Iowa DNR

Water Quality Assessments

Impaired Waters List

East Nishnabotna River IA 05-NSH-1415

from confluence with Fisher Cr. (S27, T69N, R40W, Fremont Co.) to Page/Montgomery county line

Assessment Cycle
Release Status
Result Period
2004 - 2006
Class A1 Class B(WW-1) Class HH
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 05-NSH-0020_2
Overall Use Support
Not supporting
Aquatic Life Use Support
Not supporting
Fish Consumption
Not assessed
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on: (1) results of IDNR/UHL monthly ambient monitoring conducted during the 2004-2006 assessment period near Shenandoah (STORET station 10360001 (at Hwy 59 bridge; formerly station 821008)) and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project.

Basis for Assessment

[Note:  Prior to the current (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" due to levels of indicator bacteria that violate state water quality criteria.   Although results of ambient water quality monitoring suggest “full support,” the Class B(WW1) aquatic life uses for this assessment segment remain assessed (evaluated) as "not supported" based on results of IDNR/UHL biological monitoring in 2002.   Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on (1) results of IDNR/UHL monthly ambient monitoring conducted during the 2004-2006 assessment period near Shenandoah (STORET station 10360001 (at Hwy 59 bridge; formerly station 821008)) and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project.  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli).   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected at the IDNR/UHL ambient monitoring station at the Hwy 59 bridge at Shenandoah during summer recreational seasons of 2004-2006 (250 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Thirteen of the 24 samples (54%) exceed Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E.  coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

The Class B(WW1) aquatic life uses remain assessed (evaluated) as "not supporting" due to results of biological monitoring in 2002 conducted 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 2002 BM-IBI score was 41 (fair).  The aquatic life use support was assessed (evaluated) as not supporting (=NS), based on a comparison of the BM-IBI score with biological assessment criteria established for previous Section 305(b) reports.  The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.  

The assessment type is considered “evaluated” (of lower confidence) because the size of the segment’s watershed exceeds the range of reference condition watershed sizes used to calibrate the benthic macroinvertebrate and fish bioassessment indexes.   For this assessment, the drainage area for the segment (approximately 950 square miles) exceeds the maximum drainage area cutoff (500 square miles) that IDNR has established for use of BMIBI and FIBI data.   Therefore, IDNR considers the aquatic life use impairments indicated by these data as “evaluated” assessments that are 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).

In contrast to results of biological monitoring, results of chemical/physical water quality monitoring at the IDNR/UHL station near Shenandoah continue to suggest “full support” of the Class B(WW1) uses.   Levels of ammonia-nitrogen and pH were below the respective Class B(WW1) criteria in all 36 samples analyzed during the 2004-2006 period.   One of the 36 samples violated the Class B(WW1) criterion for dissolved oxygen of 5.0 mg/l:  the sample collected on February 3, 2004 contained 4.2 mg/l of dissolved oxygen, thus violating the Class B(WW1) criterion of 5.0 mg/l.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations at this station during the 2004-2006 period for dissolved oxygen (3%) does not suggest a water quality impairment.   These guidelines allow up to 10% violations of conventional parameters such as dissolved oxygen before impairment of water quality is indicated.   In addition, levels of pesticides in the eight samples analyzed, and levels of toxic metals in the ten samples analyzed were all below their respective Class B(WW1) criteria.   These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  

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

Monitoring and Methods
Assessment Key Dates
12/4/2006 Fixed Monitoring End Date
1/5/2004 Fixed Monitoring Start Date
10/17/2002 Biological Monitoring
220 Non-fixed station physical/chemical monitoring (conventional pollutant only)
315 Regional reference site approach
330 Fish surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
320 Benthic macroinvertebrate surveys
Monitoring Levels
Biological 4
Habitat 4
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 1
BioIntegrity Poor
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation Moderate
  • Source Unknown
  • Moderate
Cause Unknown Aquatic Life Support High
  • Source Unknown
  • High