Onion Creek IA 03-SSK-6295

The assessment of the presumptive Class B(WW1) aquatic life uses is based on data collected in 2007 and 2008 as part of the DNR/SHL headwater stream sampling 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 scores were 41 (fair) and 51 (good); the 2008 FIBI score was 51(good). The 2007 BMIBI scores were 66 and 69 (both good); the 2008 BMIBI score was 61 (good). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008.The stable riffle habitat FIBI BIC for this ecoregion is 53 and the BMIBI BIC is 62. This segment passed the FIBI BIC 0/3 times and passed the BMIBI BIC 2/3 times in the last 10 years. Even though this site failed the FIBI BIC and passed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it is a small headwater stream and doesn’t fall in the calibrated watershed size.

Additionally, the 2007 and 2008 observed FIBI scores of 41, 51 and 51 are at, or near, the predicted Ecoregion Fish Habitat Index (EFHI) scores and within expectations based on physical habitat characteristics of the segment and ecoregion.

This assessment is considered “evaluated” (of lower confidence) due to the lack of a biological assessment protocol for general use-only streams. DNR’s existing protocol has been calibrated to aquatic life reference conditions in wadeable Class B(WW1) and Class B(WW2) streams. 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).

Prior to the changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this stream was classified only for general uses due to the inability of the stream to support a viable aquatic community at low-flow conditions. DNR’s existing biological assessment protocol, however, was calibrated to aquatic life reference conditions in larger wadeable streams with perennial flow. The aquatic environment of most headwater streams is one of extremes in water quality. Consequently, intermittent headwater streams tend to have biological diversity that is low relative to more stable aquatic environments of larger streams with perennial flow. Thus, the use of biological assessment methods developed for the ecologically more stable and diverse streams will likely overstate the existence of impairment in headwater streams.