16 
orien of the Present Vegetation -- It was not possible to determine 
exact ow the cattail originally became established. If it is 
borne in mind that all these potholes were dry during the aneene? two 
alternatives present themselves. 
The first is that as the water areas dried up they were gradually invaded 
by sedge and whitetop which formed a meadow. In heavily grazed areas, this 
meadow might have been grazed short and continually trampled, creating a bare 
or nearly bare area, and allowing the establishment of hardstem bulrush and/or 
cattail, This does not seem likely if one considers the statement made by one 
of the farmers on the study area that the only crops he was able to raise 
during the drought came fron the bottoms of potholes, This cultivation 
should certainly have created as much or more of a bare area than grazing by 
livestock, Jt also seems unlikely that the stock could completely remove the 
vegetation from such a lowland area. Such meadows occurred in 1949 in the 
bottoms of dried-up semipermanent potholes, .In spite of severe grazing pressure, 
they still maintained a solid cover of vegetation. 
The more probable alternative is that these potholes dried too rapidly 
for the sedge and whitetop to invade their centers, This would have left 
extensive bare mud flats furnishing seed beds for cattail and hardstem bulrush,. 
Those that have not been heavily grazed since then may have been reinvaded by 
sedge-whitetop through rhizome competition, Those that have been grazed may 
have retained cattail or hardstem-bulrush vegetation through weakening of the 
sedge and whitetop by repeated cropping. Apparently at times of receding 
water levels in late summer, new stands have been developed in some of the 
Type Al; potholes which show a lack of zonation and have dense clumps of cattail 
and hardstem bulrush (Plate VII). This may have been caused by a local removal 
of less palatable emergent vegetation by livestock during periods when the 
pastures were overgrazed,. 
III, METHODS OF STUDY 
Mapping -- Before the study area was visited, a rough sketch of it was 
made to the scale of 6" = 1 mile from an aerial photograph and was checked 
in the field during the first coverage of the area, This was marked off in 
d-inch grids so that any point on the map could be located by its grid 
coordinates. A copy was carried in the field at all times and all observations 
and objects such as potholes, ducks, and nests were located and referred to by 
their grid coordinates. 
Nest se -- As soon as the study area was selected, nest hunting 
began order that broods might be marked by dyeing the young in the egg so 
that their subsequent movements might be observed. Coverage of all water areas 
was the first job and consisted of thorough inspection of the emergent vegeta- 
tion of potholes. During this search most diving-duck nests were located and 
counts were made of the waterfowl population of the area, This coverage was 
made in the 7 days from June 12 through June 18. From then until July 12, 
most of the mornings were spent in hunting for rivereduck nests by traversing 
on foot the better nesting cover of the area, while afternoons and evenings 
were spent in observation of broods. 
In conjunction with nest hunting a certain amount of crow control was 
carried on, This was felt desirable since it seemed probable that once a 
crow had seen a hen flushed and the nest exposed, it would be likely to 
return and destroy the nest. The crow control was not intended to reduce the 
population and consisted mainly of driving off crows which tended to follow 
