220 
on 33.2 per cent of the area, slopes are in 
excess of 30 per cent (Stall et al. 1951: 
20). Agricultural misuse of the watershed 
has created a serious silting problem with- 
in the lake. In 1949, Stall et al. (1951: 
12) found that the lake had an average 
annual capacity loss of 1.29 per cent, 
representing an annual erosion of 4.36 
tons of soil per acre of watershed. 
The amount of soil loss from the water- 
shed depends primarily upon the nature of 
the soils, slopes, and land use or farming 
practices. Erosion occurs in two forms, 
sheet erosion and gully or channel erosion. 
Pasturing of some of the ravines above the 
lake contributes to the erosion. The slipping 
of earth from ravine sides into the valley 
floor occurs even where the ravines are 
entirely unpastured. Most of the soil that 
drops off the hillsides as a result of slip- 
ping eventually comes to rest on the bot- 
tom of Ridge Lake. In 1950, about 4 
acres of the upper lake area were covered 
with water of less than a foot in depth; 
this same area was covered with water 3 
to + feet in depth when the water was im- 
pounded in 1941. An effort is now being 
made to organize a soil conservation dis- 
trict that will include the lake watershed 
and that will slow down the rate of sedi- 
mentation within the basin. 
The fill impounding the water of Ridge 
Lake is a 450-foot earthen dam running 
north and south across the valley of Dry 
Run Creek; a concrete surface spillway 
70 feet wide was constructed over a nat- 
ural bank at the north end of the fill, fig. 
2. The primary overflow structure, how- 
ever, is a tower spillway designed so that 
it removes water from the bottom of the 
lake rather than from the surface. In- 
side the tower is a spillway wall parallel 
to the dam that divides the tower into an 
east and a west section; the overflow lip, 
the top of this wall, is 8 feet long; its 
elevation above sea level is 595 feet. The 
elevation of the crest of the surface spill- 
way is 596 feet above sea level, so that, 
on small rises of the lake level, all the 
overflow water passes through the tower. 
As the tower is open at the bottom, water 
from the bottom of the lake rises in the 
east section of the tower to correspond to 
the lake level. When the lake level ex- 
ceeds the overflow lip, the water spills 
over into the west section of the tower, 
Intinois NaturaAL History Survey BULLETIN 
Vol. 26, Art. 2 
the section nearer the dam, and passes 
through a rectangular concrete tunnel 5 
feet wide and 7 teet high to that part of 
Dry Run Creek on the downstream side 
of the dam. In summer, when the waters 
of the lake are thermally stratified and 
the bottom waters are devoid of oxygen, 
rises in the lake level allow stagnant 
bottom water to escape from the lake. 
A gate valve, + feet square, located at 
the base of the spillway wall in the 
tower, allows the lake to be drained. This 
gate was first closed on April 17, 1941, 
when construction of the dam was com- 
pleted. 
In 1940, a brick laboratory and a boat 
dock were constructed on the south shore 
of the lake about 800 feet up the lake 
from the dam, fig. 3. At the same time, 
two single-post concrete towers were built, 
one in the center of the lake opposite the 
laboratory and the second about two-thirds 
of the distance up the lake from the dam 
to the upper end of the basin. These 
towers, which were supplied with wooden 
floors and railings, have been used to 
support recording thermographs and other 
weather instruments. 
The maximum lake depth, 25 feet, is : 
in the old stream channel near the dam; 
the valley floor of the lake near the dam is — 
4 to 6 feet above the bottom of the old 
stream channel. From the old stream 
channel near the dam, the floor of the 
valley gradually rises until, about 2,500_ 
feet up the lake from the dam, it exceeds 
the 595-foot level of the overflow lip of 
the tower spillway. The width of Ridge 
Lake varies from about 400 feet at the 
dam to 200 feet at the upper end. The 
steep side walls of the lake basin tend to 
make much of the lake comparatively deep, 
except in the upper end, where sedimen- 
tation is now extensive. 
aS ee 
Limnological Characteristics 
Ce ee a . e 
Limited work on the limnology of Ridge 
Lake defined the range of variation of 
some physical and chemical characteristics — 
of this body of water during the summer 
periods of 1941-1951, somewhat longer in — 
1942 than in the other years; some quan-— 
titative biological sampling was done in 
1948 when Fleming (1949) was investi- 
gating the characteristic forms of plankton 
