Mittlefeldt and Bruce
Munson developed this lesson.
begin to understand the relationships between rainfall, landuse, and
turbidity (suspended particulate material) in lakes.
be significant events affecting turbidity
readings in a lake. A portion of the rain that falls on the land in
washes into the lake as runoff. The runoff carries inorganic and
materials, including soil, humus, chemicals, nutrients, and pollutants,
into the lake. Topography of the watershed, landuse in the watershed,
and saturation levels of the soil when the rain occurs determine the
amount of runoff (percent of total rainfall). The turbulent energy generated
by wind and wave action can also erode unvegetated shorelines and resuspend
shallow lake sediments.
factors that increase turbidity in lakes.
and quantify the impacts of major rainfall events on turbidity values
in a lake.
and practice strategies for collecting relevant information for real-world
thermal stratification, suspended sediment,
need to be familiar with thermal stratification: epilimnion,
thermocline; and watersheds
access to climatic data for major precipitation events, including
dates, times and rainfall amounts. The web address for the University
of Minnesotas Climatology Working Group is
http://climate.agri.umn.edu/scripts/hradius.htm. The NOAA National
Data Center for climatalogical data is http://www.nndc.noaa.gov/cgi-bin/nndc/buyOL-002.cgi.
Students will need to select a series of dates when major storms (at
least one inch of rainfall during a period of 24 hours or less) moved
through Minnesota. Alternatively, you may want to assign a date(s)
for all students.
to data on landuse surrounding WOW lakes.
handouts for the Directed Study Lesson: Handout
1, Handout 2, Handout
3, Handout 4
ArcView or similar software
requires 2 one-hour class periods plus student homework time between
class periods. (A third class period is needed if student groups present
Chemistry - suspension,
Water Quality; Conductivity;
Data Interpretation; Properties
meet the goals for this lesson by completing a directed study or an inquiry
In the directed
study lesson, students work in cooperative groups. Each group member is
assigned a professional role, and each group develops a report. Students
need printed handouts for the directed study lesson.
study lesson is found in the student section of WOW under the title: "Studying Rain Storms,
Landuse, and Lake Turbidity."
In the student
inquiry lesson students choose a date(s), analyze lake turbidity values,
and develop a written paper, oral presentation, poster, or multi-media
presentation based on their research. They may want to print directions
for the inquiry lesson.
lesson is found in the student section of WOW under the title: "Investigating
Rain Storms, Landuse, and Lake Turbidity".
this lesson by reviewing students knowledge of turbidity. Discuss
the visible differences students have observed in water clarity in lakes
and speculate on which changes have been caused by changes in turbidity.
Review the factors that can affect the degree of turbidity in lakes (physical
processes, geology, vegetative cover, hydrology, mechanics, chemistry).
You may want
to use the WOW data visualization tools to display changes in turbidity
within a lake over a period of time. The image below demonstrates turbidity
in Ice Lake.
Discuss students hypotheses of which factors have the
most significant impact on lake turbidity after a heavy rain.
WOW lakes to student lab groups (four people per group). Make sure
the students number off in their groups. Group members are assigned
roles and complete corresponding handouts according to their numbers
(Handout 1 | 2
| 3 | 4).
The following student roles ensure each group member is actively involved:
meteorologist, landuse planner, water quality analyst, and reporter.
students with a list of dates when major rainstorms have occurred
near WOW lakes. Refer them to the University of Minnesotas Climatology
Working Group website at http://climate.agri.umn.edu/scripts/hradius.htm or
the NOAA National Data Center for climatalogical data is http://www.nndc.noaa.gov/cgi-bin/nndc/buyOL-002.cgi.
After finding dates with heavy rainfall, make sure there is WOW data
available for those dates (the RUSS technology is very new and there
have been times when the RUSS is not operational). The remainder of
the first class period should be used by the groups to brainstorm
and discuss their strategies for collecting and working with the data
needed. Samples of WOW data or access to the Internet would be helpful
for each group during these discussions.
choose a date(s) following a major rain storm to analyze turbidity
values. Students also need to choose a starting point for the turbidity
and how much data to collect.
collect their data they need to consider how they want to organize it
so that it will be easily analyzed and understood. Students will collect
their data using the Internet. The turbidity data and some watershed
landuse data are available from the WOW web site.
should answer their whole group questions. Suggest that each group
designate a timer to keep track of the time during the group discussions.
need to consider how to organize and analyze their data to reach a
conclusion about how a rain storm affects turbidity values in a lake.
should decide to graph their turbidity data over time for several depths.
the different strategies used by each group in collecting and calculating
their information. Have students discuss the merits of the different
strategies. Summarize how landuse variables appear to affect turbidity
in each of the lakes after a heavy rainfall.
discuss the relationships they found among their data. They should
also discuss other factors that might relate to changes in turbidity.
end of the second class period the student groups should either turn
in a group project report, or be prepared to give an oral class report.
should prepare a final presentation. Specify a written paper, oral
presentation, poster, or multi-media presentation.
skills in using GIS information can calculate percentages of shoreline
and percentages of watershed dedicated to specific landuses using ArcView
software or similar software. Other students can devise a variety of
other strategies for calculating percentages of shoreline and watershed
dedicated to specific landuses, such as:
- Using string
to determine length of shoreline and landuse percentages.
a map on graph paper and then calculating percentages.
areas by calculating the mass of a paper map of the watershed and measuring
the mass of cutouts of each of the different landuses.
- Changes in turbidity not related to storm surges or seasonal changes in the lake
system can be signs of things getting better or worse in the lake. Predict
how your lake will change in the future and explain your prediction.
- Correlate Secchi disk readings for your lake with turbidity values.
- Discuss the term "BMP" (best management practices) and its use in
the regulatory arena.
- Assign the following additional roles: banker, DNR fisheries biologist, real
estate developer, logger, shoreline property owner. Lead a discussion
about how one could improve the clarity of the lake and how such a discussion
would be supported or not supported by each of these roles.