When It Rains it Drains
Background Information
When water falls onto the surface of the Earth, it will seep into the ground and/or
run off over the surface. The nature of the ground and the intensity and duration of the
rain govern how much water the ground will absorb and how much will run off. With
thick sandy soils and long dry periods between rainfalls, for example, most of the early
rain will seep into the ground. Once the soil is saturated (long duration rainfall), or if the
rain is falling faster than the ground can absorb it (high intensity rainfall), water will
begin to run off. On hard rocky ground not much rain will be absorbed. Rather, most
will flow downhill, over the surface, especially during high intensity rainfall.
Gravity and topography govern where runoff will collect and flow. Water flows
downhill and follows the path of least resistance toward some base level (another larger
river, lake , or ocean). As long as there is a gradient (slope) to the surface, water will
continue to flow downhill. Water that is absorbed into the ground is stored as
groundwater reservoirs. Like surface water, groundwater also flows downhill towards
streams, lakes, and the ocean but at a much slower rate. It is important to understand this
connectivity of water especially in areas where pollutants can get into the groundwater,
rivers, and lakes that are to be used for residential purposes.
Key Terms
Watershed
Water basin
Drainage basin
Drainage divide
Urbanization
Urban
Confluence
Landslide
Discharge
Dendritic
Tributary
River source
Ridges
River outflow
Pollution
Purpose
The purpose of this activity is to construct a model watershed to show students what
constitutes a watershed, and how that watershed is affected by external (weather) and internal
(geology, urbanization) conditions.
Objective
The goal of this exercise is to examine the major paths of water on the Earth's surface,
and to understand the many factors that contribute to those paths. Students will demonstrate their
understanding of a watershed by identifying the defining features of a watershed such as ridges
and valleys and predicting the flow path of water. Additionally, students will observe absorption
and runoff of rainfall within the model watershed and how pollutants contaminate water sources.
Time Needed
45 Minutes
Materials Needed (per model)
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heavy duty wrapping paper approximately 3 ft x 3 ft piece
1 black permanent marking pen
1 blue and 1 red water soluble marking pens (Overhead Projector Pens)
1 mist spray bottles
small beaker of sand, small beaker of clay (optional)
tracing paper, one per student (optional)
sponges/paper towels (for clean up)
CA State Science Standards (9-12):
Investigation and Experimentation Standard:
1.m. "Investigate a science-based societal issue,"e.g...".land and water use
decisions in California."
Earth Science, California Geology Standard:
9.c. "Students know the importance of water to society, the origins of California's
fresh water, and the relationship between supply and need."
Activity
1. Crumple a 3'x3' sheet of heavy duty wrapping paper, and allow it to slightly
unfold so that you have a rough, irregular surface, approximating a hilly
landscape. You can make slight adjustments so that the terrain looks hilly or
mountainous. The idea is to be able to identify landscape elements that
approximate valleys and ridges.
2. To determine where water would flow if it rained on your watershed model,
identify potential streams by drawing blue lines along valley paths. To determine
watershed boundaries, identify the highest ridges that separate adjacent streams
flowing away from one another by drawing black lines along their edges.
3. With your black marker, draw spots of residential locations. With the soluble red
marker pens, draw spots of pollution, representing gas tanks or septic systems, on
the sides of hills adjacent to rivers. Place small amounts of sand and clays in
flatter areas adjacent to the streams as well (optional).
4. Spray the entire crumpled paper watershed with the fine mist and note the pattern
of river flow (in blue), contaminant flow (in red), and any sand and/or mud slides
that result. Be careful to spray from a distance so that the force of the spayed
water does not flatten your landscape! The water should land on the paper, begin
to soak in, and then begin to collect and run off once the paper is slightly
saturated. Once the paper is saturated, it will begin to flatten on its own. It is best
not to overdo the “rainfall” duration and make observations quickly.
Questions
1. What is a watershed, and how can we define its boundaries?
2. Tributaries are the smaller streams that flow into the major (trunk) streams.
Count
the number of tributaries joining each of the larger streams, and note that number.
How many tributaries are there? How many major trunk streams are there? How can
you differentiate between a tributary and a major trunk stream? Which are longer?
Can you tell which are steeper?
3. Which of the rivers would have the greatest flow (called Discharge), and what
contributes to that discharge?
4. If you did not put arrows on your watershed, could you tell which direction the
streams and rivers are flowing? How?
5. What does the river system of your watershed look like, and can you suggest a name
for this pattern?
6. In your own words, how would you define a watershed or drainage basin?
7. What affects the flow of pollutants above and below the ground, and can you predict
the paths pollutants will take even if you can't see them?
8. When and where do landslides occur?
Extensions
Looking vertically down over the landscape you just created, with the blue marker and on
a separate piece of paper, draw a map view of the river system, including all the
tributaries. On this map draw the outline of the ridges in black. With arrows, show the
direction of flow of the streams and rivers.
The longest rivers, with the largest watersheds, as defined by the black lines along the
ridges, will tend to have the largest discharges. This can be affected by the nature of soil
or rock. Thick soils would absorb more water and decrease runoff to the river, reducing
river discharge, and steeper slopes drain more effectively than gentle slopes. Vegetation
would also affect runoff, though you can't tell that from our model.)
Question: How would you define a watershed? The word "drainage basin" means the
same as watershed. Write a short description of what a watershed is. Number the different
watersheds on your map in order of size. How do the sizes of the watersheds compare to
the sizes of the rivers they contain?
Question: Looking at the world map, identify 3 of the largest rivers on Earth (those that
have the largest discharges). With tracing paper, outline the watershed area for each, and
draw the major river systems within that watershed. How did you make this
determination?
On the map of San Francisco can you outline the drainage basin (watershed area) that
includes your school? What makes that difficult? (Identifying watersheds in SF will be
completed during another lesson)
Question: From your reading, identify an area that has experienced a major flood. Where
is it? Why do floods seem to periodically occur in the same areas? What makes these
places more vulnerable? What factors other than rainfall can contribute to flooding?
(Students might identify areas with high rainfall rates, large watersheds, regions that
might have been urbanized or modified by human activity like lumbering - though this is
a little sophisticated. They might identify dam or levee breaks, or other human-induced
factors. This could lead to an interesting discussion of these factors).
Citation
(adapted from the Project WET activity "Branching Out")