Iowa DNR

Water Quality Assessments

Impaired Waters List

Shell Rock River IA 02-SHL-783

from south corporate limit of Shell Rock (S12 T91N R15W Butler Co.) to confluence with Flood Cr. in S27 T93N R16W Butler Co.

Assessment Cycle
Result Period
2010 - 2012
Class A1 Class B(WW-1) Class HH
Assessment Methodology
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 02-SHL-0010_2
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on (1) results of monthly (plus) ambient water quality monitoring from January 2010 through December 2012 at the IDNR/SHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co. (STORET station 10120001); (2) results of U.S. EPA/Iowa DNR fish contaminant monitoring in 2011 and (3) IDNR/SHL biological sampling conducted in 2011 and 2012.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW) 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 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) based on results of monitoring for indicator bacteria.   Results of recent monitoring, however, suggest the continuation of low levels of indicator bacteria in this river segment.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-u) based on results of IDNR/SHL biological sampling conducted in 2011 and 2012.   The fish consumption (Class HH) uses remain assessed (monitored) as "partially supported" (IR Category 5a) due to issuance of fish consumption advisory.   The sources of data for this assessment are (1) the results of monthly ambient water quality monitoring from January 2010 through December 2012 at the IDNR/UHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co.  (STORET station 10120001); (2) results of U.S.  EPA/Iowa DNR fish contaminant (RAFT) monitoring near Clarksville in September 2011 and (3) IDNR/SHL biological sampling conducted in 2011 and 2012.   Note:  USGS conducted monitoring in this assessment segment at station 05462000 at Shell Rock in October 2010.   Too few data were produced from this monitoring during the 2010-2012 assessment period (one sample) to be of use for assessment for purposes of either Section 305(b) reporting or Section 303(d) listing.

EXPLANATION:  Despite indications of “full support”, the Class A1 uses remain assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli) in 2008 at IDNR/UHL station 10120001.   Results of recent (2010-12) monitoring, however, indicate “full support of the Class A1 uses.   The geometric means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR ambient station 10120001 at Shell Rock were as follows:  the 2010 geometric mean was 48 orgs/100 ml, the 2011 geometric mean was 50 orgs/100 ml and the 2012 geometric mean was 46 orgs/100 ml.   The geometric means for all three recreation seasons are far below the Class A1 criterion of 126 orgs/100 ml.   Only one of the 24 samples (4%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.   These results do not suggest impairment of the Class A1 uses:  all geometric means meet the Class A1 criterion, and, according to IDNR’s assessment/listing methodology, the percentage of samples exceed the single-sample maximum criterion is not significantly greater than 10%.   Because, however, the IDNR assessment/listing methodology requires that, before a bacterial impairment can be de-listed, geometric mean levels of E.  coli must all be less than the applicable state water quality criteria for two consecutive listing cycles (i.e., five consecutive years), the impairment of the Class A1 uses will remain in effect.   Of the monitoring results from this station over the last five years, only the 2008 recreation season geometric mean (254 orgs/100 ml) slightly exceeded the Class A1 criterion.   If sufficient additional data are collected during the 2013-2014 period that show continued low levels of bacteria and compliance with the Class A1 water quality criterion, this impairment may be removed.   Despite this impairment, levels of indicator bacteria in this segment have historically been, and continue to be, low relative to other Iowa streams and rivers.   The very low geometric means during the 2009 through 2012 recreation seasons suggest that this river segment typically “fully supports” its designated Class A1 primary contact recreation uses.

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting".   This evaluated biological assessment was based on data collected in 2011 and 2012 as part of the IDNR/SHL stream nutrient sampling project.   A series of biological metrics that 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 benthic macroinvertebrate taxa collected in the stream sampling reach.   The biological metrics were combined a benthic macroinvertebrate index (BMIBI).   The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2011 BMIBI score was 53 (fair) and the 2012 BMIBI scores were 51 (fair) and 69 (good).  The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.   The artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70.   This assessment is considered evaluated because the drainage area (1730 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   The BMIBI scores of 53 and 69 were natural substrate sample and failed the natural substrate BIC of 70.   The 51 score was an artificial substrate score and failed the artificial substrate BIC of 52.   Even though this site failed the BMIBI BIC (3/3), 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).  

Results of chemical/physical monitoring at station 10120001 continue to suggest “full support” of the Class B(WW1) aquatic life uses.   Monitoring at this station from 2010 through 2012 showed no violations of Class B(WW1) water quality criteria for ammonia, dissolved oxygen,  pH, chloride, or sulfate in the 36 samples analyzed.   These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  

Fish consumption uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring in 2011.  The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes.  The fish contaminant data generated from the 2011 RAFT sampling conducted on the Shell Rock River near Clarksville show that levels of mercury are sufficiently high for concern and thus justify issuance of a one meal/week fish consumption advisory that extends from the mouth of the Shell Rock River to its confluence with the Winnebago River near Rockford in Floyd County.  Results from the 2011 sampling show that the level of mercury in the sample of walleye muscle tissue (0.75 ppm) far exceeds the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol.  According to the Iowa DNR/Iowa Dept.  of Public Health fish advisory protocol, the single occurrence of contaminant above an advisory trigger level does not typically result in issuance of an advisory.  Such an advisory is issued only after follow-up monitoring confirms that contaminant levels exceed the advisory trigger level.  Because, however, the level of mercury in the 2011 sample of walleye was well above the one meal/week advisory trigger, a one meal/week consumption advisory was issued.  Follow-up monitoring will be conducted to better define the level of mercury in Shell Rock River fish and to provide information for any changes needed to the consumption advisory.  Because a one meal per week fish consumption advisory was issued for this waterbody, the fish consumption uses are assessed as “partially supported.”

Monitoring and Methods
Assessment Key Dates
12/3/2012 Fixed Monitoring End Date
8/6/2012 Biological Monitoring
9/8/2011 Biological Monitoring
8/23/2011 Fish Tissue Monitoring
1/6/2010 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
260 Fish tissue analysis
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
Monitoring Levels
Biological 3
Habitat 2
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 1
BioIntegrity Fair
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Mercury Fish Consumption Moderate
  • Source Unknown
  • High
Pathogens Primary Contact Recreation Slight
  • Source Unknown
  • Slight
Cause Unknown Aquatic Life Support Slight
  • Source Unknown
  • Slight