Mill Creek IA 06-LSR-1615

The fish kill occurred on or before August 17, 2013 and was attributed to runoff from a cattle facility on a unnamed tributary of West Branch Mill Creek. Approximately 865,940 dead fish were found up to 27.9 miles downstream of the tributary. The estimated value of the dead fish was $158,551.63 and the investigation expenses were $1,945.73. The party responsible for the kill was identified and restitution was sought and received.

According to DNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill during an assessment period indicates "partial support" of the aquatic life uses. If, however, a consent order has been issued to the party responsible for the kill and monetary restitution has been sought for the fish killed, the affected waterbody should be placed in IR Category 4d (impaired but TMDL not required). Thus, based on the 2013 fish kill and due to the receipt of restitution of the value of the fish killed and the cost of the DNR fish kill investigation, the aquatic life uses of this stream will are assessed as impaired (partially supporting) and placed in Category 4d of Iowa’s Integrated Report.

The Class B(WW2) aquatic life uses are assessed (evaluated) as “partially supporting” based on data collected in 2012 and 2016 as part of the DNR/SHL stream REMAP and biocriteria projects. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological 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 2016 BMIBI score was 45 (fair). The 2012 FIBI score was 44 (fair) and the BMIBI score was 44 (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 FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54. This segment passed the FIBI BIC 1/1 times and passed the BMIBI BIC 0/2 times in the last five 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 "monitored" 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 (BMIBI) samples collected in the previous five years (2012-2016).