LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
361 
Backwater entirely disappears at a flow of 50,000 c. f. s. a few miles above 
Oquawka. At flood stages backwater from the dam does not reach Keithsburg. 
Thus the head of the lake is approximately near Burlington. 
A characteristic feature of Lake Keokuk is that it has no real head, the river 
being transformed gradually into a lake (fig. 7) , and the overflowed islands below 
Burlington being the first noticeable signs of its existence. This lake can be divided 
into two parts, the upper extending from Burlington down to the Nauvoo-Montrose 
line and the lower extending from this line to the dam. Large 3 areas of submerged 
forested islands and low-lying shore lands are found in the upper part. Here the 
lake is divided into many bayous, channels, and sloughs and passes among wooded 
islands and former agricultural lands which are now under several feet of water. 
The dead vegetation rising above the water forms a very characteristic peculiarity 
of this part of the lake (fig. 8). 
The body of the lower part of Lake Keokuk has a comparatively straight shore, 
in several places bordered by bluffs, and compared with the upper part has fewer 
sloughs and bayous favorable for aquatic vegetation. The depth of the lake gradu- 
ally increases from Burlington to Keokuk, attaining 37 feet near the dam. During 
the investigation on July 30 the maximum depth was found to be at station No. 45. 
The bottom is covered with soft brown mud. 
In spite of favorable conditions, aquatic vegetation has not yet developed 
materially in the lake. It is almost entirely absent in the lower part, but more 
is found in the upper part, where the shallows and the protected areas on the sub- 
merged wooded islands are very favorable for the development of water-plant 
associations. Evidently the period of eight years since the dam has been constructed 
has not been long enough for a full development of aquatic vegetation. At the pres- 
ent time only one form ( Sagittaria longifolia) seems to grow in profusion along the 
shore and on the overflowed islands (fig. 9) . One can find also many Ceratophyllum 
demersum on the shallows and long filaments of Lyngbya sp., which cover the trunks 
of the trees and other objects under the water. A characteristic of this section is 
the rich development of duckweed (Lemna), which is found in such abundance 
that sometimes it covers several acres of water surface with a dense green layer. 
When the water rises Lemna is washed away from the submerged areas and is 
carried down to the dam, for min g small floating islands. 
The velocity of the current in Lake Keokuk at the mean stage decreases from 
2.3 feet per second at Keithsburg to 0.3 foot per second near the dam. At inter- 
mediate points the velocity is as follows: 1.90 f. s. at Burlington, 1 f. s. at Dallas, 
0.58 f. s. just below Nauvoo, and 0.34 f. s. 2 miles above the dam. It can be seen 
that in regard to the current also there is a great difference between the lower and 
upper parts of the lake. In the lower part, where the water is almost stagnant, the 
conditions are more stable than in the upper part, where the lake is more like the 
river. The conditions just mentioned exist only at the average and low stages of 
the river; at time of overflow they disappear almost entirely. 
There is a marked difference between Lake Keokuk and Lake Pepin. There 
is no boundary between the river and the head of Lake Keokuk, while in Lake 
Pepin the inflow is reduced to a comparatively narrow stream. Probably when 
Lake Pepin extended as far northward as Red Wing and the Mississippi had not 
61999°— 24 2 
