KEOKUK DAM 
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Green algae. — (a) The volume of green algas increased with the season. This was 
especially to be noted in the main body of water and with the approach to the dam 
at Keokuk. (b) In chutes with running water and over submerged land there was 
an increase of green algae, (c) In the tributaries without current and filled only with 
back water from the lake the amount of green algae decreased with distance from the 
lake. ( d ) In tributaries with running water there were no green algae except within 
the immediate influence of the lake. The species of green algae in the main channel 
were Converva sp., or convervoid forms, with very slight admixture of Pediastrum and 
Polyedrium. With the passing of the river water over submerged lands and into 
dead-water tributaries or arms species of green algae characteristic of stagnant con- 
ditions were found. Thus, Pediastrum boryanum and Eudorina would appear with 
considerable regularity, and at times Scenedesmus, Volvox, Glosterium, and 
Staurastrum. 
Blue-green algae. — Blue-green algae made their first appearance in late July but 
were consistently present thereafter in the main channel and in chutes with running 
water derived from the river or lake. In “dead-water” arms of the lake, blue-green 
algae were not encountered. They were likewise absent from tributaries with running 
water. Apparently food conditions are so changed by chemical action or by the 
metabolism of green or blue-green algae that, when not kept constant by renewal of 
river water, blue-green algae can not exist. The species of blue-green algae were few. 
Anabaena circinalis and A. jlos-aqux were present and prevailed as the dominant 
blue-green algae until mid-August; while encountered once thereafter, they had 
virtually disappeared by August 20. Clathrocystis was likewise present from the 
beginning in small amounts, equaled Anabaena by mid-August, and thereafter, with 
one or two exceptions, was the sole blue-green alga recognized. Nostoc was found 
over submerged land in the region above Montrose from August 10 to 21. Consider- 
able amounts of Lyngbia were found in water with weedy and stagnant conditions, 
but these localities were very limited in area. An increase in this and similar forms 
may be expected in the regions where water Persicaria becomes thickly established. 
FISH FOOD 
As has been seen, the old rock bed of the river is now covered with silt, but the 
present lake extends over what was formerly land, and here there must have been, and 
must still be, much decomposition of land vegetation with consequent depletion of the 
oxygen supply. On the other hand, one of the chief effects of winds and waves and 
of the growth of aquatic plants is the reoxygenation of the water. It is quite certain, 
too, that much of the submerged organic matter has been utilized as food by fishes 
and by other organisms, notably May-fly nymphs, which in turn serve as food for fish. 
The most conspicuous form of acquatic vegetation that appeared in the early 
years was duckweed (including several species), which was sometimes thick enough 
in 1915 and 1916 to give a green appearance to large areas of the surface of the lake. 
Of the larger sorts of exclusively aquatic vegetation the only other kind noticed was 
the hornweed (Ceratophyllum), which the fishermen found on their lines, and of 
which thick masses attached themselves to structures immediately above the lock. 
The simpler single-celled plants, both floating and attached, were noted, but casual 
observation did not show these to be present in any remarkable abundance. It is 
worth noting that the only large aquatic plants occurring abundantly away from the 
shores were forms that do not strike roots into the soil and therefore do not bring into 
