Credits
Bill Mittlefeldt and Bruce Munson developed this lesson.

Goals
Students will begin to understand the relationships between rainfall, landuse, and turbidity (suspended particulate material) in lakes.

Introduction
Storms can be significant events affecting turbidity readings in a lake. A portion of the rain that falls on the land in a watershed washes into the lake as runoff. The runoff carries inorganic and organic 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.

Outcomes
Students will:

1. Identify factors that increase turbidity in lakes.
2. Describe and quantify the impacts of major rainfall events on turbidity values in a lake.
3. Develop and practice strategies for collecting relevant information for real-world problem solving.

Keywords
Landuse, watershed, turbidity, thermal stratification, suspended sediment, algae, clarity, phytoplankton, transparency

Prerequisites
Students need to be familiar with thermal stratification: epilimnion, metalimnion, hypolimnion, thermocline; and watersheds

Materials/Resources/Software

• Internet access to climatic data for major precipitation events, including dates, times and rainfall amounts. The web address for the University of Minnesota’s 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.
• Access to data on landuse surrounding WOW lakes.
• Student handouts for the Directed Study Lesson: Handout 1, Handout 2, Handout 3, Handout 4
• Optional: ArcView or similar software

Time Required
The lesson requires 2 one-hour class periods plus student homework time between class periods. (A third class period is needed if student groups present oral reports.)

Curriculum Connections
Biology - turbidity, eutrophication, stratification, water clarity
Chemistry - suspension, solutions

WOW Curriculum Links
Modeling Water Quality; Conductivity; Data Interpretation; Properties of Water

Procedure
Students can meet the goals for this lesson by completing a directed study or an inquiry lesson.

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.

The directed 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.

The inquiry lesson is found in the student section of WOW under the title: "Investigating Rain Storms, Landuse, and Lake Turbidity".

Knowledge Base
Introduce 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.

Experimental Design
Discuss students’ hypotheses of which factors have the most significant impact on lake turbidity after a heavy rain.

Directed Study
Assign 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.

Provide students with a list of dates when major rainstorms have occurred near WOW lakes. Refer them to the University of Minnesota’s 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.

Student Inquiry
Students choose a date(s) following a major rain storm to analyze turbidity values. Students also need to choose a starting point for the turbidity graph and how much data to collect.

Data Collection
Before students 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 and landuse data are available from the WOW web site.

Data Management and Analysis

Directed Study
Students should answer their whole group questions. Suggest that each group designate a timer to keep track of the time during the group discussions.

Student Inquiry
Students 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. Most students should decide to graph their turbidity data over time for several depths.

Interpretation of Results

Directed Study
Compare 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.

Student Inquiry
Have students discuss the relationships they found among their data. They should also discuss other factors that might relate to changes in turbidity.

Reporting Results

Directed Study
At the 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.

Student Inquiry
Students should prepare a final presentation. Specify a written paper, oral presentation, poster, or multi-media presentation.

Teacher Notes
Students with 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.
• Superimposing a map on graph paper and then calculating percentages.
• Approximating areas by calculating the mass of a paper map of the watershed and measuring the mass of cutouts of each of the different landuses.

Extensions

1. 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.
2. Correlate Secchi disk readings for your lake with turbidity values.
3. Discuss the term "BMP" (best management practices) and its use in the regulatory arena.
4. 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.