Watershed Weekly
Series 1 Watersheds
Episode 3:
Runoff
Grade level: 5-12
Rationale: This episode focuses on what happens to the
water after it falls from the sky. It discusses how water
running down the landscape impacts the geology,
chemistry and biology of “the land on which we stand”.
Materials and Preparation:
 Watershed Weekly episode 3
 Vocabulary:
• sediment
• runoff
• pollution
• riparian zone
• erosion
• Universal Solvent
• Solution
 Resources
• Project Wild Aquatic, Council for
Environmental Education, 2000
o Where Does the Water Run? p. 21
• Project Wet
o Capture Store and Release, p 133
o Get the Groundwater Picture, p. 136
o Just Passing Through, p 166
o The Pucker Effect, p. 338
Procedure:
• Questions for listening:
o How does water affect rocks and soils
o Chemicals dissolve in water and are carried by water. This can have
both positive and negative affects on living things. Give an example
of a positive affect and a negative affect
o Why is water called the Universal Solvent?
o How does water affect organisms living in and out of the water?
o Explain why life is easier for water organisms than for land organisms.
• After listening:
o Organizer or Concept Map based on the affects of water on geology,
chemistry and biology.
o Can you draw a food chain that starts in the water and ends on the
land?
o Take a stream field trip to identify erosion by water, riparian
vegetation, aquatic organisms, etc.
o Using the watershed model from the first episode, students could
experiment with the effect of precipitation on a variety of substances.
What happens if I put a lump of sand in the watershed? What happens
if I put Mud? Clay? Salt? Bread? Food coloring? Some things break
into smaller particles. Some things dissolve. Different substances
react differently to precipitation, but they all tend to go downhill.
Some substances, like the bread, would begin to grow organisms if
you left them for a week or so.
Script - Runoff
Our last Watershed Weekly explored the water cycle, Planet Earth’s vast water recycling
system. Now we will focus on that part of the cycle which we can experience most
directly- when water is traveling through our local watersheds. This is the time when
water has its greatest influence on the geology and chemistry of the land. This is also the
time when water is available for use by plants and animals, humans included.
So, water has a unique role in shaping the geology, chemistry and biology of our
watersheds. We’ll take a closer look at all three of these topics, one at a time.
After a few days of rain, head out to a nearby creek. See the cloudy water carrying
sediment? It’s been carried here from upstream. Hear those rocks tumbling beneath the
surface? They have been freed by an increased volume of water that will soon collapse
undercut sections of bank. You are watching water alter geology on a small scale.
The force of falling water striking the soil causes tiny soil particles to be dislodged and
carried downhill. The size of particles thus moved in this way is proportional to the size
of the falling rain drops. Thus water influences the development and composition of soils.
The total amount of soil movement is dependent on the duration and intensity of rainfall.
In flood events truly amazing amounts of sediment, and indeed, larger sized particles are
transported over land. As water gathers in streams and rivers its erosive power is
magnified. Streaming over the surface of the land, these ribbons of water pry loose and
carry along enormous amounts of suspended clay and sand particles, and tumble rocks
and gravel along. As they collide, larger particles are broken into smaller fractions.
Boulders break into rocks, rocks to gravel, gravel to sand. In flowing over the land,
rivers literally sculpt the face of the Earth, carving canyons and, bit by bit, moving
mountains. The Grand Canyon of the Colorado River is a spectacular example of this
work but any little creek or river in your hometown will show you the same effects.
Water is called the “universal solvent” because of its ability to carry chemicals in
solution wherever it goes. A solution is a mixture of water and one or more chemicals,
and any natural body of water carries a mix of chemicals along. In some cases these are
naturally derived from minerals in the soil since rain and meltwater infiltrate the soil and
carry off available compounds. Biologically processed chemicals, nutrients like nitrogen
and phosphorus and more complex compounds are also absorbed by water and taken
downstream through watersheds. When toxic chemicals find their way into our water,
usually as a result of human activity, the quality of water is impaired, or as most of us
would say, polluted. Water pollution puts at risk the biological health of watersheds.
The propensity of water to contain and deliver vital nutrients makes the immediate
vicinity of watercourses particularly rich biologically as plants find not only the water
they need but the natural chemicals necessary to their survival. A richer plant cover
outlines any healthy stream or river, as an aerial photo of most Southeast Alaska
watersheds will demonstrate. Here, where water is not often a limiting factor, the
strongest growth of trees is along still along the margin of waterways.
Our tremendous runs of salmon are the most dramatic evidence of the biological effect of
water on the land. The richness of fish migration is really just the most visible sign of the
vitality of streams, rivers and lakes. But from the abundance of microscopic plants and
animals at the bottom of the aquatic food chain to the 1200 lb. bear carrying a sockeye to
shore for lunch it is evident that water is a biological hotbed. Life evolved in water and
has secondarily colonized the land. Life on land carries great challenges and the
buoyancy that creatures find in water allows them to devote their energy to pursuits like
finding food and mates. Life in water is physically easier and water is rich in available
nutrients. For these reasons the total density of life is higher in the water. In the case of
streams and rivers, this wealth in turn fosters the rich plant growth we’ve already
mentioned and attracts animals to the bounty. These areas, called riparian zones, are
biological treasure troves.
Now you can see that water moving over the land is integral to its geology, how it
changes its chemistry and how it affects its biology. Our next Watershed Weekly
examines the interactions of humans with watersheds, the land on which we stand.