National Weather Service United States Department of Commerce

Effective March 15, 2016, the National Weather Service (NWS) , in partnership with the Illinois State Water Survey, City of Ottawa, IL, and FEMA has implemented new flood inundation maps to the Advanced Hydrologic Prediction Service (AHPS) web portal for a portion of the Illinois and lower Fox rivers in LaSalle county around Ottawa, Illinois.  The inundation mapping extends approximately 3 miles upstream and 2.5 miles downstream of the Ottawa river gage at the Ottawa high school on the Illinois River,  as well as 4 miles upstream on the lower Fox River.  The maps indicate the potential inundation within portions of Ottawa in LaSalle county. Note: Flooding in Ottawa IL is a result of the combination of Illinois River in flood and high flows on the Fox River.  Additional map layers are provided to represent these combined conditions. Also, gage heights for the Ottawa river gage are reported in elevation above mean sea level.

Ottawa IL flood inundation map example

AHPS inundation maps provide information on the spatial extent and depth of flood waters in the vicinity of National Weather Service river forecast points. Users will be able to display flood inundation maps for various levels ranging from minor flooding through major flooding.  Flood inundation maps, combined with river observations and NWS forecasts, enhance the communication of flood risk and provide users additional information to better mitigate the impacts of flooding. 

NWS flood inundation information is provided as a series of maps.  Each map graphically displays the flooding expected for a specific river level, starting from the NWS-established flood stage for a given location and incrementally produced for increasing river levels up to the highest level observed at the location.  The set of all maps is referred to as a “map library.”

Map libraries are provided on interactive web pages (see example above), and are also available for download in either compressed shapefiles or Keyhole Markup Language (kml) files. The web graphics include roadways, streets, buildings, airports, etc., to allow users to assess areas that are likely to be impacted by floodwaters.

More detailed information about the Web interface is available at:

The Illinois River at Ottawa flood inundation maps and associated geospatial data (shapefiles) can be accessed through
the AHPS web portal:

Click on the "Inundation Mapping" tab

A complete list of flood inundation map locations is online at:

If you have any questions or comments, please contact:

William Morris
Service Hydrologist
National Weather Service Chicago/Romeoville, IL
phone: 815-834-0600 x493