Major flooding occurred in northeastern Illinois during September 13–16, 2008, following extended storm activity. Rainfall recorded at select Illinois State Water Survey (ISWS), National Weather Service (NWS), and U.S. Geological Survey (USGS) rain gages in northeastern Illinois, ranged from 2.39 to 10.51 inches throughout a 51-hour period during September 12–14, 2008. The rainfall resulted in extensive urban drainage and riverine flooding, causing the evacuation of thousands of residents, millions of dollars in damages, hundreds of road closings, and two water-related fatalities in the greater Chicago area. Nine counties in northeastern Illinois (16 counties throughout the State) were declared Federal disaster areas. USGS streamgages recorded new record-peak streamflows at 13 locations as a result of the heavy rainfall. Four streamgages had a calculated annual exceedance probability (AEP) ranging from 0.2 to 1 percent, and one streamgage had a calculated AEP of less than 0.2 percent. During this flood event, USGS crews made 48 streamflow measurements at 45 streamgages. After the high-water had subsided, USGS crews set 230 high-water marks in over 40 communities along 131 miles of streams. The elevations for 117 high-water marks along approximately 100 miles of streams were measured by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) and the Illinois Department of Natural Resources–Office of Water Resources (IDNR–OWR). Flood peak water-surface profiles for select streams are plotted from the high-water mark data.