Assessment Comments

Assessment based on IDNR/SHL biological monitoring in 2010 and on the results of the statewide assessment of freshwater mussels in Iowa (Arbuckle et al. 2000). Potential causes & sources of impairment were applied to all sites in study.

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 (2012) 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 remain assessed (evaluated) as "not supported" based on information from the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by Arbuckle et al.  (2000).   As part of this study, sampling results from 1998 and 1999 (Arbuckle et al.   2000) were compared to results from stream sites surveyed in 1984 and 1985 by Frest (1987).   On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s.   Sources of data for this assessment include (1) results of the report “Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams and (2) results of monitoring at one site (Buffalo Creek 4; STORET station 15100004) conducted as part of a Clean Water Act Section 319 water quality project from June through December 2008.


EXPLANATION:  For purposes of Section 303(d) listing, the assessment of the Class B(WW2) aquatic life uses remains based on the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey.   For stream segments having four or more species reported for the 1984-95 survey, results of this comparison were used by staff of the Iowa DNR TMDL & Water Quality Assessment Section to assess the degree to which the aquatic life uses of the sampled stream segments are supported.   Greater than a 50% decline in species richness from the 1984-85 to the 1998-99 period suggests an impairment of the aquatic life uses.   Species richness of freshwater mussels at the three sample sites in this segment of Buffalo Creek  was 4, 6, and 5, respectively, in the 1984-85 period (average of approximately 7 species) and was 0, 2, and 0, respectively, in the 1998-99 period (average of less than one  species) for an average percent change of minus 89%.   Based on these results, the aquatic life uses are assessed (monitored) as “not supported.”  As presented by Arbuckle et al.   (2000), the potential causes of declines in species richness of Iowa's freshwater mussels include siltation, destabilization of stream substrate, stream flow instability, and high instream levels of nutrients (phosphorus and nitrogen).   Their study also suggested the importance of stream shading provided by riparian vegetation to mussel species richness.   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 type, this waterbody remains in IR Category 5b.

In addition to the mussel study, the aquatic life uses were evaluated using 2010 IDNR/SHL biological monitoring data.   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 1994-2004.  

This 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).   However, despite this change in assessment methodology and type, this waterbody remains in IR Category 5b and remains on Iowa’s 2012 Section 303(d) list of impaired waters.