Iowa DNR

Water Quality Assessments

Impaired Waters List

South Skunk River IA 03-SSK-934

from the Ames Water Works dam in River Valley Park at Ames (S36 T84N R24W Story Co.) to the Co. Rd. at north line of S6 T85 R23W Story Co (approximately 1 mile NNE of Story City)

Assessment Cycle
Result Period
2002 - 2004
Class B(WW) HQR Class A
Assessment Methodology
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 03-SSK-0030_2
Overall Use Support
Aquatic Life Use Support
Fish Consumption
Primary Contact Recreation
Assessment Comments

Assessment is based on results of (1) IDNR/UHL city monitoring upstream from Ames from 2002-04, (2) IDNR/UHL biological monitoring in 2000 and 2003, and (3) U.S.EPA/IDNR fish contaminant monitoring in 2004. Biological monitoring results: 2000 FIBI = 40 (fair) 2003 FIBI = 54 (good), 44 (fair); 2003 BMIBI = 76 (excellent), 65 (good). FIBI BIC = 32; BMIBI BIC = 62.

Basis for Assessment

SUMMARY:  The Class A uses are assessed (monitored) as “partially supported” due to high levels of indicator bacteria that violate state water quality criteria.   The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported " based on results of (1) biological monitoring in 2000 and 2003 by IDNR/UHL and (2) ambient chemical/physical monitoring by IDNR/UHL from 2002-2004.   Fish consumption uses are assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 2004.   The sources of data for this assessment include (1) the results of monthly monitoring from 2002 through 2004 at the IDNR/UHL ambient city monitoring station located upstream from Ames at the Sleepy Hollow canoe access (station 10850003), (2) results of IDNR/UHL biological monitoring in 2000 and 2003; and (3) results of U.S.  EPA/IDNR fish contaminant (RAFT) monitoring in 2004.  

EXPLANATION:  The Class A uses are assessed as "partially supported" based on results of monitoring for indicator bacteria (E.  coli) during recreational seasons of 2002 through 2004.   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 (128 orgs/100ml) is approximately equal to the Iowa Class A water quality criterion of 126 orgs/100ml.   Nine of the 24 samples (38%), however, exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting, if levels of E.  coli exceed the state’s single-sample maximum criterion in more than 10% of the samples, 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”.   This assessment is consistent with the previous (2004) assessment based on results of fecal coliform monitoring at this station during recreational seasons of 2000 through 2002 (see assessment developed for the 2004 reporting cycle for more information).

The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" based on results of both chemical/physical and biological monitoring.   Results of chemical/physical water quality monitoring during the 2002-04 period at the IDNR ambient station upstream from Ames showed no violations of Class B(WW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the approximately 32 samples collected, for toxic metals in the 32 samples collected, or for pesticides and other toxic organic compounds in the approximately 10 samples analyzed.   These results suggest “full support” of the Class B(WW) aquatic life uses.    

Results of biological monitoring also suggest “full support” of the aquatic life uses of this river segment.   This assessment was partially based on results of IDNR/UHL biological monitoring in 2000 and 2003.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological sampling data.   The biological metrics are based on the numbers and types of fish species and benthic macroinvertebrate taxa 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 community index of biotic integrity (BMIBI).   The indices rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The DNR/UHL FIBI average (n=3) was 46 (fair) and the BMIBI average (n=2) was 70.5 (good).   The aquatic life use support was assessed as Fully Supporting (= FS), based on a comparison of the F-IBI score with biological assessment criteria established for previous Section 305(b) reports.   The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.   The FIBI BIC for this ecoregion (non-riffle sites) is 32 and the BMIBI BIC for this ecoregion is 62.

Fish consumption remain assessed (monitored) as "fully supported" based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring conducted in 2004 near Story City.   The composite samples of fillets from common carp and smallmouth bass had low levels of contaminants.   Levels of primary contaminants in the composite sample of common carp fillets were as follows: mercury: 0.069 ppm; total PCBs: 0.09 ppm; and technical chlordane: 0.031 ppm.   Levels of primary contaminants in the composite sample of smallmouth bass fillets were as follows:  mercury: 0.109 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 2004 RAFT sampling conducted in this assessment segment:  the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.

Monitoring and Methods
Assessment Key Dates
12/8/2004 Fixed Monitoring End Date
9/16/2004 Fish Tissue Monitoring
9/18/2003 Biological Monitoring
9/16/2003 Biological Monitoring
1/16/2002 Fixed Monitoring Start Date
9/14/2000 Biological Monitoring
330 Fish surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof
430 Sediment analysis
230 Fixed station physical/chemical (conventional plus toxic pollutants)
242 Non-fixed station physical/chemical (conv + toxicants during key seasons and flows)
260 Fish tissue analysis
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
Monitoring Levels
Biological 4
Habitat 4
Physical Chemistry 4
Toxic 0
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 3
BioIntegrity Good
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation Moderate
  • Source Unknown
  • Moderate