Trinkle Creek IA 06-BOY-6351

This evaluated biological assessment of the presumptive Class B(WW1) aquatic life uses was based on data collected in 2013 and 2014 as part of the IDNR/SHL headwater stream sampling project.    A series of biological metrics that 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 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 2013 FIBI score was 18 (poor) and the BMIBI score was 58 (good).    The 2014 FIBI scores were 20 (poor), 34 (fair) and the BMIBI scores were 54, 55 (both fair) and 58, 64 (both 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 FIBI BIC for this ecoregion is 31 and the BMIBI BIC for this ecoregion is 54.    Even though this site failed the FIBI BIC (1/3) and passed the BMIBI BIC (5/5), 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.

This biological assessment is considered “evaluated” (of lower confidence) due to the lack of a biological assessment protocol for headwater streams.   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.   IDNR’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.   For these reasons, headwater stream reaches that show a failure to meet regional expectations for aquatic biota (fish or aquatic macroinvertebrates) will not be added to Iowa’s Section 303(d) list of impaired waters (Category 5 of the Integrated Report).   The assessment type for these assessed waters will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to “monitored" (indicating an assessment with relatively high confidence).   Such waters are typically placed in either Category 2b or 3b of the IR and are added to the state’s list of “waters in need of further investigation”.