Walnut Creek IA 05-NSH-1450

Assessment Comments

Assessment remains based on 1998 IDNR Fisheries data: FIBI = 28 (fair). FIBI BIC = 31.

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/EXPLANATION:  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 remain assessed (evaluated) as “fully supporting” based on biological data collected in 1998 as part of an IDNR Fisheries stream sampling project: Manchester research station.   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 (FIBI).   The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 1998 evaluated FIBI was 28 (fair).   The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI 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 31 and this site passes the BIC with the use of the uncertainty adjustment value (UAV) of 7 points applied to single sample assessments.  

The sample data from this segment lack the necessary documentation or metadata to determine whether the data meet quality and completeness criteria required of “monitored” assessments.   Thus, FIBI scores calculated from IDNR Fisheries Bureau sampling data collected in this segment provide useful, but limited evidence of impairment.   With respect to FIBI sample results, the following criteria are considered: sampling conditions; sampling gear; crew size; and length of stream sampled.   One or more of these sample descriptors are not available for the Fisheries Bureau data.

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 a single sample collected in 1998.   Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   Despite this change in assessment methodology and type, this waterbody remains in IR Category 2a.