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
2002 - 2004
Class B(WW)
Assessment Type
Integrated Report
Category 3b
Legacy ADBCode
IA 05-NSH-0020_2
Overall Use Support
Not supporting
Aquatic Life Use Support
Not supporting
Fish Consumption
Not assessed
Assessment Comments

Assessment is based on results of (1) IDNR/UHL biological (biocriteria) sampling in 2002 and (2) IDNR/UHL monthly ambient water quality monitoring at Hwy 59 near Shenandoah from 2002-04.

Basis for Assessment

SUMMARY:  Although results of ambient water quality monitoring suggest “full support”, the Class B(WW) aquatic life uses for this assessment segment remain assessed (evaluated) as "not supported" based on results of 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 (plus) ambient monitoring conducted during the 2002-2004 assessment period near Shenandoah (STORET station 10360001 (formerly station 821008)) and (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project.  

EXPLANATION:  The Class B(WW) aquatic life uses were 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 suggest “full support” of the Class B(WW) uses.   Levels of ammonia-nitrogen and pH were below the respective Class B(WW) criteria in all of the 42 samples analyzed during the 2002-2004 period.   One of the 41 samples violated the Class B(WW) 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(WW) 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 percentages of violations at this station during the 2002-2004 period for dissolved oxygen (2%) do 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 12 samples analyzed, and levels of toxic metals in the 11 samples analyzed were all below their respective Class B(WW) criteria.   These results suggest that the Class B(WW) 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
12/6/2004 Fixed Monitoring End Date
10/17/2002 Biological Monitoring
1/7/2002 Fixed Monitoring Start Date
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
230 Fixed station physical/chemical (conventional plus toxic pollutants)
Monitoring Levels
Biological 3
Habitat 0
Physical Chemistry 3
Toxic 3
Pathogen Indicators 0
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
Cause Unknown Aquatic Life Support High
  • Source Unknown
  • High