Vol. 25, No. 5 
Page 5 
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Wetland units are outlined and measured on the photographs and then delineated 
by classes and subclasses (based on life forms of vegetation) and dominance 
types (based on dominant plant species). 
We will modify this method somewhat in our classification of the bottomland 
lakes in the Illinois River valley; due to the physical characteristics of 
these lakes, aerial photographs will not be necessary. Sedimentation in these 
lakes since the turn of the century has changed their bottom profiles into 
platter-shaped basins and eliminated all but a few scattered beds of aquatic 
vegetation. At normal water levels, the present average depth of bottom and 
lakes is only 0.6 m (2.0 ft). When water levels are at the wooded shorelines, 
it takes little reduction in water levels to expose large areas of the lake 
basins as mud flats. Because of the nearly uniform characteristics o t e 
bottomland lakes, we can use available data on surface areas at different 
contour intervals for wetland-classification. By systematically sampling 
selected areas of mud flats and integrating the results with da^a in our files, 
we can determine with reasonable accuracy the composition and extent of moist 
soil plant communities under given environmental conditions. This method may 
not be as accurate as measuring all wetland units on aerial photographs. 
However, a one-time measurement of a dynamic system is only accurate for a 
specific time under existing circumstances. Our approach w,1 allow the 
classification of the Illinois Valley wetlands under “average conditions and 
will provide more useful information under varying conditions than would the 
use of aerial photographs. 
