I did this trip on the first clear day in over a week. That is, it had rained every day for a while before I took this photo. So I caught it with a heavy flow in the river. This theory is confirmed by a couple of Goggle Photos that show more typical river flows: Oct 2016 (note a fisherman is standing in the river) and Dec 2009.
I wonder if this dam replaced a "rolling dam" (drowning machine) such as the one in Wilmington, IL. The short sharp drop onto a gentle slope looks like it might avoid the rolling boil and erosion created by the dams that simply have a sharp drop.
A comparable view in Apr 2016 shows a similar heavy river flow. But a heavy flow in Spring is more expected than this heavy flow in January. (Some more Apr 2016 photos.)
I made a short video to capture the sound of the water flow and the turbulence.
including bald eagles.
The warning sign indicates this dam is an old fashioned "rolling dam." The PVC pipe is for fish line disposal.
sluice gate at the entrance and outlet.... The gates were raised and lowered depending on the river's level and the mill's power requirements." [Forest Preserve interpretive sign]