Lake size 



(Acres) 



Slope ^ 

 (Percent) 



Less than 5 



28 

 25 

 22 



19 

 15 

 10 



5-10 

 10-15 

 15-20 

 20 -40 



More than 40 



1 



Average of the two steepest measured slopes . 



Such a table may be used to classify lakes into two groups as more or less than 15 feet 

 deep, on the basis of average bank slope measurement, without actual plotting of the 15 -foot 

 depth contour. Eliminating the plotting of the underwater contour considerably reduces the work 

 time required per lake. Although the time saving does resiilt in somewhat reduced accuracy, 

 80 percent of the lakes in the study were classified correctly by this method. Lakes that fail to 

 meet these minimum criteria of slope and lake size must be measured with additional slope 

 readings and the 15 -foot depth contour must be plotted. 



In a recreation inventory, it is often desirable to know what proportion of a deep lake is 

 relatively shallow. As mentioned earlier, fish are not likely to survive over winter in lakes 

 less than 15 feet deep but are largely dependent on food produced in the shallow zone of less than 

 this depth. 



Two methods can be used to determine the proportion of shallow zone area. The first, 

 contour plotting, requires that slopes be measured on photos, and that the 15 -foot depth line be 

 plotted. The shallow area is then determined by the dot -counting technique described earlier. 

 The second method, formula computation, still requires bank slope measurements but elimi- 

 nates the plotting and dot -counting chore by using a formula to compute the proportion of shallow 

 area. The proportion can easily be converted into acreage if a dot count of the total lake area 

 has been made. 



Shallow area estimation by plotting contour. --Once the 15 -foot depth line has been drawn 

 on a photo, either from unmeasured estimates or from projection of 5 to 10 measured bank 

 slopes per lake, it is easy to obtain the shallow zone area of each lake by the dot -count pro- 

 cediires described earlier. 



Contour plotting with parallax measurements was found to be the most accurate of the two 

 methods when results were compared with field measurements. Without parallax measurements, 

 only 47 percent (16 out of 34) of the photo estimates of the shallow zone area were within ±25 per- 

 cent of field measurements. However, estimates based on parallax measurements and projec- 

 tions of bank slopes were within ±25 percent of field measurements 80 percent of the time (41 

 out of 51). 



Formula computation of shallow area. - -The formula method of estimating shallow zone 

 area uses 5 to 10 bank slope measurements and projections. The equation is as follows: 



Estimating Shallow Area of Lakes 



10 



