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Water: Lakes

Site Selection for the Survey of the Nation’s Lakes

Technical Fact Sheet

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Design Objectives - select lakes so that the study will:

  • Determine the proportion of lakes (+/-5%) in the conterminous U.S. that exceed a threshold of concern using selected indicators with 95% confidence.

  • Determine the proportion of lakes (+/-15%) in a specific eco-region grouping that exceed a threshold of concern using selected indicators with 95% confidence.

The selection of sampling locations for the Survey of the Nation's Lakes was completed using a probability based design. Rules for selection were developed to meet certain distribution criteria, while ensuring that the design yeilded a set of lakes that would provide for statistically valid conclusions. With input from the states and other partners, EPA used the following framework to guide the site selection process:

  • The National Hydrography Dataset (NHD) was used to derive a list of lakes for potential inclusion in the survey.

  • For purposes of this survey "lakes" refers to natural and man-made freshwater lakes, ponds, and reservoirs greater than 10 acres (4 hectares) in the conterminous U.S. , excluding the Great Lakes.

  • The sample size was set to include 1,000 lake sampling events. The result was the inclusion of 909 discrete lakes, with 91 of the lakes to be scheduled for revisits. An ‘oversample' of additional lakes was also done so that any state wishing to conduct a state scale survey could be accommodated.

  • The design was constructed to include a representative subset of the lakes that were included in the National Lake Eutrophication Study (NES), conducted by EPA in 1972. This will allow for an extrapolation of changes to the full set of NES lakes.

  • Lake selection for the survey provided for 5 size class categories, as well as spatial distribution across the lower 48 states and 9 aggregated Omernik Level 3 ecoregions.

  • 10 acres (4 hectares) was set as the minimum size for inclusion in the Lakes Survey. Lakes from 2.5-10 acres in size (1-4 hectares) were included in the selection process so that states may elect to include these smaller lakes in state scale efforts.


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Distribution of Lakes in Survey

Lake Size Category

# of Lakes Selected

Total # of Lakes in the US

10-25 acres
(4-10 hectares)

104

68,559

25-50 acres
(10-20 hectares)

185

24,902

50-125 acres
(20-50 hectares)

184

16,488

125-250 acres
(50-100 hectares)

172

6,134

> 250 acres
(>100 hectares)

264

7,356

Total

909

123,439

Total number of lake visits: 1,000

909 unique lakes
91 lakes for repeat sampling

Number of Lakes from 1972-76 National Lake Eutrophication Study (NES): 113

Number of Lakes per state:

Range: 4-41
Median: 18

Number of lakes per ecoregion:

Range: 84-119
Median: 101



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April 2006
EPA 841-F-06-002

For more information, visit:
http://www.epa.gov/owow/lakes/lakessurvey/

Or contact:
Susan Holdsworth
holdsworth.susan@epa.gov
(202)566-1187
EPA’s Monitoring Branch (4503T)
1200 Pennsylvania Ave., NW
Washington, DC 20460



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