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Biological Stream Ratings

In 1984, a Biological Stream Characterization (BSC) Work Group convened to develop a multi-tiered classification of streams based primarily on fish communities. The use of letter grades "A" through "E" for evaluated reaches established a means of communicating levels of biotic integrity to diverse stakeholders. In 1992, the Illinois Natural History Survey (INHS) published a list of biologically significant streams (BSS) for the purpose of conserving biodiversity across the state. The BSS process expanded on BSC "A" rated streams by adding additional information on endangered and threatened species and mussel diversity.

Despite the lack of regular updates, both BSC and BSS processes generated products that are used extensively by diverse stakeholders. As Illinois resource managers move forward with implementing the Wildlife Action Plan, current stream ratings generated with diverse aquatic biological information and a process by which ratings can be updated were needed. The IL Department of Natural Resources - Office of Resource Conservation, in partnership with INHS, initiated a project to combine, update, and enhance the two previous approaches for rating Illinois streams. This revised process incorporates biological data from 1997 through 2007. For additional information see FAQ

Revised System Enhancements

The most significant differences among the past and present rating systems are:

  • Stream reaches are now rated for diversity, integrity, and biological significance depending on available data.
  • The present rating system includes information on taxonomic groups that were not included previously and limits ratings to stream sizes where existing biotic indices work (e.g., wadeable streams that are mostly third to seventh order).
  • Data other than fish can be used to generate the highest rating for a stream segment.
  • Ratings are applied to valley segments rather than a set distance upstream or downstream of a sample site, which results in ratings at a finer spatial scale than before.
  • BSS designation is based on information from at least two different taxonomic groups.

Stream Rating Report

Mapping Tool

Stream Rating Data

Please Note: These data are in ESRI "File Geo-Database" format. You must use ArcMap 9.3 or higher or ArcGIS Pro to view this file geodatabase. Make sure that you refer to the field aliases for the reference names used in the metadata, etc. 

Ann Marie Holtrop, Chief

Division of Natural Heritage
One Natural Resources Way
Springfield, IL 62702
(217) 785-4325