Iowa DNR

Water Quality Assessments

Impaired Waters List

Walnut Creek IA 03-SSK-953

mouth (S5 T82N R23W Story Co.) to confluence with unnamed tributary in SE 1/4 S34 T83N R24W Story Co.

Assessment Cycle
Release Status
Cycle Period
2012 - 2014
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
9/1/2016 10:59:00 AM
10/4/2016 3:58:23 PM
Assessment conducted in accordance with Iowa's 2016 IR methodology
Use Support
Class A1
Not Assessed
Class BWW2
Partially Supported
Biological: low fish IBI
Support Level
Partially Supported
Impairment Code
5b-t - Biological - tentative
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
TMDL Priority
Tier IV
General Use
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment Summary

The presumptive Class A1 (primary contact recreation) uses remain "not assessed" (IR 3a) due to the lack of information upon which to base an assessment.   The Class B(WW2) aquatic life uses are assessed (evaluated) as "partially supported" (IR 5b-t) based on results of IDNR/SHL biological sampling in 2007 and 2011; this impairment is appropriate for Category 5b of Iowa’s Integrated Report.   In addition, a fish kill occurred in this stream segment in March 2003.   The kill was attributed to natural causes (winterkill).  

Assessment Explanation

The Class B(WW2) aquatic life uses are assessed (evaluated) as “partially supporting” based on data collected in 2007 and 2011 as part of the IDNR/SHL stream biocriteria project.   A series of biological metrics which 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 that were 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 2007 FIBI score was 29 (fair) and the BMIBI score was 77 (excellent).   The 2011 FIBI scores were 36 (fair) and 17 (poor).   The 2011 BMIBI scores were 68 (good) and 48 (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 riffle habitat FIBI BIC for this ecoregion is 53 and the non-riffle habitat FIBI BIC for this ecoregion is 32.   The BMIBI BIC for this ecoregion is 62.   This segment passed the FIBI 0/3 times and passed the BMIBI BIC 2/3 times in the last eight 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 "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 recent five-year period to be considered “monitored”.   This segment had multiple samples collected in the previous eight years (2007-2014); however, the multiple samples were not collected during a recent five-year period.   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).   However, despite this change in assessment methodology and type, this waterbody remains in IR Category 5b-t and remains on Iowa’s Section 303(d) list of impaired waters.  

In addition to the impairment suggested by results of biological monitoring, the occurrence of a fish kill in this stream segment in March 2003 also suggests a potential impairment of the Class B(WW2) aquatic life uses.   This kill occurred on or before March 27, 2003; the kill was attributed to natural causes (winterkill).   An estimated 550 fish were killed; no estimate of the value of the fish killed was provided.   The location point for this kill is Section 1 of T82N, R24W, Story County.   According to the IDNR investigation, fish killed included creek chubs, other minnows, darters, crayfish and frogs; approximately 300 feet of stream was affected by the kill.   Live fish observed approximately 400 feet above the fish kill area.   Most of the fish observed had been dead for a long period of time.   Another potential contributor to the kill may have been stream contamination some type of fuel accelerant used on a burn pile located along the stream.  

According to IDNR’s assessment/listing methodology, 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 IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”  Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5).   Waterbodies affected by such fish kills are placed in IR subcategories 2b or 3b and are added to the state list of waters in need of further investigation.  According to Iowa DNR’s assessment methodology for Integrated Reporting, if no additional kills have been reported for a non-303(d)-impaired stream for over 10 years, any fish kill-related impacts in this segment have likely dissipated, and the aquatic life uses will be considered “not assessed” (IR 3a).  Thus, this assessment is moved to IR Category 3a for the 2016 cycle.

Monitoring and Methods
Assessment Key Dates
7/17/2007 Biological Monitoring
7/19/2011 Biological Monitoring
7/20/2011 Biological Monitoring
3/27/2003 Fish Kill
140 Incidence of spills and/or fish kills
150 Monitoring data more than 5 years old
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
330 Fish surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof