Silver Creek IA 01-WPS-363

Assessment Comments

Assessment is based on 1998 & 2004 biocriteria data: FIBI = 40, 44 (fair), BMIBI = 56, 54 (fair). FIBI BIC = 36; BMIBI BIC = 51.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.   EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now presumptively designated for Class A1 (primary contact recreation) uses.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current (2010) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses remain "not assessed" due to the lack of information upon which to base an assessment.   The Class B(WW2) aquatic life uses are assessed as “fully supported” based on results of IDNR/UHL biological (biocriteria) monitoring in 1998 and 2004.

EXPLANATION:  The assessment of the Class B(WW2) aquatic life uses was based on data collected in 1998 and 2004 as part of the IDNR/UHL stream biocriteria project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria 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 1998 FIBI score was 40 (fair) and the BMIBI score was 56 (fair).   The 2004 FIBI score was 44 (fair) and the BMIBI score was 54 (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 1994-2004.   The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51.   This segment passed the FIBI and BMIBI BICs 2/2 times in the last 11 years.

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 between 2004 and 2008 to be considered “monitored”.   This segment had multiple samples collected in the previous 11 years (1998-2008); however, the multiple samples were not collected during 2004-2008.   Despite this change in assessment methodology and type, this waterbody remains in IR Category 2a.