Iowa DNR
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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
2014
Release Status
Final
Result Period
2010 - 2012
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 based on: (1) results of IDNR/SHL routine monthly ambient monitoring conducted near the mouth of the Rock River during the 2010-2012 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 sampling by IDNR/SHL in 2003, 2011 and 2012.

Basis for Assessment

[Note:  Prior to the 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" (IR 5a) due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (monitored) as “partially supported” (IR 5b) based on occurrence of a pollutant-caused fish kill in August 2002 and (evaluated) as “partially supported” (IR 3b-u) based on results of biological sampling by IDNR/SHL in 2003, 2011 and 2012.   Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment.   The sources of data for this assessment include (1) results of IDNR/SHL routine monthly ambient monitoring conducted near the mouth of the Rock River during the 2008-2010 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 sampling by IDNR/SHL in 2003, 2011 and 2012.

EXPLANATION:  The 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 24 samples collected during the recreational seasons of 2010 through 2012 at the Rock River near Hawarden were as follows:  the 2010 geometric mean was 803 orgs/100 ml, the 2011 geometric mean was 136 orgs/100 ml, and the 2012 geometric mean was 201 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Eleven of the 24 samples (46%) 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.”  

Regarding support of the Class B(WW1) aquatic life uses, monitoring at the IDNR/SHL ambient monitoring station near Hawarden showed no violations of Class B(WW1) water quality criteria for dissolved oxygen (minimum = 6.7 mg/L), pH (range = 7.8 to 8.5) or ammonia-nitrogen (maximum = 0.95 mg/L) in the 36 samples analyzed during the 2010-2012 assessment period.  

Results of biological monitoring in 2003.  2011 and 2012 suggest “partial support” of the Class B(WW1) aquatic life uses of this river segment.   This assessment is based on biological sampling conducted in 2003, 2011 and 2012 as part of the IDNR/SHL stream REMAP and large river projects.   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).   The 2011 BMIBI score was 34 (fair) and the 2012 BMIBI score was 44 (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 1994-2008.   The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54.   This segment passed the FIBI BIC 1/1 times in 2003 and passed the BMIBI BIC 0/2 times in the last five years.   This assessment is considered evaluated because the drainage areas (1591 and 1680 mi2) above the sampling sites were greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site passed/failed 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).

Despite results of ambient water quality 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 (evaluated) 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 indicates an impairment of the aquatic life uses.   Because the data upon which these assessment are based are now more than five years old, the assessment type is changed from “monitored” (a higher confidence assessment) to “evaluated” (a lower confidence assessment).   As water quality data age, they are less able to represent current water quality conditions.   Despite the age of the data and the change from a “monitored” to an “evaluated” assessment type, any impairments previously identified for this assessment segment remain on Iowa’s section 303(d) list.  

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

Monitoring and Methods
Assessment Key Dates
12/11/2012 Fixed Monitoring End Date
10/11/2012 Biological Monitoring
9/7/2011 Biological Monitoring
1/11/2010 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
150 Monitoring data more than 5 years old
Monitoring Levels
Biological 4
Habitat 4
Physical Chemistry 3
Toxic 1
Pathogen Indicators 3
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
Unionized Ammonia Aquatic Life Support Moderate
  • Source Unknown
  • Moderate
Organic enrichment/Low DO Aquatic Life Support Moderate
  • Source Unknown
  • Moderate
Pathogens Primary Contact Recreation Moderate
  • Source Unknown
  • High
Cause Unknown Aquatic Life Support Not Impairing
  • Source Unknown
  • Not Impairing