Iowa DNR

Water Quality Assessments

Impaired Waters List

Thompson River IA 05-GRA-1351

from the Iowa/Missouri state line to confluence with Long Cr. in SW 1/4, S8, T69N, R26W, Decatur Co.

Assessment Cycle
Release Status
Result Period
2002 - 2004
Class B(WW) Class A
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 05-GRA-0040_0
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of (1) IDNR/UHL monthly water quality monitoring from 2002-04 near Davis City, (2) U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2003, and (3) IDNR/UHL biological (biocriteria) monitoring in 2001.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as “not supported" due to levels of indicator bacteria (E.  coli) that violate state water quality criteria.   The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" based on results of IDNR/UHL biological monitoring in 2001.   Fish consumption uses are assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 2003.   This assessment is based on results of (1) IDNR/UHL monthly ambient monitoring conducted during the 2002-2004 assessment period at Davis City (STORET station 10270001 (formerly station 784089)), (2) IDNR/UHL biocriteria monitoring in 2001, and (3) results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring in 2003.  

EXPLANATION:  The Class A uses were assessed 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 2006 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 the 2006 Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2002 through 2004 (152 orgs/100ml) exceeded the Iowa Class A water quality criterion of 126 orgs/100ml.   In addition, 7 of the 24 samples (29 %) exceeded Iowa’s single-sample maximum value 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 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).   Also, these EPA guidelines state that if more than 10% of the samples exceed the state’s single-sample maximum criterion, the primary contact recreation uses should be assessed as “partially supported”.   According to IDNR’s assessment/listing methodology, these results suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum value, thus suggesting that the Class A uses should be assessed as “partially supported/impaired”.   Thus, both the geometric mean and the percentage of violations of Iowa’s single-sample maximum criterion (33%) suggest impairment of the Class A uses of this river segment.

Results of ambient water quality monitoring at the IDNR/UHL station at Davis City showed no violations of Class B(WW) water quality criteria during the 2002-2004 assessment period for dissolved oxygen (minimum value = 5.7 mg/l), pH (range = 7.3 to 8.5 units), or ammonia-nitrogen (maximum value = 0.70) in the 36 samples analyzed.   Levels of pesticides in the seven samples analyzed were all below the analytical level of detection, and none of the ten samples analyzed for toxic metals exceeded the respective Class B(WW) criteria.   These results suggest "full support" of the Class B(WW) aquatic life uses.  

Results of IDNR/UHL biocriteria monitoring suggest that the Class B(WW) uses are "fully supported".   Data were collected in 2001 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 (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 2001 FIBI score was 32 (fair) and the BMIBI score was 43 (fair).   The aquatic life use support was assessed as fully supported (=FS), 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 33 and the BMIBI BIC for this ecoregion is 41.   This assessment is considered evaluated because the drainage area (607 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 passed both the FIBI and BMIBI BICs, 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.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring northwest of Davis City in 2003.   The composite samples of fillets from channel catfish and flathead catfish had low levels of contaminants.   Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.048 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   Levels of primary contaminants in the composite sample of flathead catfish fillets were as follows: mercury: 0.074 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm.   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.   Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa.   In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol.   This protocol went into effect in January 2006 (see for more information on Iowa’s revised fish consumption advisory protocol).   Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses.   This scenario, however, does not apply to the fish contaminant data generated from the 2003 RAFT sampling conducted in this assessment segment:  the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.

Monitoring and Methods
Assessment Key Dates
12/1/2004 Fixed Monitoring End Date
8/23/2003 Fish Tissue Monitoring
1/7/2002 Fixed Monitoring Start Date
10/17/2001 Biological Monitoring
8/19/1997 Fish Tissue Monitoring
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)
230 Fixed station physical/chemical (conventional plus toxic pollutants)
260 Fish tissue analysis
315 Regional reference site approach
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 Fair
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation High
  • Source Unknown
  • High