Iowa DNR

Water Quality Assessments

Impaired Waters List

Iowa River IA 02-IOW-641

from confluence Timber Cr. (S3 T83N R17W Marshall Co.) to confluence with Asher Cr. in S27 T84N R18W Marshall Co.

Assessment Cycle
Result Period
1994 - 1996
Assessment Type
Integrated Report
Category 0
Legacy ADBCode
IA 02-IOW-0060_4
Overall Use Support
Aquatic Life Use Support
Fish Consumption
Not assessed
Primary Contact Recreation
Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring downstream from Marshalltown.

Basis for Assessment

Used results from the DNR quarterly WQ monitoring station on the Iowa R.  near the Marshalltown Water Works for developing assessments of support of designated uses.  Support of Class B(WW) aquatic life uses assessed as FST due to lack of violations for Class B(WW) WQ criteria for conventional and toxic parameters in the 8 samples collected over the 1996-97 period.   Assess support of the Class A primary contact uses as PS due to high levels of fecal coliform bacteria in the 5 non-flow affected samples collected during summers of 1996 and 1997:  geometric mean of 184 orgs/100 ml < 200 orgs/100 ml WQS, but > 10% of samples (40%) exceeded 400 orgs/100 ml =PS (see page 3-34 of Supplement to the EPA Guidelines for preparation of the 1998 Section 305(b) reports).   Addendum:  In summer 1997, reports of a reddish/brown coloration in the Iowa River between Eldora and Tama was reported by the public.   Investigations by DNR Field Offices 2 (Mason City) and 5 (Des Moines) indicated that the cause of the unusual coloration was a bloom of algae (diatoms, possibly Stephanodiscus or Cyclotella sp.).   Water quality sampling showed to apparent negative impact to levels of DO or ammonia in the river; no fish kills or other water quality problems observed.

Monitoring and Methods
Assessment Key Dates
420 Water column surveys (e.g. fecal coliform)
230 Fixed station physical/chemical (conventional plus toxic pollutants)
Monitoring Levels
Biological 0
Habitat 0
Physical Chemistry 3
Toxic 0
Pathogen Indicators 0
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
Pathogens Overall Use Support Moderate
  • Source Unknown
  • Moderate
Pathogens Primary Contact Recreation Moderate
  • Source Unknown
  • Moderate