Iowa DNR

Water Quality Assessments

Impaired Waters List

Rock River IA 06-BSR-1534

mouth (S1, T95N, R48W, Sioux Co.) to confluence with Little Rock R. in S35, T98N, R46W, Lyon Co.

Assessment Cycle
Release Status
Result Period
2002 - 2004
Class B(WW)
Assessment Type
Integrated Report
Category 5b
Legacy ADBCode
IA 06-BSR-0030_0
Overall Use Support
Aquatic Life Use Support
Fish Consumption
Not assessed
Assessment Comments

Assessment is based on results of (1) biological monitoring conducted in 2000 by IDNR Fisheries, (2) IDNR/UHL biological (REMAP) monitoring in 2003, (3) IDNR/UHL monthly ambient water quality monitoring conducted from 2002-04, (4) IDNR/UHL ambient monitoring in support of TMDL development in 2002-03, and (5) an IDNR investigation of a fish kill in August 2002.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses remain assessed (monitored) as “partially supported” based on occurrence of a pollutant-caused fish kill in August 2002.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.   The sources of data for this assessment include (1) results of IDNR/UHL routine monthly ambient monitoring conducted near the mouth of the Rock River near Hawarden during the 2002-2004 assessment period at the County Road B-40 bridge north of Hawarden (STORET station 10840001 (formerly station 975005)), (2) results of ambient chemical/physical water quality monitoring conducted near Rock Valley by IDNR/UHL conducted during the 2002-04 assessment period at TMDL station 7 (STORET station 11840002), (3) results of an IDNR investigation of a fish kill in August 2002, (4) results of biological monitoring by IDNR Fisheries in 2000 and (5) results of biological monitoring by IDNR/UHL in 2003 as part of the REMAP project.

EXPLANATION:  Monitoring at the IDNR/UHL ambient monitoring station near Hawarden showed no violations of Class B(WW) water quality criteria for pH or ammonia-nitrogen in the 36 samples analyzed during the 2002-2004 assessment period.   Monitoring results for dissolved oxygen, however, showed that one of the 39 samples analyzed (3%) violated the Class B(LR) water quality criterion for dissolved oxygen: the level of dissolved oxygen in the sample (4.1 mg/l) was less than 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), if 10% or less of samples exceed criteria for conventional parameters (e.g., dissolved oxygen and pH), aquatic life uses should be assessed as “fully supported”.   Levels of pesticides in the seven samples analyzed, and levels of toxic metals in the 10 samples analyzed, were all below their respective Class B(WW) water quality criteria.  

In addition, results of IDNR/UHL chemical/physical water quality monitoring conducted at TMDL Station 7 near Rock Valley from March 2002 through December 2003 suggest good water quality in this stream.   None of the 21 samples collected had levels of dissolved oxygen, pH, or ammonia-nitrogen that violated their respective Class B(WW) quality criteria (these samples were not analyzed for toxic metals, or pesticides).  

Results of biological monitoring in 2000 and 2003 also suggest “full support” of the Class B(WW) aquatic life uses of this river segment.   This assessment is based on biological monitoring conducted (1) in 2000 by the IDNR Fisheries Bureau and (2) in 2003 as part of the DNR/UHL stream REMAP project.   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 2000 IDNR Fisheries FIBI score was 50 (fair).   The 2003 FIBI score was 57 (good) and the 2003 BMIBI score was 71 (good).   The FIBI average was 53.5.   The aquatic life use support was assessed (evaluated) as Fully Supporting (= 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 43 and the BMIBI BIC for this ecoregion is 54.   This assessment is considered evaluated because the drainage area (1592 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.

Despite results of ambient water quality and biological monitoring that suggest “full support” of the Class B(WW) uses, the occurrence of a fish kill in this river segment in August 2002 suggests that these uses should remain assessed as only “partially supported.”  A fish kill occurred on this stream on August 8, 2002.   The kill followed heavy rainfall; no specific cause was determined, but runoff from feedlots is suspected as the cause and source of this kill.   Approximately 1.75 miles of river was affected, and an estimated 930 fish were killed.   According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill within the most recent three-year period indicates an impairment of the aquatic life uses.  

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

Monitoring and Methods
Assessment Key Dates
12/7/2004 Fixed Monitoring End Date
8/6/2003 Biological Monitoring
8/8/2002 Fishkill
1/8/2002 Fixed Monitoring Start Date
1/1/2000 Biological Monitoring
230 Fixed station physical/chemical (conventional plus toxic pollutants)
315 Regional reference site approach
330 Fish surveys
140 Incidence of spills and/or fish kills
240 Non-fixed station physical/chemical (conventional + toxicants)
220 Non-fixed station physical/chemical monitoring (conventional pollutant only)
320 Benthic macroinvertebrate surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof
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 2
BioIntegrity Fair
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Organic enrichment/Low DO Aquatic Life Support Moderate
  • Source Unknown
  • Moderate
Unionized Ammonia Aquatic Life Support Moderate
  • Source Unknown
  • Moderate