I've heard often that there should be another USGS gauge that measures river flow between Algonquin and Montgomery. I found a chart that converts the gauge height reading at SE to a flow (in cfs). Hope this helps, guys...
http://il.water.usgs.gov/nwis-w/IL/data.components//rating.cgi?statnum=05551000
http://il.water.usgs.gov/nwis-w/IL/data.components//rating.cgi?statnum=05551000