a stream. For example, the velocity of water in the middle of a stream is different than the velocity of water at the edges of a stream. In a straight stream channel, the highest water velocity is in the center of the channel, and water velocity slows at the edges of the channel. In a meandering (curved) stream channel, water velocity is fastest on the outside of a bend and slowest on the inside of a bend.


There are two main types of water flow, laminar flow and turbulent flow. These are illustrated in the figure below. Flow becomes more turbulent as water flows over rocks and other large sediments. The turbulent flow slows water velocity because it takes a given particle longer to cover a certain distance. Stream channel roughness decreases downstream as sediment becomes smaller, causing less turbulent flow.


Discharge Rate

Discharge is the volume of water that passes a given point during a specific time interval. It is usually measured in m3/s (cubic meters per second) or ft3/s (cubic feet per second). The discharge of a stream varies during the seasons. For example, discharge downstream could increase when melting snow adds more water to tributaries or when a major storm occurs. Discharge is directly related to water velocity:

As discharge increases, either velocity increases or the width or depth of the channel must increase. This can lead to flooding downstream. As water changes depth, it can also lead to changes in temperature and light penetration which can lead to changes in habitat for living organisms.

Watersheds contain sediment, loose insoluble materials such a rock fragments, soil, and organic matter. Sediment is often formed by the erosion of the stream channel itself or by eroded material from the surrounding land (watershed). Sediment can be suspended in and carried by the moving water. The amount of material that can be suspended is a function of water velocity. For a sediment particle to be transported, the water velocity must be higher than the settling velocity of the particle. Most sediment is suspended when water velocities are highest. If the water velocity of a stream channel decreases, the ability of water to keep particles in suspension is lowered. Sediment particles begin to settle out of the water, depositing on the bottom of the stream channel. This process is called sedimentation. Streams ultimately deposit most of the material they carry as water velocity decreases, forming watershed structures like alluvial fans and deltas.



Turbidity is a measure of the cloudiness of water and is due to suspended particles in the water. Sedimentation and turbidity are indirectly proportional. Turbidity increases with an increase in water velocity or an increase in turbulent flow, whereas sedimentation increases with a decrease in water velocity or a decrease in a turbulent flow.

Turbidity is one measure of water quality in lakes and streams. Turbidity may increase due to increased nutrient input from the watershed, increased erosion, increases in bacteria or algae, heavy rain, or increased boating. Increases in turbidity can have a negative impact on water ecosystems. Increased sediment in the water can hold heat, raising the temperature of the water. Turbidity decreases the ability of sunlight to penetrate the water, leading to decreased photosynthesis by submerged vegetation and ultimately decreased dissolved oxygen content of the water. Remember, oxygen gas is one of the products of the photosynthesis reaction:



Dissolved Oxygen

One of the most crucial abiotic factors to the health of watershed ecosystems and overall water quality is dissolved oxygen content. Dissolved oxygen is the amount of oxygen that is dissolved in water. It is measured in units of milligrams (mg) of oxygen per liter (L) of water (mg/L). Aquatic organisms like fish, mollusks, and insect larvae depend on dissolved oxygen to live. They extract the dissolved oxygen with their gills.

Oxygen dissolves into water in two ways: through diffusion at the air/water interface and from production from the photosynthesis of aquatic plants (see illustration above). The amount of oxygen that stays dissolved is dependent on several abiotic factors, such as the temperature of the water. As water temperature increases, the concentration of dissolved oxygen decreases. Water velocity also has an effect on dissolved oxygen content. As water velocity increases, there is more turbulent flow and an increased surface area of water is exposed to air. This increase in surface area increases the amount of dissolved oxygen in the water.

Even though increased velocity generally increases the turbidity of flowing water, it also drastically increases the surface absorption of atmospheric oxygen. The surface churning and splashing of turbulent water flow both trap O2 from the air and significantly increase the water’s surface area for absorption of the gas.



Salinity is the concentration of ions or salts dissolved in water. Typical ions found in water include sodium, magnesium, calcium, and chloride. These ions enter the water through erosion of rocks and through the mixing of seawater with fresh water in an estuary. Salinity has a direct effect on many aquatic plants and animals. As the salinity of water increases, many aquatic organisms cannot maintain their internal ionic balance. Salinity also has an indirect effect on water quality. As salinity increases, more ions enter solution. Water has a higher affinity for chloride and other ions than for oxygen, so less oxygen is able to dissolve in the water. Dissolved oxygen content decreases with increasing salinity.

