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mapped for turbidity. The turbidity values indicated in the 
ERTS frame in Figure 1 were so mapped as were the values for 
suspended solids (Scherz, Sydor, et al., December, 1973). 
LAKE TURBIDITY CAUSED BY LIVING MATTER 
The turbid water in Lake Superior near Duluth 
shown in Figure 1, is inorganic red clay. An analysis of its 
reflectance intensity on different bands shows that red clay 
has a unique and identifiable spectral fingerprint. Green 
algal material in lakes also has its unique spectral finger 
print, can be so identified, and can be handled by the technique 
shown in Figure 2. 
When algae is the suspended matter causing the 
turbidity, generally speaking, the algal concentration is high 
est in the most enriched or eutrophic lakes. Since algal con- 
3tal centration is related to turbidity and turbidity related to 
shown ERTS image brightness, then image brightness can be used as an 
index to algal concentration. 
Ely Area Lake Test Site ; The first application of 
this image correlation technique to lake eutrophication was in 
the Boundary Waters Canoe Area (BWCA) in northeastern Minnesota. 
This area is shown in the North Western portion of the ERTS 
frame in Figure 1. 
The BWCA is the nation^ largest wilderness preserve 
and has more recreational use than all of the other national 
wildernesses combined. The BWCA is acclaimed for its aesthetic 
value, especially its pristine water quality. However, the in 
tense recreational pressures coupled with the rapid cultural 
development of the surrounding private lands is threatening the 
delicate chemical and biological balance of the lakes. Most of 
the lakes in this area are of the tannin variety. During the 
summer of 1973, University of Wisconsin researchers, in conjunc 
tion with the Forest Service, sampled three lakes near the time 
of an ERTS overpass to test the potential of using ERTS for lake 
classification. 
The three lakes sampled were: (1) Shagawa Lake, 
classified as eutrophic (enriched), (2) Ensign Lake, somewhat 
more clear and classified as mesotrophic and (3) Snowbank Lake, 
a very clear lake, classified as an oligotrophic lake. Labora 
tory reflectance values were obtained from the water samples from 
these lakes. An analysis was made for solids and turbidity. 
These values are presented graphically in Figure 3.