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
2002 - 2004
Class B(LR)
Assessment Methodology
Assessment Type
Integrated Report
Category 5b
Legacy ADBCode
IA 03-SSK-0058_0
Overall Use Support
Aquatic Life Use Support
Assessment Comments

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

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses remain assessed (monitored) as "partially supported"  based on results of IDNR/UHL biological (biocriteria) sampling in 1999.   This is the same assessment as that developed for the 2002 and 2004 reporting cycles.   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 2006 Integrated Report.   Note:  In the absence of the impairment of Class B(LR) 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(LR) aquatic life uses remains based on data collected in 1999 as part of the DNR/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 (F-IBI) and a benthic macroinvertebrate index (BM-IBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 1999 Fish IBI score was 42 (fair) and the BM-IBI score was 59 (good).  The aquatic life use support was assessed as partially supporting (=PS), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2002 Section 305(b) report.  The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.

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 (2002-2005) 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
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