Walnut Creek IA 04-LDM-1069

This evaluated biological assessment was based on data collected in 2004 and 2005 as part of the IGSB/SHL 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 fish species collected in the stream sampling reach.   The biological metrics were combined to make a fish community index of biotic integrity (FIBI).   The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2004 FIBI score was 22 (poor) and the 2005 FIBI score was 29 (fair).   The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI score 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 36.   This segment passed the FIBI BIC 0/2 times in the last 11 years.   Even though this site failed to meet the FIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it is a small general use stream and doesn’t fall in the calibrated watershed size.  

According to IDNR’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).   IDNR 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.   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.

Results of chemical/physical water quality monitoring conducted by USGS near Vandalia from May to September 2013 do not suggest water quality problems in this assessment segment.  No violations of Class B(WW1) water quality criteria occurred in the 15 samples analyzed from May to September 2013 for ammonia, dissolved oxygen, pH, and temperature or in the 12 samples analyzed for chloride and sulfate.