This site gives the best information about water levels/gauges for the entire Rock River and some of its tributaries.
http://www2.mvr.usace.army.mil/WaterControl/new/layout.cfm

This site is very good information about water levels/gauges for the Rock River just in Illinois:
http://waterdata.usgs.gov/il/nwis/current/?type=flow

This site is good information about water levels/gaugesfor the central Rock River and as an overall site for the river:
http://www.americanwhitewater.org/content/River/detail/id/10167/#tab-flow