Cedar Creek IA 04-RAC-1156

Assessment Comments

Assessment remains based on 2002 IDNR/SHL biological sampling data: FIBI = 43 (fair), BMIBI = 64 (good) and on 1998 IDNR Fisheries data: FIBI = 42 (fair). Non-riffle habitat FIBI BIC = 32, BMIBI BIC = 62.

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, 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 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" (IR 3a) 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” (IR 2a) based on biological data collected in 1998 and 2002 as part of the IDNR/SHL REMAP and IDNR Fisheries sampling projects.   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 that were 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 Fisheries FIBI score was 42 (fair).   The 2002 FIBI score was 43 (fair) and the BMIBI score was 64 (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 1994-2008.   The non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC for this ecoregion is 62.   This segment passed the FIBI BIC 2/2 times in the last 15 years and passed the BMIBI BIC 1/1 times in 2002.  

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 over a five-year period to be considered “monitored”.   This segment had multiple FIBI samples collected in the previous 15 years (1998-2012); however, the multiple samples were not collected during a recent five-year period.   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.  

Threats to the continued support of the Class B(WW2) uses include over-pasturing of the riparian corridor and channelization in headwater reaches.   Nutrient impacts in this upper (Class B(WW2)) reach seem to be less obvious than in the lower (Class B(WW1)) reach of Cedar Creek.