388 
Pekin than above Peoria, and the Sphaeriidae less so. The larger Gas- 
tropoda (Viviparidae and Pleuroceridae) made up about the same per- 
centage of the average weight of collections as above Peoria (88.6%). 
In the 4-7-foot zone the Sphaeriidae showed the heaviest poundages 
(66.2% of totals), and the insect larvae and the larger snails were rela- 
tively much less abundant than in channel collections. 
Borrom Fauna, 4—7—root ZONE, WESLEY TO PEKIN, 1915, AVERAGE 
Viviparidae 
Small 
Insects, 
and : gaat oda worms, Total 
Pleuroceridae Sphaeriidae Crustacea 
Number per sq. yard 20.0 272.5 56.5 349.0 
< 4 collec- 
tions 
Pounds per acre 60.0 136.7 9.6 206.3 
4 collec- 
tions 
Per cent. of total, 
(By weight) 29.2% 66.2% 4.6% 
(c) PEKIN TO CopPpERAS CREEK Dam (16.2 MILEs) 
Hydrography. Following the swift run from the foot of Peoria 
Lake to Pekin, where the low-water slope in 1901 was 2.00 inches per 
mile, the effect of the dam at Copperas Creek became very distinct be- 
low Pekin at those low levels, and the average slope of water surface 
between Pekin and the dam fell to 0.14 inch per mile. Although this 
average low-water decline is not much more than half that between 
Chillicothe and the foot of Peoria Lake (0.26 inch per mile for 18.2 
miles), at ordinary spring flood-levels the slope rate is multiplied ten 
times or more (1.40 inches per mile at gage 18 ft., Peoria), and the aver- 
age flood velocity (114.40 feet per minute at gage 18 it., Peoria) rises to 
‘a figure more than double that between Chillicothe and Peoria.* 
This circumstance—a consequence of the fact that at flood stages the 
sweep of the current tends to follow the old lines of slope of water sur- 
face as they existed before the low-crested dam was put in—accounts 
for the generally well-scoured channel floor that we find throughout this 
reach of 16.2 miles, not even excepting its lowermost portion just above © 
the dam. 
The only important stretch of soft mud bottom in the channel in 
the 16 miles is the deposit occupying less than a mile of channel length 
just above the mouth of the Mackinaw. With the exception of that and 
of about a mile of dirty sand which ends a mile above the dam, the gov- 
* Table, p. 377. 
