Iowa DNR

Water Quality Assessments

Impaired Waters List

Floyd River IA 06-FLO-1552

mouth to West Branch Floyd R. in S2, T91N, R46W, Plymouth Co.

Assessment Cycle
Release Status
Release Period
2012 - 2014
9/2/2016 10:04:31 AM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Last Updated
9/27/2016 2:38:10 PM
Use Support
Class A1
Primary contact
Partially Supporting
Bacteria: Indicator Bacteria, E. coli
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
Class B(WW-1)
Aquatic Life
Warm Water Type 1
Partially Supporting
Biological: low aquatic macroinvertebrate IBI
Impairment Code
3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
Class HH
Human Health
Not Assessed
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment 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 (evaluated) as "partially supported" (IR 3b-u) based on results of biological monitoring in 2001, 2013 and 2014 that suggest potential impairment of the Class B(WW1) aquatic life uses. Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment. This assessment is based on results of (1) IDNR monthly ambient monitoring conducted during the 2012-2014 assessment period at the County Road C-70 bridge north of Sioux City (STORET station 10750001 (formerly station 950110)) and (2) IDNR/SHL biological sampling in 2000, 2013 and 2014.

Assessment Explanation

[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 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).]

The Class A1 (primary contact recreation) are assessed as "partially 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 2012 through 2014 at the Floyd River near Sioux City were as follows: the 2012 geometric mean was 235 orgs/100 ml, the 2013 geometric mean was 190 orgs/100 ml, and the 2014 geometric mean was 89 orgs/100 ml.  Two of the three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml.  Eleven of the combined 24 samples (46%) exceeded Iowa’s 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 the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (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 "partially supported" 

Regarding support of the Class B(WW1) aquatic life uses, monitoring at the IDNR/SHL monthly ambient monitoring station north of Sioux City showed no violations of Class B(WW1) water quality criteria for 36 Ammonia samples (maximum = 1.7 mg/L), 36 Dissolved Oxygen samples (minimum = 7 mg/L), 36 pH samples (range = 7.2 to 8.9), 36 Temperature samples (maximum = 27.9° c), 36 Chloride samples (maximum = 140 mg/L), or 36 Sulfate samples (maximum = 170 mg/L) occurred during monitoring from January 2012 to December 2014.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

Biological sampling results suggest that the aquatic life uses should be assessed (evaluated) as partially supporting based on IDNR/SHL biological sampling in 2000, 2013 and 2014.   This evaluated biological assessment was based on data collected in 2000, 2013 and 2014 as part of the IDNR/SHL large river sampling 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 2000 FIBI score was 45 (fair).   The 2013 and 2014 BMIBI scores were 15 (poor) and 35 (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 31 and the BMIBI BIC for this ecoregion is 54.   This segment passed the FIBI BIC in 2000 and failed the BMIBI BIC (0/2) in the last five years.   This assessment is considered evaluated because the drainage area (886 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 the FIBI BIC and failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the site used for the assessment 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).

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


Monitoring and Methods
Assessment Key Dates
1/5/2012 Fixed Monitoring Start Date
12/2/2014 Fixed Monitoring End Date
10/4/2000 Biological Monitoring
9/4/2013 Biological Monitoring
10/7/2014 Biological Monitoring
230 Fixed station physical/chemical (conventional plus toxic pollutants)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
330 Fish surveys
380 Quan. measurements of instream parms-- channel morphology-- floodplain-- 1-2 seasons-- by prof
420 Water column surveys (e.g. fecal coliform)