
\ 
intermittent areas are nearly all small; open-water areas are generally large; 
and there is a general trend from intermittent to open water with an increase 
in size. If all potholes that have had their size reduced through division 
by road grades, or their permanence decreased by drainage, were omitted, the 
correlation between size and permanence among the remainder would be even 
closer than is indicated by table 3. Seasonal variations in water depths in 
different wetland classes are shown in figure 8. 
Since size, depth, permanence, and density of plant cover are all closely 
correlated, and since there were too few potholes on the study area to per- 
mit treatment of each factor separately, most of the data concerning waterfowl 
use will be presented on the basis of pothole size. 
Table 3 excludes 39 completely drained areas but includes 24 potholes, 
with an aggregate area of 89.5 acres, that have been affected to some extent 
by drainage. Most of this partial drainage was done before 1945, and either 
has been later reduced in effectiveness or was never intended to be complete, 
having been done to protect road grades or to prevent extensive flooding of 
fields. Four of the areas so affected are still open water, and five are still 
deep marshes. In all but three cases, drainage since 1945 has been complete 
and the potholes are no longer capable of holding water. Thus, 63 potholes 
totaling 117.5 acres have been at least affected by drainage. There is no 
tile drainage presently functioning on the study area, all drainage being 
accomplished by relatively short open ditches that lead the water into lower- 
lying potholes. This is typical of drainage in many types of pothole country. 
A description of the water areas in this region would not be complete 
without a discussion of their variability. Figures 9 and 10 show the number 
of potholes in each wetland type which held water during the four seasons of | 
study. This number is cumulative, and the top of the band denotes the total - 
number of all classes that held water. These data are based on the 270 pot- 
holes (69% of total) for which complete records for the four years are avail- 
able. The percentage of these 270 that held water was applied to the total 
number of 391 potholes. Although minor fluctuations are masked and the ex-= 
tremes of fluctuation are dampened because the points on the curve represent 
averages for the period of each coverage, these graphs give an excellent 
picture of general trends. 
There is a tremendous difference in the surface-water picture between 
spring and fall on the study area. This is largely dependent on the type of 
season and on sudden showers which may relieve a dry spell in a matter of 
hours, refilling nearly all water areas. This occurred in early June of 1951. 
In 1950, precipitation during the growing season was about average, while 
1951 and 1953 were wet in June and July. The growing season of 1952, with 
the exception of June, was extremely dry. These differences are reflected 
in the rate of decrease in the number of water areas. For instance, in 
early July of 1953, there were nearly twice as many potholes containing 
water as there were during the same period in 1950. Further differences may 
be noted between the years in the number of potholes at the outset of the 
season. This determines, to a large extent, the condition of the habitat 
during the waterfowl nesting season. On May 10, 1950, about 275 potholes 
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