Walnut Creek IA 05-NSH-1449

The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supporting” based on biological data collected in 1996 and 1998 by DNR/SHL and DNR Fisheries. 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 1996 FIBI scores were 20 and 23 (both poor). The 1996 BMIBI scores were 46, 52 and 54 (all fair). The 1998 FIBI was 29 (fair). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), 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 1994-2008. The FIBI BIC for this ecoregion is 31 and the BMIBI BIC is 54. This segment passed the FIBI BIC 0/3 times and passed the BMIBI BIC 1/3 times in the last 21 years.

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 19 years; 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). 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).

In addition, the data upon which this biological assessment is based are now well over ten years old. As water quality data age, they are less able to represent current water quality conditions. As data age beyond ten years, their ability to represent current water quality conditions is increasingly suspect. Despite the age of these data, the aquatic life uses of this assessment segment will remain “partially supporting/evaluated” and will remain on Iowa’s list of waters in need of further investigation (IR Category 3b-c). Additional monitoring is needed in this assessment segment to update status of its aquatic communities.