Not only does salinity impact the dissolved oxygen level of water, but it affects water density as well. Saltwater is denser than freshwater. The difference in density of saltwater versus freshwater can be observed in studies of estuaries. Estuaries are locations where rivers (freshwater) meet the saltwater of the ocean.  When the tide is coming in (at high tide, twice a day), it rushes from the ocean into the river. The river, of course, flows in the opposite direction. This situation can cause seriously dangerous conditions for boats and ships entering or leaving the estuary. In addition to the large bodies of water colliding with each other head-on, the denser saltwater of the ocean dives down and under the less dense freshwater of the river, as shown below.

The forces that exist at the point where the saltwater and freshwater meet (indicated by the X in the figure above) can be extreme. Large logs broken loose from logging barges and even sizable boats can be drawn down and submerged in seconds if they hit the shearing water interface at the wrong angle. Coast Guard stations are often built near the mouth of such rivers to help rescue sailors and boaters that get entrapped in these dangerous waters.


  • Watershed Facts
  • Learn the Lingo
  • Get Focused



Plant Nutrients

Normally, one thinks of nutrients as something good. We consume food to gain both calories for energy as well as nutrients. Animals need nutrients like small amounts of metals and salts. The need for nutrients often involves the requirement of trace (extremely small) amounts of elements like phosphorous, nitrogen compounds, calcium, iron, magnesium, zinc, and so on. Plants need nutrients as well. For example, they need nitrogen and phosphates, and metals like magnesium. As you know, chlorophyll is an essential pigment molecule required for photosynthesis. But, did you know that an ion of magnesium is incorporated into each chlorophyll molecule? Without magnesium, chlorophyll can’t capture the Sun’s energy and most life on Earth would be impossible! Notice the single magnesium (Mg) ion at the very center of the chlorophyll molecule below.

The study of freshwater systems is called limnology. Limnology considers both the biotic and abiotic characteristics of freshwater systems – lakes, rivers, and streams. Limnologists study natural watersheds to understand the proper balance of biotic and abiotic factors for a healthy habitat. They also identify and study issues and problems associated with the human use of watersheds. One of the important issues in these terms is eutrophication.


Eutrophication is caused by excessive nutrients in a body of water. As you might guess, excessive amounts of plant nutrients will cause water (aquatic) plants to grow. In fact, it causes them to grow too much. This type of unnatural and excessive plant growth is what eutrophication is all about. Look at the algal bloom in the pond water shown below (click on the image to enlarge).

Excessive nutrient runoff from the surrounding watershed can cause such extensive algal blooms to occur. This type of overgrowth is very bad for lakes and ponds. The mat of algae prevents sunlight from penetrating into the water, thus preventing other, deeper plants from growing. Even worse, this type of eutrophication often results in depletion of oxygen from the water as the algae die and sink to the bottom and are decomposed by oxygen-consuming bacteria. Eventually, as the oxygen runs out, other bacteria that use sulfur instead of oxygen to decompose the mass of dead algal cells begin to dominate. This often results in the release of hydrogen sulfide gas (H2S), which has a strong rotten-egg smell associated with low water quality. 

The use of fertilizers on land near bodies of water can lead to eutrophication. This is because rain washes much of the phosphates and nitrogen contained in fertilizers directly into the surrounding waters. Cattle and other herd animals also contribute to eutrophication through the high nitrogen content of their waste products.


The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.


Investigation 1:
  • Watershed: a land area that slopes downward and drains water into one primary waterway or reservoir; made of a series of brooks, streams, creeks, and river.
  • Velocity: the time it takes for a given particle to travel a given distance; measured in meters per second.
  • Discharge: the rate at which a volume of water flows past a point over some unit of time.


Investigation 2:
  • Sedimentation: the process of depositing sediment.
  • Turbidity: a measure of the cloudiness of water.
Investigation 3:
  • Estuary: a place where a river runs into an ocean and fresh and saltwater mix, for example, a bay or salt marsh.
  • Salinity: the concentration of ions (or salts) dissolved in water.

Get Focused

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:


Investigation 1:
  • How do the abiotic factors of water velocity and discharge affect a watershed?


Investigation 2:
  • How do the abiotic factors of water velocity, sedimentation, and turbidity affect a watershed?
  • How does water velocity affect sedimentation and turbidity in a watershed?
Investigation 3:
  • How do the abiotic factors of temperature, water velocity, salinity, and dissolved oxygen affect a watershed?