KEOKUK DAM 
95 
20 feet or more, back, sides, or underparts up, to be carried off as soon as they fell. Presumably 
no injuries were received, as no dead or injured fish were observed in the river below. No fish, 
as previously indicated, were drawn in from the slack water between the easterly spillways and 
the westerly, although similar eddies prevailed here. 
This remarkable aggregation of a fish, presumed to be one of the most migratory 
in the river, seemed to be especially significant, and we expected that it would be 
repeated in the following year. However, this expectation was not realized, and it 
will be seen later that other observations give ground for the belief that the herring 
is established both above and below the dam. (Coker, 1930.) 
In spite of the closest observation in the spring and summer of 1915 and 1916 
no extraordinary gatherings of fish other than gar pikes and carp were observed 
by Stringham. An extraordinary aggregation of carp that occurred near the lock 
on July 17, 1916, is illustrative. The fish were just below the short section of the 
dam connecting the lock and the power house, or at the bottom of the chute used 
for passing over the drift that accumulates in the fore bay above the lock. Thousands 
of carp were visible at any moment. In this case the occasion of the assemblage of 
carp was fairly obvious. On July 13 and 14 there had occurred a noteworthy flight 
of May flies, and millions of them had drowned in the lake. On the 15th the pre- 
vailing wind was from the southeast, but on the 16th it was from the north and on 
the 17th from the northeast. An enormous and noisome mass of May flies, May-fly 
casts, and duckweed had drifted toward the lock and was flowing in a steady stream 
through the chute. The carp were snapping up the May flies and duckweed at the 
surface. Nine of the carp were opened and the stomachs of seven were found to 
contain principally adult May flies, with some duckweed and the remains of the 
weed; two were empty. Early the next morning both fish and May flies had dis- 
appeared. Later observations on the 19th, 20th, and 21st indicated a recurrence of 
carp in noticeable numbers associated with a north wind and the presence of the 
floating food. 
While the dam serves as a barrier to upstream movements of fish so far as the 
dam structure proper is concerned, it is of interest to know if it is in any way a barrier 
to downstream movements. Observations upon this point are not complete or satis- 
factory. Fish can and do pass over the spillways, but we do not know that this 
occurs at all frequently. It is probable that the lake itself operates as a sort of 
barrier, in that deep still water provides such habitats for fish in downward migration 
as to inhibit the tendency to move down the river. Certainly, so far as a downward 
migration is the result of drifting with the current, the relatively slack waters of the 
lake would serve as an automatic check. 
During the latter half of July and August, 1916, when May flies were very 
abundant immediately above the dam, fish were frequently seen to break the 
surface, but, on the whole, fish were not often in evidence in the surface waters 
of the lake near the dam. On August 2, 1915, a number of fish were seen above 
the western end of the dam, swimming near enough to the surface to make con- 
spicuous wakes. Commonly they swam down near the crest of the dam and then sud- 
denly turned back, but sometimes they were seen to go over the spillways. Such 
observations could be made only when the fish swam close to the top of the water. It 
is reasonable to infer that this occasion was not an isolated one and that the fish do 
go over the spillways from time to time. Several experiments mentioned in another 
connection show that the goujon ( Leptops olivaris), the carp, and the paddlefish 
may go down the spillways and through the turbulent waters below without injury. 
