Prairie Creek IA 04-UDM-1796

The Class B(WW2) aquatic life uses remain assessed (evaluated) as "partially supported" due to a fish kill in August 2006. The kill occurred on our before August 21, 2006 one mile upstream of Dolliver State Park and was attributed to natural causes (low dissolved oxygen). The partial fish kill of about 200 to 300 minnows, chubs and shiners occurred in a meandered section of the stream through a wooded area. It is speculated that day time oxygen levels in this section of stream were not sufficient to carry through the night time hours and the weaker fish succumbed. Live fish were observed in the same area as dead fish. Live fish were observed both above and below the fish kill segment. Manure or fertilizer releases are not suspected as a reason for the fish kill because ammonia nitrogen levels were not elevated in the stream. No estimate of the value of the fish was made.

The continuance of the IR Category 3b listing is also based on DNR's assessment methodology that states the following: the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the DNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated” and will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation. Thus, this assessment segment remains in Category 3b of Iowa’s Integrated Report.

Data collected in 2011 and 2015 as part of the DNR/SHL stream biocriteria project also suggest "partial support" of the Class B(WW2) uses. The 2015 FIBI score was 34 (good) and the BMIBI score was 61 (good). The 2011 FIBI score was 57 (good) and the BMIBI score was 49 (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 biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The riffle habitat FIBI BIC for this ecoregion is 53 and the BMIBI BIC for this ecoregion is 62. This segment passed the FIBI BIC 1/2 times and passed the BMIBI BIC 0/2 times in the last six 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 five-year period to be considered “monitored”. This segment had multiple samples collected in the previous six years (2011-2016). 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).