BULLETIN OF THE BUREAU OF FISHERIES.


Rock Island Rapids is almost the same; the average slope from Minneapolis to Le
Claire, at the head of the rapids, is about 0.35 foot per mile; from Rock Island, Ill.,
below the rapids, to the head of Keokuk Lake at Oquawka, Ill., 0.38 foot per mile.
In Lake Pepin the river has a fall of less than 0.2 foot in 24 miles.
    A peculiar characteristic of the river is the great number of islands. Between
St. Paul and the mouth of the Missouri, 658 miles, there are about 540 big enough
to be marked and enumerated on the map. Many of these are more than 10 miles
long and of irregular shape. They split the river into many sloughs and form many
bays and channels, most of which are too shallow to be reached even in a small
flat-bottom river launch. The character of the river is clearly shown on the picture
(fig. 1) taken from the top of Queens Bluff, 10 miles below Homer, Minn. Often
the entrance to a slough is barred by sand deposits checking the flow of the water
and forming a closed bay or a shallow temporary pond. Many lakes and ponds
are found also on the islands, but in the warm season, when the river is at its lowest,
usually in July and August, they almost entirely dry up. In many places the banks
are covered with a soft dark-brown mud and a sparse aquatic vegetation is found
along the river except in the section between Prairie du Chien and Homer, where
beautiful water lilies grow here and there in great profusion along both sides of the
river.
    In the southern extremity of this section the flow of the river is obstructed by
the Keokuk Dam. This dam, built in 1913, has been fully described in technical
and in biological literature (R. E. Coker, 1914; Mississippi River Power Co., 1913)
and it is unnecessary to repeat the descriptions here, although some data must be
given. The dam extends from the Illinois side at Hamilton to the Iowa side at
Keokuk. It is 4,278 feet long, and with abutments, power house, lock, and dry
dock forms an uninterrupted barrier about 1 mile long. Its height is 53 feet.
The water flows through 119 spillways, but ordinarily only a few of them are in use
simultaneously. The difference between the water levels above and below the dam
is about 35 feet at mean flow. Keokuk Lake extends about 60 miles northward
from the dam and is from 1 to 2 miles wide. Its lower part covers the area where
formerly the Des Moines Rapids existed. The formation of this artificial lake
caused the submersion of about 25,000 acres of low-lying shore land and islands.
In order to facilitate navigation not less than 5,000 acres of timber and brush near
Fort Madison were cleared and burned. Dead trees cover many of the overflowed
islands and rising above the water form a strange and desolate picture characteristic
of the upper part of Lake Keokuk.
     Even these introductory statements are sufficient to show that the upper
Mississippi is of particular interest because of its hydrobiological conditions.
     The first point we have to take into consideration is that a barrier, almost
impassable (in an upward direction) for all water animals, divides the river into two
parts; the second is that a new lake has been formed. An immense quantity of
water is held in check, and consequently there arises the possibility of the develop-
ment of a lake (or pond) fauna rather than a river fauna. The biological develop-
ment of a new basin can be followed from the earliest stage of its existence; more-
over, in a newlake, we have the opportunity to introduce the organisms that have
the most practical value and by this means to control the natural process of the


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