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
1998 - 2000
Assessment Type
Integrated Report
Category 0
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 IDNR monthly ambient water quality monitoring near the mouth of the Rock River in 2000 & 2001.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed as "fully supported / threatened."  Fish consumption uses remain "not assessed."  This assessment is based on results of IDNR monthly ambient monitoring conducted near the mouth of the Rock River during the 2000-2001 biennial period at the County Road B-40 bridge north of Hawarden (station 10840001 (formerly station 975005)).   EXPLANATION:  Monitoring at the IDNR station near Hawarden showed no violations of Class B(WW) water quality criteria for dissolved oxygen (minimum value = 5.2 mg/l) or pH (range = 7.5 to 8.4) in the 24 samples analyzed during the 2000-2001 biennial period.   In addition, levels of toxic metals in the four samples analyzed, and levels of pesticides in the six samples analyzed, were all below the analytical level of detection.   One of the 24 samples collected, however, violated the Class B(WW) criterion for ammonia-nitrogen.   The sample collected on April 3, 2001 contained 0.70 mg/l of ammonia-nitrogen; this level slightly exceeded the Class B(WW) temperature/pH-dependent chronic criterion of 0.69 mg/l.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-18), however, one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) set does not indicate an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, this violation suggests that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened."  Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.

Monitoring and Methods
Assessment Key Dates
9/5/2001 Fixed Monitoring End Date
10/5/1999 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
Monitoring Levels
Biological 0
Habitat 0
Physical Chemistry 3
Toxic 0
Pathogen Indicators 0
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 0
BioIntegrity N/A
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Unionized Ammonia Overall Use Support Not Impairing
  • Source Unknown
  • Not Impairing
Unionized Ammonia Aquatic Life Support Not Impairing
  • Source Unknown
  • Not Impairing