CONSTRUCTING AND DEMONSTRATING A
STREAM TABLE WATERSHED MODEL
OBJECTIVES
 Understand the relationship between urbanization and stream flow in the San
Antonio River Basin
 Create simple flood maps to compare flooding in urban vs. natural areas
 Demonstrate that the force of water can cause movement of a toy car
TOPICS
BACKGROUND MATERIAL
 Flooding
Every river has a certain amount of water in it, even during
 Floodplains
periods when it is not raining, called the base flow. In rivers
 Flood safety
such as the San Antonio, whose headwaters are springs, base
 Impervious surface
flow is the water that enters the river from sources beneath the
 Runoff
TEKS
ALIGNMENT
Grade 7 Science:
1A, 2A, 2C, 2D, 2E,
3B, 3C, 4A, 8A, 8C
ground. A significant source of base flow in the San Antonio
River is also return flows from wastewater treatment plants. In
other rivers, such as the Rio Grande, base flow may also
include snow melt from glaciers. Stream flow levels are
recorded at stations throughout the San Antonio River Basin,
shown in the map on Teacher Sheet 1.
If rain falls faster than it can infiltrate into the soil, the result is
surface runoff. One of the biggest problems with surface
runoff is that land uses that prevent water from infiltrating into
the soil can lead to increased flooding. The amount of water
that runs off can be increased if much of the watershed is
covered by an impervious surface (surface that does not
allow water to infiltrate), thus also increasing flooding. An
example of a type of area high in impervious surface is urban
areas, where streets and houses are designed to move water
off of the land quickly and to the closest river or stream.
People usually try to protect themselves from flooding by
building walls between themselves and the floodwaters. In
the San Antonio River basin, there are several examples of this.
Olmos Dam in Bexar County traps floodwaters above the
headwaters of the San Antonio River and prevents flooding in San Antonio. There are an
additional 26 small dams in the north and eastern parts of Bexar County that collect runoff and
prevent floodwaters from flowing into Salado Creek, Calaveras Creek and Martinez Creek.
The City of Kenedy, in Karnes County, is also protected from flooding by 13 small dams that
prevent runoff from entering Escondido Creek. Maps of these retention sites are shown in
Teacher Sheet 2
Another way of protecting people and property from floods is to avoid building houses or
businesses within the floodplain. A floodplain is the area along the edges of a stream that is
covered with water when the river floods. But depending on the intensity of a rain event, the
amount of flooding that occurs can be highly variable. Many floodplain maps show the 100year flood event. The 100-year flood event is not necessarily a flood that occurs once every
100 years, but a flood that has a 1% probability of occurring in any given year.
Flooding can still be dangerous, even if your house is not in a floodplain and you don’t live
near a creek or stream. Many streets in the San Antonio River Basin have low water crossings
where a street intersects the pathway of a creek or stream. During flooding events, these low
water crossings may become completely covered with flowing water; yet some people still try
to cross these areas in their cars. Since a car’s tires are filled with air, the force of the water
can pick up a car fairly easily. Once the car’s tires are lifted off the road, the car does not
have any traction and it can be swept away.
The San Antonio River Authority is constantly involved in flood control projects to minimize
flooding in the San Antonio River Basin. For up-to-date information about these projects,
please visit www.sara-tx.org.
KEY TERMS
Base Flow is the water that
happening in any given
is in a river during dry
Floodplain is the area
periods; the source of
along the edge of the
base flow is usually
stream that is subject to
Urban refers to a city or
groundwater
flooding
town
Flood of Record is the
100-Year Flood is a flood
Urbanization is the process
largest flood about which
that has a 1% chance of
of creating and enlarging
people have knowledge
year
cities and towns
PROCEDURES
A. Before class, set up one stream table for about
every five students.
MATERIALS
B. Ask the students if any of them can remember
when the last flood happened in their area. Ask
For each group of students
them what they think causes a flood, and write
you will need:
their answers on the board.
 Stream Table
C. For the stream table activity:
1. Have the students construct a stream channel
 Stream table sand
about one-half inch deep in the center of the
 Two 250mL graduated
stream table. The stream channel can either
containers
be straight or have one or two meandering
bends. The land surrounding the stream
 Three 500mL graduated
channel represents the stream’s watershed.
containers
Place a layer of felt cloth over the watershed
to simulate natural groundcover and to
 Felt cloth
prevent excessive erosion and new channel
 Aluminum foil
formation when water is poured onto the
 Ruler
watershed.
2. Pour two 500-mL containers of water onto the
 Colored toothpicks
watershed at the head of the stream (do not
 Matchbox toy car
pour it directly onto the stream) and have the
 Student Sheets
students mark the extent of flooding by
placing toothpicks into the soil at the margins
of the stream flow.
3. Using the graph paper provided in Student
Sheet 1, have the students sketch to scale a map of their stream showing the extent of
the lateral movement of the water. This sketch represents a floodplain map showing
the extent of flooding in a natural watershed.
4. Cover the layer of aluminum foil to represent impervious cover, being careful not to
disturb the toothpicks.
5. Repeat steps 2 and 3, using a different colored pencil to draw the extent of flooding.
The floodplain map that is created will show the extent of flooding in the watershed
dominated by impervious cover compared to the extent of flooding in a natural
watershed.
6. Using a strip of aluminum foil, have the students construct a “low-water crossing” across
the main stream, and place a Matchbox-sized toy car on the bridge. Pour three 500mL
containers of water onto the head of the stream and observe the results on the toy car.
D. Lead the students in a discussion about the topics that they demonstrated using the stream
tables. Explain to the students what a floodplain is. Then discuss floodplain maps, and
what a 100-year flood is. Discuss the impact of impervious surface versus natural surface
on the extent of the flooding.
E. Finally, discuss what the students observed with the low-water crossing and the car
demonstration. You can use the following calculations to assist you: Each gallon of water
weighs 8.33 pounds, so a cubic foot of water weighs 62.31 pounds. Multiply the amount of
cubic feet per second of water that is flowing by 62.31 to get the force (in lbs) that is hitting
an object every second (i.e. 100cfs = 6,241 lbs of force).
GUIDING QUESTIONS






What does the word “floodplain” mean?
What is the “100-year flood?”
What did the water in the beakers represent?
How did the floodplain sizes compare between the natural watershed and the one with
impervious surface?
What happened to the toy car when it was caught in the “flood?”
How did your models accurately depict a watershed? How were they different?
EVALUATION
Obtain a 100-year floodplain map for the area near your school, either through the San
Antonio River Authority website or another source. Then, have the students write a report
about 100-year flood events as it relates to their community. Have the students answer some
of the following questions: What is a 100-year flood event? Where is the 100-year floodplain in
your community? Are there houses or businesses in this area? How would you solve the
problem of flooding to protect people’s lives and livelihoods? Would you move people out of
the floodplain or try another method of flood control?
STUDENT SHEET 1
TEACHER SHEET 1
TEACHER SHEET 2
TEACHER SHEET 3
REFERENCES
Black, P. E. 1996. Watershed Hydrology, Second Edition. Ann Arbor Press, Chelsea, MI.
449pp.
Schlesinger, W. H. 1997. Biogeochemistry: An Analysis of Global Change, Second
Edition. Academic Press, San Diego, CA. 588pp.
U.S. Geological Survey. 1996. Groundwater Atlas of the United States: Oklahoma,
Texas. U.S.G.S Publication HA 730-E.
Wetzel, R. G. 1983. Limnology, Second Edition. Saunders College Publishing, Orlando,
FL. 857pp.