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.
5/17/2019 8:10:18 AM
7/10/2019 2:57:53 PM
Assessment conducted in accordance with Iowa's 2018 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" 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" based on results of DNR/SHL biological sampling conducted in 2012, 2013 and 2014. This assessment is based on results of DNR/SHL monthly ambient monitoring conducted during the 2014-2016 assessment period at the West Fork Little Sioux River at the Highway 141 bridge approximately 1 mile east of Hornick (STORET station 10970002) and on 2012, 2013 and 2014 DNR/SHL biological sampling conducted near Hornick.

Assessment Explanation

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 23 samples collected during the recreational seasons of 2014 through 2016 at the West Fork Little Sioux River near Hornick (STORET station 10970002) were as follows: the 2014 geometric mean was 351 orgs/100 ml, the 2015 geometric mean was 633 orgs/100 ml, and the 2016 geometric mean was 614 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Sixteen of the combined 23 samples (70%) 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 DNR’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."

Results of chemical/physical water quality monitoring at the West Fork Little Sioux River near Hornick (STORET station 10970002) suggest “full support” of the aquatic life uses. Monitoring showed no violations of Class B(WW2) water quality criteria for 35 Ammonia samples (maximum = 0.2 mg/L), 35 Dissolved Oxygen samples (minimum = 6.7 mg/L), 35 pH samples (range = 7.5 to 8.5), 35 Temperature samples (maximum = 23.9 °C), 35 Chloride samples (maximum = 24 mg/L), or 35 Sulfate samples (maximum = 100 mg/L) occurred during monitoring from January 2014 to December 2016. 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.

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 DNR/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 (2012-2016).

This aquatic life assessment is now considered "monitored" based on a change in the 2010 DNR assessment methodology. DNR now requires a segment have two or more biological samples collected from the segment in multiple years in the most recent five year period (2012-2016) to be considered “monitored”.

Monitoring and Methods
Assessment Key Dates
1/8/2014 Fixed Monitoring Start Date
12/7/2016 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