360 
BULLETIN OF THE BUREAU OF FISHERIES. 
narrow stream less than 1,000 feet wide. The outlet above the mouth of the Chip- 
pewa River is 1,400 feet wide. There are also two small deltas in Lake Pepin, one 
formed by the Rush River near Maiden Rock, the other by the Isabel River near 
Bay City. 
Bluffs and terraces form the shores of the lake (fig. 6). On the low shore lines, 
especially on the Minnesota side, the waves and currents have deposited sand and 
formed spits, some of them inclosing triangular swampy areas (see fig. 4). These 
capes (Point au Sable, Central Point, point at Lake City, and others) reach far put 
into the water and form a very characteristic feature of the lake. 
The fall from Red Wing, 5 miles above the head of the lake, to Reads Landing, 
on the outlet 28 miles below, is only 0.5 foot, about 0.02 foot per mile. In the 
middle part of the lake there is no fall of the water at all. At the head of the lake, 
3 miles above island No. 28 (see fig. 4) the slope is 0.26 foot for 3 miles; between 
island No. 28 and Wacouta Point it is only 0.07 foot for 3 miles. At the foot of the 
lake above the mouth of the Chippewa River (bench mark No. 237) the slope is 0.25 
foot per 3 miles, and just below Reads Landing it is 1.65 feet per 3 miles. 
The shore line of Lake Pepin is comparatively straight and there are few sloughs 
and bayous favorable for aquatic vegetation. Most of the banks are rocky or 
covered with sand. Water plants are found very close to the sandy spits where 
they are protected from waves (Point au Sable, Central Point), and in the lower 
shallow part near Pepin Village and the delta of the Chippewa River. Here Potam- 
ogeton crispus and americanus, Ruppia occidentalis, and Vallisneria spiralis grow in 
great profusion. A large, shallow area near Bay City in the northern part of the 
lake has very sparse vegetation. At the rocky shore line all stones are covered with 
sponges ( S pongilla fragilis ) . 
LAKE ST. CROIX. 
Lake St. Croix, 21 miles above Red Wing, is similar to Lake Pepin and of the 
same origin. The only difference is that instead of the main stream a tributary was 
dammed. The deposits of the Mississippi obstructed the mouth of the St. Croix 
River, which filled out its valley and formed a lake 23 miles long and from one- 
quarter to 1J miles wide. 
LAKE KEOKUK. 
Lake Keokuk, as has already been said, is a newly formed lake spreading from 
the Keokuk Dam northward as far as Burlington or Oquawka and covering the area 
of the former Des Moines Rapids. According to the contract between the United 
States Government and the Mississippi River Power Co. the level of Lake 
Keokuk must be maintained at 515-525 feet above Memphis datum. Consequently 
the influence of the dam disappears at Oquawka, where the natural mean stage is 
about 525 feet above Memphis datum. (See Table 3.) 
According to information received from the Mississippi River Power Co. the 
rise of water level above the dam at the mean flow of 50,000 c. f. s. caused by Keokuk 
Dam is as follows: 
Feet. 
Keokuk Dam 35. 3 
Fort Madison 12. 7 
Burlington 4.4 
Oquawka 1.0 
Keithsburg 0. 0 
