Dibble Creek IA 01-TRK-203

Assessment Comments

Assessment based on 1998, 2004 and 2013 IDNR/SHL biological sampling results: FIBI = 57 (good), 44 (fair), 57 (good); BMIBI = 76 (excellent), 58 (good), 77 (excellent). FIBI BIC = 52; BMIBI BIC = 61.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general 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 and for Class B(WW1) aquatic life uses.   According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are designated as Class A1 and Class B(WW1) waters.   Thus, for the current (2014) assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]

SUMMARY/EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses remain "not assessed" (IR Category 3a) due to the lack of information upon which to base an assessment.   The presumptive Class B(WW1) aquatic life uses remain assessed (evaluated) as “fully supporting” (IR Category 2a).   This evaluated biological assessment of the presumptive Class B(WW1) uses was based on data collected in 1998, 2004 and 2013 as part of the IDNR/SHL stream biocriteria 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 1998 FIBI score was 57 (good), the 2004 FIBI score was 44 (fair) and the 2013 FIBI score was 57 (good).   The 1998 BMIBI score was 76 (excellent), the 2004 BMIBI score was 58 (good) and the 2013 BMIBI score was 77 (excellent).   This segment passed the FIBI BIC 2/3 times and passed the BMIBI BIC 2/3 times in the last 16 years.   The aquatic life use support was assessed (evaluated) as fully supporting (=FS), 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 1994-2004.   The FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61.   Even though this site passed the FIBI and BMIBI BIC's, 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 aquatic life assessment is now also considered "evaluated" based on a change in the 2010 IDNR assessment methodology.   IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a five-year period to be considered “monitored”.   This segment had multiple samples collected in the previous 16 years (1998-2013); however, the samples were not collected over a five-year period.