The U.S. Geological Service’s National Water Information System maintains a network of stream gauges in partnership with state and local agencies. From the USGS “Current Conditions for Illinois” webpage:
“Streamgages are used nationwide to predict and address drought and flood conditions by monitoring water availability. The USGS and over 850 Federal, State, and local agencies cooperatively fund the USGS streamgaging network, which consists of over 8,000 streamgages.
Streamflow in Illinois is monitored in cooperation with State, County, and other Federal agencies.”
Here are links to the stream gauges on the Fox River:
FOX RIVER (TAILWATER) NEAR MCHENRY, IL (stream gauge downstream of the McHenry Lock and Dam)
FOX RIVER (TAILWATER) AT ALGONQUIN, IL (stream gauge downstream of the Algonquin Dam)
FOX RIVER AT SOUTH ELGIN, IL (TAILWATER) (stream gauge downstream of the S. Elgin Dam)