UP/MoPac/C&EI Trestle top just below the skyline.
a satellite image, you can see the stairway that lets you walk down the concrete embankment to the water's edge. This picture was taken from the bottom of that stairway. It appears that the visible Tainter gates were slightly open but the other one may have been closed.
the wheelchair accessible fishing dock --- the wooden platform by the river. The bridge behind the roof is IL-16, and we can see the top of the train trestle through the trees in the left background.
Note the sign indicates the spillway capacity is designed to handle 162,500 cubic feet per second.
This river was included in the Flood Control Act of 1938. But WWII precluded any action. Floods in 1943, 44, 46, 47, 48, 49, 50, 51, 57, and 58 motivated the Flood Control Act of 1958. Construction started in 1963 and the lake was open in 1970 "for millions of visitors to enjoy." The exhibit has a picture of a flood and the water is just below the roofs of the houses. It also mentions the force of the river's flow would push buildings. Holding back flood waters not only helps this river, but also the Mississippi River.
One of the signs provided the information:
The crest is 643 feet above sea level and 110 feet above the stream bed. The overall length is 1,025 feet long.(Sorry about the numbers not being right justified, I wrote HTML in the 1990s, and I have learned that I have become rather rusty.)
Stage Storage Water area Elevation Length Width Shoreline Flood 474,000 25,300 625.3 30 1.5 376 Summer 210,000 11,100 599.7 20 1 172 Winter 152,000 9,096 594
The sign uses both the terms Recreational and Summer for the high normal level and Conservation and Winter for the low normal level. Since a fisherman I talked to about water levels used the terms Winter and Summer, that is what I used in the table. He said the lake was at the normal winter level.
This is my opinion as to how the dam would be operated.
I wonder how far the gates are raised for the normal max flow rate. That would be the rate that fills the river channel and floods wetlands. it would be a fraction of the max spillway capacity. The max spillway capacity is designed to save the dam and would probably cause flooding downstream.
During the Winter. the level is allowed to fall to 594 so that the lake can absorb the Spring rains and snow melt without exceeding the normal max flow rate and not rising more than six feet. After the wet season, they reduce the flow to the normal minimum and allow the level to go up to 600 feet because that is what the fishing docks, boat launches, etc. are designed for. If they would have to exceed the max normal flow to maintain 600 feet, then they close the recreational areas and allow the level to rise. If the lake level reaches 626.3 feet, then they would increase the flow up to the 162,500 cfs capacity of the spillway to maintain that level. If, at 162,500 cfs, the level rises above 626.3 feet, then they loose a lot of sleep at night because if it would rise another 17.7 feet, it would breech the crest of the dam. Actually, looking at the spillway, the earth berms on the side would be compromised before the crest is reached. By definition, the level should not exceed 626.3 feet because the spillway is supposed to be designed to handle the worse case scenario.
To put 162,500 cfs in context, the maximum capacity of the Oroville Dam is 160,000 cfs. [Video] The Oroville Dam's spillway was passing less than half its capacity when it broke apart.
The other side of the coin is the years that have little snow and rain. That is why they don't allow the level to fall below 594 feet in the Winter. During the Summer, they would use the sluice gates to draw down the lake to maintain a minimum flow in the river.