Walnut Creek IA 03-SSK-953

The Class B(WW2) aquatic life uses remain assessed (evaluated) as “partially supporting” based on data collected in 2007 and 2011 as part of the DNR/SHL 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 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 2007 FIBI score was 29 (fair) and the BMIBI score was 77 (excellent). The 2011 FIBI scores were 36 (fair) and 17 (poor). The 2011 BMIBI scores were 68 (good) and 48 (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 riffle habitat FIBI BIC for this ecoregion is 53 and the non-riffle habitat FIBI BIC for this ecoregion is 32. The BMIBI BIC for this ecoregion is 62. This segment passed the FIBI 0/3 times and passed the BMIBI BIC 2/3 times in the last 10 years. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008.

This aquatic life assessment is now considered "evaluated" based on a change in the 2010 DNR assessment methodology. DNR now requires a segment have two or more biological samples collected from the segment in multiple years over a recent five-year period to be considered “monitored”. This segment had multiple samples collected in the previous 10 years (2007-2016); however, the multiple samples were not collected during a recent five-year period. According to DNR’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). DNR 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-t and remains on Iowa’s Section 303(d) list of impaired waters.