South Fork Iowa River IA 02-IOW-748

Assessment Comments

Assessment based on 2003, 2007 & 2012 IDNR/SHL biological sampling data: FIBI = 67 (good), 73, 77 (both excellent), BMIBI = 37 (fair), 69, 73 (both good) (FIBI BIC = 53; BMIBI BIC = 62) and results of a fish kill investigation in August 2009.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008, this segment is also now presumptively designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses remain "not assessed" (IR 3a) due to the lack of information upon which to base an assessment.   The Class B(WW1) remain assessed (evaluated) as "partially supported" (IR 3b) based on results of a fish kill investigation in August 2009.   The kill occurred on or before August 11, 2009.   The kill affected approximately 9,394 fish along a 6.8 mile stretch of the South Fork of the Iowa River southwest of Owasa.   The cause of the kill was unknown, but was suspected to be a natural cause.   Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this assessment segment.

EXPLANATION:  According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired.”  If a cause of the kill was not identified during the IDNR investigation, 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 (IR Category 5).   Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.   Thus, this assessment segment was placed in Category 3b of Iowa’s 2010 Integrated Report and remains in Category 3b for the current report.  

While the fish kill suggests potential impairment of the aquatic life uses, data collected in 2003, 2007 and 2012 as part of the IDNR/SHL stream biological sampling project suggests "full support" of the aquatic life uses.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2003 FIBI score was 73 (excellent) and the BMIBI score was 73 (good).   The 2007 FIBI score was 77 (excellent) and the BMIBI score was 69 (good).   The 2012 FIBI score was 67 (good) and the BMIBI score was 37 (fair).   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 1994-2008.   The FIBI BIC for this ecoregion is 53 and the BMIBI BIC for this ecoregion is 62.   This segment passed the FIBI BIC 3/3 times and passed the BMIBI BIC 2/3 times in the past 10 years (2003-2012).  

This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology.   IDNR now requires a segment have two or more biological samples collected from the segment in multiple years during the recent five-year period to be considered “monitored”.   This segment had multiple samples collected in the previous 10 years (2003-2012); however, the multiple samples were not collected during a five-year period.   Despite this change in assessment methodology and type, this waterbody remains in IR Category 3b based on the 2009 fishkill.