Assessment Comments

Assessment is based on IDNR/SHL biological sampling in 2010 and on the results of the statewide assessment of freshwater mussels in Iowa by Arbuckle et al. (2000) and by Iowa DNR in 2008 and 2013.

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:  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 previous IR 5b impairment of Class B(WW1) aquatic life uses due to declines in the freshwater mussel community of this segment is de-listed based on results of new (2013) mussel surveys.    The Class B(WW2) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b) based on results of IDNR/SHL biological monitoring in 2010.   Sources of data for this assessment include (1) information in the report “Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams, (2) results IDNR/SHL biological monitoring in 2010, and (3) results of Iowa DNR mussel surveys in 2013.

EXPLANATION:  The Class A1 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 (evaluated) as "partially supporting" based on the results of IDNR/SHL biological monitoring in 2010.   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 2010 FIBI score was 53 (good) and the BMIBI score was 40 (fair).  The aquatic life use support was assessed as partially supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports.    The non-riffle habitat FIBI BIC for this ecoregion is 44 and the BMIBI BIC for this ecoregion is 52.   This segment passed the FIBI BIC and failed the BMIBI BIC in 2010.   The biological impairment criteria were determined from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.  

This (2010) aquatic life assessment is considered "evaluated" because there were not two or more samples collected from this segment in multiple years.   According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report).   IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).   Thus, this assessment is placed in IR Category 3b for the current IR cycle.


Results of IDNR surveys of freshwater mussels at three sites in this assessment segment in 2013 suggest that, based on IDNR's assessment methodology, the biological impairment based on the apparent decline of freshwater mussels no longer exists.  The following is a summary of these surveys:

--Total number of live freshwater mussel species in this segment from surveys conducted in 1984 (Frest (1987), in 1998 (Arbuckle et al.  2000), and IDNR Watershed Improvement Section staff (J.  Kurth) in 2008 and 2013:

                                                          1984           1998          2008      2013
No.  of live mussel species:                 7                  2               1           5
              

The original impairment was based on a comparison of sampling results from 1998 and 1999 (Arbuckle et al.  2000) to results from stream sites surveyed in 1984 and 1985 by Frest (1987).  Impairment was based on the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey.  According to the IDNR assessment methodology, greater than a 50% decline in species richness from the 1984-85 to the 1998-99 period suggested an impairment of the aquatic life uses.  The degree of decline in the number of freshwater mussel species in this segment between 1984 and 1998 (80%) suggested “partial support” (impairment) of the Class B(WW) aquatic life uses for purposes of Section 303(d) listing.  Results of the IDNR's surveys in 2013, however, show recovery in the number of freshwater mussel species in this assessment segment, with less than a 50% difference in the current (2013) versus historical (1984) mussel species richness (28%).   Due to the lack of a protocol for identifying biological thresholds that indicate "full support" of a freshwater mussel community, this impairment is moved from Category 5b of Iowa's Integrated Report to IR Category 3a (insufficient information to develop an assessment of support of uses).