626 FRANK LEVERETT 
River, and its lower part, below Stillwater, is known as Lake St. 
Croix. A few miles farther down is Lake Pepin, where for a length 
of 25 miles the Mississippi is ponded by a barrier built up from sedi- 
ment brought in by the Chippewa and Zumbro rivers. The level 
has thus been raised at least 40 feet at the lower end of the lake. 
There appears to have been aggradation by the present stream 
along much of the course from Lake Pepin to the head of the 
rapids below the mouth of the Wapsipinicon. 
The channel across these rapids seems to have been inadequate 
to carry the great volume of water from the glacial lakes, for part 
of it flowed southward to Rock River through channels now occupied 
by Meredosia and Cattail sloughs (Fig. 2), and thence down Rock 
River Valley to the Mississippi below the rapids. Peat deposits 
now fill these channels to such height that only the highest floods 
pass through them, but the beds of the channels are about as low 
as the low-water level of the present river. The rapids thus seem 
to have been cut down but little since the waters of the glacial 
lakes ceased flowing over them. The Des Moines rapids, above 
Keokuk, took the entire flow of the glacial lake waters. The 
river at the head of the rapids is 50-60 feet lower than the surface 
of the Wisconsin deposits of sandy gravel immediately above them. 
. It is probable that much of this lowering is due to the waters of 
the glacial lakes. 
The Illinois Valley was cut to a very low gradient by the waters 
of Lake Chicago. Under present conditions there is a fall of 
slightly less than 30 feet in the 220 miles below La Salle. But as 
the mouth of the Illinois is immediately above the mouth of the 
Missouri the sediment from that stream may have raised the level 
of the mouth of the Illinois since the waters of Lake Chicago ceased 
flowing down the Illinois Valley. The amount of aggradation thus 
produced is a difficult matter to estimate. Possibly it is as great 
as that below Lake Pepin, some 30 to 4o feet. Attention was 
directed above to the influence of the sediment of the Missouri 
on the gradient of the Mississippi below its mouth. 
