Iowa DNR

Water Quality Assessments

Impaired Waters List

West Fork Little Sioux River IA 06-LSR-1598

mouth (S12 T84N R45W Monona Co.) to confluence with a small unnamed tributary near Climbing Hill in S16 T87N R45W Woodbury Co.

Assessment Cycle
Release Status
Data Collection Period
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
9/6/2016 1:26:13 PM
9/22/2016 7:56:12 AM
Assessment conducted in accordance with Iowa's 2016 IR methodology
Use Support
Class A1
Partially Supported
Bacteria: Indicator Bacteria- E. coli
Support Level
Partially Supported
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 BWW2
Fully Supported
General Use
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" (IR 5a) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported" (IR 2a) based on results of IDNR/SHL biological sampling conducted in 2012, 2013 and 2014. The extensive channel modifications on this river segment, however, remain a concern regarding the full support of aquatic life uses. This assessment is based on results of IDNR/SHL monthly ambient monitoring conducted during the 2012-2014 assessment period on West Fork Little Sioux River at the Highway 141 bridge approximately 1 mile east of Hornick in (STORET station 10970002) and on 2012, 2013 and 2014 IDNR/SHL biological sampling conducted near Hornick.

Assessment Explanation

[Note 1:  For the 2002 report, the previous waterbody segment for the West Fork Little Sioux River (IA 06-LSR-0120-0), which extended 32 miles from its mouth to confluence with Mud Creek near Moville in Woodbury Co., was split into two subsegments:  (1) mouth to confluence with an unnamed tributary 0.5 miles north of Climbing Hill (Woodbury Co.   - this one) (IA 06-LSR-0120-1) and (2) unnamed tributary 0.5 miles north of Climbing Hill to Mud Creek near Moville, Woodbury Co.   (IA 06-LSR-0120-2).    See previous Section 305(b) from this subsegment for the assessments of the original 32-mile river reach.]

[Note 2:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008 (see, this segment is now presumptively designated for Class A1 (primary contact recreation) uses.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

[Note 3:  Prior to the 2016 Section 305(b) cycle, this segment was designated presumptively for Class A1 (primary contact recreation) uses.  Due to changes in Iowa’s surface water classification that were approved by then Iowa Department of Natural Resources on June 17, 2015 (see, this segment is now designated as Class A1 (primary contact recreation) uses and no longer considered as presumptive. Thus, for the current assessment, the available water quality monitoring data will be assessed under the Class A1 and Class B(WW2) water quality criteria.]

The Class A1 (primary contact recreation) uses are assessed (monitored) 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 West Fork of the Little Sioux River at Hornick were as follows: the 2012 geometric mean was 157 orgs/100 ml, the 2013 geometric mean was 188 orgs/100 ml, and the 2014 geometric mean was 133 orgs/100 ml.  All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml.  Nine of the combined 24 samples (38%) 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"

Monitoring at the IDNR station east of Hornick showed no violations of Class B(WW2) water quality criteria for 36 Ammonia samples (maximum = 1.1 mg/L), 36 Dissolved Oxygen samples (minimum = 6.7 mg/L), 36 pH samples (range = 7.5 to 8.9), 36 Temperature samples (maximum = 25.9° c), 36 Chloride samples (maximum = 25 mg/L), or 36 Sulfate samples (maximum = 100 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(WW2) aquatic life uses. The extensive habitat alterations due to channel straightening in this stream system suggest a concern regarding full support of the aquatic life uses.    This conclusion is based on a stream use assessment conducted by IDNR staff in 1995 (see assessments developed for the 1996, 1998, and 2000 reporting cycles for more information).

In agreement with the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "fully supporting".   This evaluated biological assessment was based on data collected in 2012, 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 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 2012 BMIBI score was 29 (poor), the 2013 score was 61 (good) and the 2014 score was 58 (good).   The aquatic life use support was assessed (evaluated) as fully supporting (=FS), 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 BMIBI BIC for this ecoregion is 54.   This segment passed the BMIBI BIC 2/3 times in the last five years (2010-2014).

This aquatic life assessment is now considered "monitored" based on a change in the 2010 IDNR assessment methodology.  IDNR now requires a segment have two or more biological samples collected from the segment in multiple years in the most recent five year period (2010-2014) to be considered “monitored”.


Monitoring and Methods
Assessment Key Dates
1/5/2012 Fixed Monitoring Start Date
12/2/2014 Fixed Monitoring End Date
9/5/2012 Biological Monitoring
7/30/2013 Biological Monitoring
8/18/2014 Biological Monitoring
230 Fixed station physical/chemical (conventional plus toxic pollutants)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
330 Fish surveys
380 Quantitative physical habitat assessment
420 Indicator bacteria monitoring