Iowa DNR
ADBNet

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
2008
Release Status
Final
Result Period
2004 - 2006
Designations
Class A1 Class B(WW-1) Class HH
Assessment Type
Monitored
Integrated Report
Category 5a
Legacy ADBCode
IA 06-BSR-0030_0
Overall Use Support
Not supporting
Aquatic Life Use Support
Partial
Fish Consumption
Not assessed
Primary Contact Recreation
Not supporting
Documentation
Assessment Comments

Assessment is based on: (1) results of IDNR/UHL routine monthly ambient monitoring conducted near the mouth of the Rock River during the 2004-2006 assessment period at the County Road B-40 bridge north of Hawarden (STORET station 10840001 (formerly station 975005)), (2) results of an IDNR investigation of a fish kill in August 2002 and (3) results of biological monitoring by IDNR/UHL in 2003 as part of the REMAP project.

Basis for Assessment

[Note:  Prior to the current (2008) Section 305(b) cycle, this river segment was designated only for Class B(WW1) 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 http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and the 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 Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) 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 during the 2004-2006 assessment period at the County Road B-40 bridge north of Hawarden (STORET station 10840001 (formerly station 975005)), (2) results of an IDNR investigation of a fish kill in August 2002 and (3) results of biological monitoring by IDNR/UHL in 2003 as part of the REMAP 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 north of Hawarden during summer recreational seasons of 2004-2006 (499 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Sixteen of the 24 samples (67%) 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).  

Regarding support of the Class B(WW1) aquatic life uses, monitoring at the IDNR/UHL ambient monitoring station near Hawarden showed no violations of Class B(WW1) water quality criteria for pH or ammonia-nitrogen in the 36 samples analyzed during the 2004-2006 assessment period.   Monitoring results for dissolved oxygen, however, showed that one of the 35 samples analyzed (3%) violated the Class B(WW1) water quality criterion for dissolved oxygen:  the level of dissolved oxygen in the sample collected on February 4, 2004 (4.1 mg/l) was less than 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), if 10% or less of samples exceed criteria for conventional parameters (e.g., dissolved oxygen and pH), then the 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(WW1) water quality criteria.  

Results of biological monitoring in 2003 also suggest “full support” of the Class B(WW1) aquatic life uses of this river segment.   This assessment is based on biological monitoring conducted 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 2003 FIBI score was 57 (good) and the 2003 BMIBI score was 71 (good).   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(WW1) 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 was 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
12/5/2006 Fixed Monitoring End Date
1/6/2004 Fixed Monitoring Start Date
8/6/2003 Biological Monitoring
8/8/2002 Fishkill
Methods
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)
140 Incidence of spills and/or fish kills
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 Good
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation High
  • Source Unknown
  • High
Unionized Ammonia Aquatic Life Support Moderate
  • Source Unknown
  • Moderate
Organic enrichment/Low DO Aquatic Life Support Moderate
  • Source Unknown
  • Moderate