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
Result Period
2006 - 2008
Class A1 Class B(WW-2)
Assessment Methodology
Assessment Type
Integrated Report
Category 5b-t
Legacy ADBCode
IA 03-SSK-0058_0
Overall Use Support
Aquatic Life Use Support
Primary Contact Recreation
Not assessed
Assessment Comments

Assessment is based on results of (1) IDNR/UHL biological (biocriteria) monitoring in 1999 & 2007 and (2) an IDNR investigation of a fish kill in March 2003.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008 (see, this segment is now presumptively designated for Class A1 (primary contact recreation) uses.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current (2010) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are "not assessed" 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"  based on results of IDNR/UHL biological (biocriteria) sampling in 1999 and 2007.   In addition, a fish kill occurred in this stream segment in March 2003.   The kill was attributed to natural causes (winterkill).   This impairment is appropriate for Category 3b of Iowa’s 2010 Integrated Report.   Note:  In the absence of the impairment of Class B(WW2) uses based on results of biological monitoring in 1999 (IR Category 5b (=303(d) list)), this waterbody would be placed into Category 3b (at least one other use "evaluated" as "impaired") due to the occurrence of the fish kill in March 2003.

EXPLANATION:  The impairment of the Class B(WW2) aquatic life uses was based on data collected in 1999 and 2007 as part of the IDNR/UHL 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 1999 FIBI score was 39 (fair) and the BMIBI score was 58 (good).   The 2007 FIBI score was 29 (fair) and the BMIBI score was 78 (excellent).    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 BMIBI BIC for this ecoregion is 62.   This segment passed the FIBI 0/2 times and passed the BMIBI BIC 1/2 times in the last 10 years.   The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.

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 between 2004 and 2008 to be considered “monitored”.   This segment had multiple samples collected in the previous 10 years (1999-2008); however, the multiple samples were not collected during 2004-2008.   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 and remains on Iowa’s 2010 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(LR) 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 will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.

Monitoring and Methods
Assessment Key Dates
7/17/2007 Biological Monitoring
3/27/2003 Fishkill
7/15/1999 Biological Monitoring
7/15/1999 One-time Chemical Monitoring
320 Benthic macroinvertebrate surveys
330 Fish surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof
220 Non-fixed station physical/chemical monitoring (conventional pollutant only)
315 Regional reference site approach
140 Incidence of spills and/or fish kills
Monitoring Levels
Biological 4
Habitat 4
Physical Chemistry 1
Toxic 0
Pathogen Indicators 0
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 1
BioIntegrity Fair
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Cause Unknown Aquatic Life Support Moderate
  • Source Unknown
  • Moderate