Assessment Comments

Assessment is based on results of (1) biological monitoring conducted in 2000 by IDNR Fisheries: Fish IBI = 50 (fair), (2) results of IDNR/UHL monthly ambient water quality monitoring conducted from 2000-02, (3) IDNR/UHL ambient monitoring in support of TMDL development, and (4) an IDNR investigation of a fish kill in August 2002.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (monitored) as “partially supported” based on occurrence of a fish kill in August 2002.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.   The sources of data for this assessment include (1) results of IDNR/UHL routine monthly ambient monitoring conducted near the mouth of the Rock River near Hawarden during the 2000-2002 assessment period at the County Road B-40 bridge north of Hawarden (STORET station 10840001 (formerly station 975005)), (2) results of ambient chemical/physical water quality monitoring conducted near Rock Valley by IDNR/UHL from March through November 2002 as part of TMDL monitoring (TMDL station 7; STORET station 11840002), (3) results of an IDNR investigation of a fish kill in August 2002, and (4) results of biological monitoring by IDNR Fisheries in 2000.

EXPLANATION:  Monitoring at the IDNR ambient monitoring station near Hawarden showed no violations of Class B(WW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 36 samples analyzed during the 2000-2002 assessment period.   Levels of pesticides in the nine samples analyzed were all below the analytical level of detection.   In addition, results of chemical/physical water quality monitoring conducted near Rock Valley from March through November 2002 by IDNR/UHL in support of TMDL development also suggest good water quality in this stream.   None of the 10 samples collected had levels of dissolved oxygen, pH, or ammonia-nitrogen that violated state water quality standards (these samples were not analyzed for toxic metals or pesticides).  

Similarly, results of biological monitoring in 2000 conducted as part of a DNR Fisheries stream sampling project (Manchester research station) also suggest "full support" of the Class B(WW) aquatic life uses.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the Fisheries sampling data.   The biological metrics are based on the numbers and types of fish species that were collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (F-IBI).   The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2000 evaluated Fish IBI = 50 (fair).   The aquatic life use support was assessed (evaluated) as Fully Supporting (=FS), based on a comparison of the F-IBI score with biological assessment criteria established for previous Section 305(b) reports.   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.

Despite results of ambient water quality and biological monitoring that suggest full support of the Class B(WW) uses, the occurrence of a fish kill in this river segment in August 2002 suggests that these uses are only “partially supported.”  A fish kill occurred on this stream on August 8, 2002.   The kill followed heavy rainfall; no specific cause was determined, but runoff from feedlots is suspected as the cause and source of this kill.   Approximately 1.75 miles of river was affected, and an estimated 930 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 an impairment of the aquatic life uses.  

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