Rock Creek IA 02-CED-586

Assessment Comments

Assessment based on results of (1) an IDNR/UHL biocriteria sampling in 2002: FIBI = 72 (excellent), BMIBI = 71 (good) and (2) a fish kill and investigation in July 2002.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" due to the results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment include (1) results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project and (2) report of a fish kill that occurred in July 2002.  

EXPLANATION:  Results of biological monitoring in August 2002 suggest that the aquatic life uses should be assessed (monitored) as “fully supported.”  This assessment was based on biological data collected in August 2002 as part of the DNR/UHL stream biocriteria project.   The 2002 FIBI score was 72 (excellent) and the BMIBI score was 71 (good).   The aquatic life use support was assessed as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports.  The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.   The riffle site FIBI BIC for this ecoregion is 65 and the substrate BMIBI BIC for this ecoregion is 70.   The biological assessment was conducted on August 21, 2002, approximately one month following the July fish kill (see below).   Thus, the potential exists that the kill influenced results of the biological assessment and may be related to the comment on the field sheet that numbers of fish were relatively low.  

A fish kill occurred on in this stream segment on July 27, 2002.   The kill occurred after a rainfall event that followed a period of hot, dry weather.   No cause or source was determined, although area feedlots were a suspected source of organic matter that potentially degraded water quality and caused the kill.   Approximately 1 mile of stream was affected, and an estimated 970 fish were killed.   According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill within the most recent three-year period indicates ongoing impairment of the aquatic life uses.    Also, according to IDNR’s assessment methodology, if a cause of a fishkill was not identified during the IDNR investigation (= "unknown"), or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”  Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (Category 5 of the Integrated Report).   Waterbodies affected by such fish kills would be placed in IR subcategories 2b or 3b and could be added to the state’s list of “waters in need of further investigation”.   However, the biological sampling was conducted in the same stream segment after the fishkill occurred and the results of that sampling suggest "full support" of aquatic life uses.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.