LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI.' 359 
There are a great many of these lakes. Martin (1916) counted over 200 of them 
in an area of about 20 square miles in the Wisconsin section between Lynxville and 
De Soto, only the lakes that had no connection with the river being counted, the 
sloughs and bays being excluded. It seems that the number of lakes in other parts 
of the river is not less than in this section. Many of them have a rich aquatic 
vegetation, and as they slowly become filled with detritus they gradually become 
swamps. All stages of this process can easily be observed in many points of the 
Mississippi flood plain. 
The depth of the Mississippi River between St. Paul, Minn., and Alexandria, 
Mo., in the main channel varies from 5 to 37 feet, though the depth at any given 
place is subject to many fluctuations, depending on the stage of the water. The 
deepest points found during the present investigation were 27 feet in the main 
channel near Prairie du Chien, Wis., on August 14, and 25 feet above the mouth of 
the Chippewa River on August 30. The depth found at most of the river stations 
varied from 9 to 15 feet. 
LAKE PEPIN. 
In the northern part, about 50 miles below St. Paul, the river fills out its gorge, 
covering the whole flood plain from bluff to bluff, and forming the so-called Lake 
Pepin (figs. 5 and 6), which covers an area of 38| square miles and has a depth of 
about 35 feet. The maximum depth of 56 feet, shown on Mississippi River Com- 
mission chart No. 180, occurs at the very foot of the lake and covers only a small 
area. Lake Pepin owes its origin to the Chippewa River, a small tributary entering 
the Mississippi from the east. The delta of the Chippewa extending into the main 
stream lies at the southeastern end of the lake and is now covered with modern 
flood-plain deposits. It has dammed the Mississippi River, leaving a narrow outflow 
opposite Reads Landing, and the river above the delta has overflowed its banks and 
has filled out the whole gorge. Owing to the slope of the Chippewa, which is con- 
siderably greater than that of the Mississippi, it has been able to deposit more 
material than even the great Mississippi could carry away, hence the formation of 
the delta. The elevation at the source of the Chippewa River is about 1,500 feet 
above sea level. At Chippewa Falls, 62 miles above its mouth, it is 806 feet, 141 
feet higher than at its mouth (Herron, 1917), making a slope of about 2.3 feet per 
mile. On the other hand, the fall of the Mississippi in the section from St. Paul to 
Reads Landing, Minn., 77£ miles, is about 21 feet, or 0.27 foot per mile. Thus the 
fall of the Chippewa River is about ten times that of the Mississippi River, and as a 
result the Chippewa River has formed a sand bar which acts as a dam almost 3 miles 
wide and which the Mississippi could not break. 
At the northern end of Lake Pepin the Mississippi has built its own delta, which 
is still growing. Apparently the lake originally extended as far north as Red Wing, 
about 5 miles upstream from the present head of the lake. The northern part of 
the lake near Bay City, Wis., is now very shallow and almost entirely filled with 
silt and sand; the former northern channel (see fig. 4) has been reduced to a depth 
not exceeding 1.5 feet. Below Red Wing there are three large lakes and several 
small ones, all between the channels in the delta. In August, 1921, they were 
partially dry and covered with water plants, and the northern channel was impas- 
sable. The delta of the Mississippi River has reduced the inlet of the lake to a 
