Iowa DNR

Water Quality Assessments

Impaired Waters List

West Nishnabotna River IA 05-NSH-1441

from confluence with Silver Cr. (S21, T71N, R41W, Mills Co.) to confluence Farm Cr. in S9, T73N, R40W, Mills Co.

Assessment Cycle
Release Status
Result Period
2010 - 2012
Class A1 Class B(WW-1) Class HH
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 05-NSH-0080_1
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Not assessed
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of IDNR/SHL monthly ambient water quality monitoring conducted during the 2010-2012 assessment period east of Malvern (STORET station 10650001) and on 2012 IDNR/SHL biological sampling conducted near Malvern.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008 (see, and due to the completion of a Use Attainability Analysis in 2007, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5a) due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" based on results of 2012 IDNR/SHl biological sampling data.   Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on results of IDNR/UHL monthly ambient water quality monitoring conducted during the 2010-2012 assessment period east of Malvern (STORET station 10650001 (formerly station 822310)) and on 2012 IDNR/SHL biological sampling conducted near Malvern.   Water quality conditions in this segment appear to have improved during the 2011-12 period.   Levels of indicator bacteria dropped from the thousands (from 915 to 1,218 orgs/100 ml) during the 2008-10 period to the low hundreds (196 and 169 orgs/100 ml) during 2011 and 2012.   Also, the maximum ammonia concentration dropped from 2.6 mg/l during the 2008-10 period to 0.17 during the 2010-12 period.

EXPLANATION:  The Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria.   The geometric means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at the West Nishnabotna River near Malvern were as follows:  the 2010 geometric mean was 1,218 orgs/100 ml, the 2011 geometric mean was 196 orgs/100 ml, and the 2012 geometric mean was 169 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Thirteen of the 24 samples (54%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”  

Based on just the water quality sampling data, the Class B(WW1) aquatic life uses should be assessed (monitored) as “fully supporting.”  Results of routine ambient monitoring at the IDNR/SHL station near Malvern showed no violations of Class B(WW1) water quality criteria during the 2010-2012 assessment period for dissolved oxygen (minimum = 7.3 mg/L), pH (range from 7.9 to 8.8 units),temperature (maximum = 31.4C), chloride, sulfate, and ammonia-nitrogen (maximum = 0.17 mg/L) in the 36 samples analyzed.   These results suggest "full support" of the Class B(WW1) aquatic life uses.  

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting".   This evaluated biological assessment was based on data collected in 2012 as part of the IDNR/SHL stream large river sampling project.   A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach.   The biological metrics were combined a benthic macroinvertebrate index (BMIBI).   The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2012 BMIBI score was 39 (fair).  The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI score with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.   The BMIBI BIC for this ecoregion is 54.   This assessment is considered evaluated because the drainage area (1092 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it 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).    

Fish consumption uses remain "not assessed" due to the lack of fish tissue monitoring in this river segment.

Monitoring and Methods
Assessment Key Dates
12/3/2012 Fixed Monitoring End Date
9/4/2012 Biological Monitoring
1/4/2010 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
Monitoring Levels
Biological 3
Habitat 2
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
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
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Slight
Cause Unknown Aquatic Life Support Not Impairing
  • Source Unknown
  • Not Impairing