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
Result Period
2008 - 2010
Class A1 Class B(WW-2)
Assessment Methodology
Assessment Type
Integrated Report
Category 5p
Legacy ADBCode
IA 06-LSR-0120_1
Overall Use Support
Not supporting
Aquatic Life Use Support
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of IDNR/UHL ambient monthly monitoring near Hornick from 2008-10.

Basis for Assessment

[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 (2010) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not 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 IDNR/UHL ambient water quality monitoring from 2008 through 2010.   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/UHL monthly ambient monitoring conducted during the 2008-2010 assessment period on West Fork Little Sioux River at the Highway 141 bridge approximately 1 mile east of Hornick in (STORET station 10970002).  

EXPLANATION: The presumptive Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria.   The geometric means of indicator bacteria (E.  coli) in the 21 samples collected during the recreational seasons of 2008 through 2010 at the West Fork of the Little Sioux River at Hornick were as follows:  the 2008 geometric mean was 1762 orgs/100 ml, the 2009 geometric mean was 354 orgs/100 ml and the 2010 geometric mean was 386 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Fourteen of the 21 samples (67%) exceeded the 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 a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (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 “impaired.”  

Monitoring at the IDNR station east of Hornick showed no violations of Class B(WW2) water quality criteria in the 32 samples collected during the 2008-2010 assessment period for dissolved oxygen (minimum value = 7.2 mg/l), pH (range = 7.8 to 8.4 units), or ammonia-nitrogen (maximum value = 1.5).   (Note: toxic metals and pesticides were not analyzed at this site from 2008 through 2010.)  These results 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).

Monitoring and Methods
Assessment Key Dates
12/8/2010 Fixed Monitoring End Date
1/8/2008 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
Monitoring Levels
Biological 0
Habitat 0
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 0
BioIntegrity N/A
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Other habitat alterations Aquatic Life Support Not Impairing
  • Bank or Shoreline Modification/Destabilization
  • Channelization
  • Not Impairing
  • Not Impairing
Pathogens Primary Contact Recreation High
  • Source Unknown
  • Moderate