KEOKUK DAM 
89 
Dams have relation to fish life, however, in other ways than through the creation 
of regions of impounded waters. They constitute more or less effective barriers or 
obstructions in the course of the river and thus may interfere with the free passage of 
migratory fish from one portion of a stream to another. This, in fact, is the effect of 
dams that is most ever present in the public mind, and it is certainly one of great 
significance whenever anadromous or broadfy migratory fishes are concerned. 
Now it is impossible to strike a fair balance in any offhand way between losses 
and gains to the fishery resulting from the construction of a dam. Except as the 
conditions may be investigated carefully and the actual facts ascertained, at least 
in a number of representative cases, broad general statements are unwarranted. 
For any stream and for the particular location on the stream we must know if there 
are fishes affected that are of distinctly migratoiy habit, and if their migrations are 
of such an extent as to be interrupted seriously by the dam; and we should measure, 
also, as best we can, the actual compensatory benefits to fish life. 
If the principal fishery of a stream is based upon the salmon, and it is known 
that the salmon must pass in considerable numbers beyond the site of the dam in 
order to propagate itself, the answer is obvious— the dam is ruinous to the salmon 
fishery unless the fish can be passed over it by means of proper devices. The effect 
of the lake in modifying the temperature of the water flowing over the dam is also a 
factor in this case. (Ward, 1927.) In the case of streams of another region the 
problem may be more complex, as when the fishery is based not upon one but upon 
many species, each of which may have its own peculiar habits as regards reproduc- 
tion and migration. 
The investigation now being reported upon was based upon the dam constructed 
across the Mississippi River between Keokuk, Iowa, and Hamilton, 111., one of the 
greatest dams yet constructed for power development exclusively and one that 
affected one of the principal streams of the United States. Such effects as this dam 
might have would be felt locally both above and below the dam; they might also be 
felt at points hundreds of miles away from the dam, above it and, perhaps, below it 
as well. The investigation was begun in 1913 and was continued, with some inter- 
ruptions, to 1927. There were several reasons why a period of years was required 
for the study. In the first place, we knew little regarding the extent of the necessary 
migrations of the common fishes of the Mississippi River; in the second place, the 
catches of fish under normal conditions vary widely from year to year, and the con- 
clusions drawn from one year’s observation might well be entirely misleading; finally, 
it was recognized that the effects, both good and bad, might display themselves only 
after a period of years. The diminution or the increase in abundance of a particu- 
lar fish might be directly attributable to the dam, although not observable in a prac- 
tical way until after several years. We do not now assume that all effects of the 
construction of the dam at Keokuk have yet been realized, but we believe that there 
is ample warrant for the presentation of a report and for a statement of the con- 
clusions that are drawn. 
Circumstances beyond the power of the Bureau of Fisheries to control led to 
the virtual termination of field observations at Keokuk in 1917 and to delay in com- 
pleting all the studies necessary for report. A considerable portion of our data, 
however, was thoroughly organized and put in preliminary report form in 1918. 
The delay in publication has not been unfortunate, because it is now evident that 
had conclusions based upon statistical reports been published in 1918 it would have 
