Iowa DNR

Water Quality Assessments

Impaired Waters List

Nishnabotna River IA 05-NSH-1412

IA/MO line to (S26 T67NR42W Fremont Co.) to confluence of E. Nishnabotna and W. Nishnabotna rivers in S2 T67N R42W Fremont Co.

Assessment Cycle
Result Period
2008 - 2010
Class A1 Class B(WW-1) Class HH
Assessment Methodology
Assessment Type
Integrated Report
Category 5p
Legacy ADBCode
IA 05-NSH-0010_0
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on: (1) USGS ambient water quality monitoring conducted on the Nishnabotna River at Hamburg from February 2008 to November 2010, (2) IDNR/UHL biological (REMAP) monitoring in 2003, and (3) EPA/IDNR fish contaminant (RAFT) monitoring in 2002.

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 results of an Use Attainability Analysis, 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 presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to violations of the state criteria for indicator bacteria.   This is a new impairment for this assessment segment.   The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supported” based on results of biological monitoring in 2003.   The fish consumption uses remain assessed (evaluated) as “fully supported” based on results of fish contaminant monitoring in 2002 and based on comparison of USGS ambient water quality data to Iowa’s human health (HH) criteria.   The sources of data for this assessment include the results of (1) USGS ambient water quality monitoring conducted on the Nishnabotna River at Hamburg (station 06810000) from February 2008 to November 2010, (2) IDNR/UHL biological (REMAP) monitoring in 2003, and (3) EPA/IDNR fish contaminant (RAFT) monitoring in 2002.  

EXPLANATION:  The presumptive 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 14 samples collected during the recreational seasons of 2009 through 2010 at the Nishnabotna River above Hamburg were as follows: the 2009 geometric mean was 5,341 orgs/100 ml and the 2010 geometric mean was 2,600 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   All fourteen of the samples (100%) 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.”  

The Class B(WW1) aquatic life uses remain assessed (evaluated) as "partially supported" based on results of IDNR/UHL biological (REMAP) monitoring in 2003.   A series of biological metrics that 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 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 2003 FIBI score was 5 (poor) and the BMIBI score was 54 (fair).   The aquatic life use support was assessed (evaluated) as Partially Supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.   The FIBI BIC for this ecoregion is 31 and the BMIBI BIC for this ecoregion is 54.   This assessment is considered evaluated because the drainage area (2,818 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 FIBI BIC and passed 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).  

In contrast to the results of biological monitoring that suggest only “partial support” of the Class B(WW1) uses, results of water quality monitoring by the U.S.  Geological Survey above Hamburg from 2008-2010 suggest “full support” of these uses.   No violations of state water quality criteria occurred in the 30 samples analyzed for ammonia (maximum = 0.78 mg/L), pH (range = 7.3-8.7), and dissolved oxygen (minimum = 6.6 mg/L) during this period.   No violations of Class B(WW1) criteria occurred in the 30 samples analyzed for the pesticides chlorpyrifos and dieldrin.   These results suggest "full support" of the Class B(WW1) aquatic life uses.  

NOTE:  IDNR began collecting data at this site in October, 2010.   However, there were only 3 samples collected in the 2008-2010 assessment period, which is not enough upon which to base an assessment.

Fish consumption uses remain assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring near Hamburg in 2002.   The composite samples of fillets from common carp had low levels of contaminants.   Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.085 ppm; total PCBs: 0.093 ppm; and technical chlordane: 0.03 ppm.   [Note:  Typically, samples of both bottom-feeding fish (e.g., common carp) and predator species (e.g., largemouth bass) are collected at RAFT status sites such as the Nishnabotna River at Hamburg.   Predator species, however, are naturally rare in rivers of southwestern Iowa, and RAFT status samples from these rivers typically contain only the bottom-feeder sample.]  The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.   The fish contaminant data generated from the 2002 RAFT sampling conducted in this assessment segment show that the levels of contaminants do not exceed any of Iowa’s advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.

One of 35 samples analyzed for dieldrin at USGS station 06810000 during the 2006-2008 period exceeded the Iowa human health-fish (HH-fish) criterion of 0.00054 ppb.   The level of dieldrin in the sample collected on June 11, 2008 was a remarked data value estimated at 0.002 ppb.   According to USGS, their estimated values represent legitimate data points that, while estimated, nonetheless can be used for comparison to water quality criteria, especially given the order of magnitude difference between the USGS estimated value for dieldrin (0.002 ppb) versus Iowa’s human-health (fish) criterion for dieldrin of 0.00054 ppb.   Levels of dieldrin in the remaining 34 samples collected during the 2006-08 period were all reported as less than the level of detection of 0.009 ppb.   For purposes of Section 305(b) assessment, Iowa DNR views the Human Health criterion for dieldrin as analogous to a chronic criterion for a toxic parameter.   Thus, impairment of the HH use by dieldrin would be suggested if more than one of the samples violated this criterion.   Because only one of the 33 samples violated this criterion, this violation does not indicate impairment of the HH-fish use.   Results of the most recent fish contaminant sampling in this river segment conducted in 2002 as part of the U.S.  EPA/IDNR RAFT program showed 11 ppb of dieldrin in the composite sample of common carp fillets.   Although Iowa does not have an advisory trigger level for dieldrin, the level of dieldrin seen in the 2002 sampling is well below the Iowa’s previous advisory trigger of 300 ppb of dieldrin (i.e., the U.S.  FDA action level for dieldrin).   Levels of dieldrin in Iowa fish have declined significantly since the early and mid-1980s when levels of 300 ppb to 500 ppb were not uncommon.

Monitoring and Methods
Assessment Key Dates
11/30/2010 Fixed Monitoring End Date
2/12/2008 Fixed Monitoring Start Date
8/12/2003 Biological Monitoring
9/9/2002 Fish Tissue 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)
320 Benthic macroinvertebrate surveys
260 Fish tissue analysis
Monitoring Levels
Biological 4
Habitat 4
Physical Chemistry 3
Toxic 3
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 Not Impairing
  • Source Unknown
  • Not Impairing
Pathogens Primary Contact Recreation High
  • Source Unknown
  • High