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.

This is a DRAFT assessment

Draft assessments may change without notice

Open Data Collection Period Modal

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2022 - 2024
Stream/River Water Quality Data	CY 2022 - 2024
Lake Water Quality Data	CY 2020 - 2024
Biological Sampling Data	CY 2020 - 2024*
Fish Kill Data	CY 2020 - 2024
Fish Tissue Sampling Data	CY 2020 - 2024
Continuous Dissolved Oxygen Sampling Data	CY 2020 - 2024
*Biological sites sampled in both 2019 and 2023 were also assessed.

Open Segment Map
View 2026 assessment summary page

Cycle: 2026
Current Phase: Public Comment
Overall IR: 5 - Water is impaired or threatened and a TMDL is needed.
Trend: Stable
Created: 10/27/2025 3:09:43 PM
Updated: 2/11/2026 9:35:27 AM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2004

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: Iowa DNR-rivers

Class BWW1

Aquatic Life - Warm Water Type 1

WINOFI

Biological: low aquatic macroinvertebrate IBI

Support Level

Further Investigation Needed

Impairment Code

3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.

Cause Magnitude

Moderate

Status

New

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2026

Impairment Rationale

Potential Impairment

Data Source

Biological monitoring: Iowa DNR WQMA

Class HH

Human Health -

Not Supported

Fish Consumption Advisory: Mercury

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2014

Impairment Rationale

Fish consumption advisory in effect: no more than 1 meal/week

Data Source

Fish contaminant monitoring: Iowa DNR

TMDL Priority

Tier IV

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed as "not supported" due to levels of indicator bacteria (E. coli) that exceeded state water quality criteria. The Class BWW1 use was assessed as "WINOFI" based on biological sampling data. The Class HH use remains assessed as “not supported” due to the historic existence of a one meal per week fish consumption advisory for mercury. Note: A TMDL covering the bacteria impairments for all segments, with the segment code prefix "IA 02-IOW" in the Iowa River Basin, was approved by EPA in 2018. Due to the completion of the TMDL, the IR category for the Class A1 use was changed from 5a (impaired; TMDL needed) to 4a (impaired; TMDL approved). Because not all impairments identified for this segment are covered by the TMDL, this segment remains in Category 5 of Iowa's Integrated Report (impaired; TMDL needed).

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2022-2024	10640002	Iowa River near Marshalltown
Iowa DNR	6	BIO	See Below	921	Iowa River	near Marshalltown
Iowa DNR	6	FT	See Below	1003	Iowa River	at Marshalltown
Iowa DNR	6	FT	See Below	1371	Iowa River (U)	Riverview Park Marshalltown

Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples per Year (2022/ 2023/2024)

2022 Geometric Mean

2023 Geometric Mean

2024 Geometric Mean

Violates Annual Geometric Mean?

# 2022 SSM violations / SSM Violations needed for impairment

# 2023 SSM violations / SSM Violations needed for impairment

# 2024 SSM violations / SSM Violations needed for impairment

Violates Annual SSM Significantly >10% Rule?

Assessment Type

Support Level

10640002

9/8/8

374

106

340

YES

5 / 3

1 / 3

4 / 3

YES

MONITORED

NOT

Class A1 – pH:

Site ID	Data Source ID	# Samples / # Years	Minimum Value	Maximum Value	Mean Value	# Violations	Violations needed for impairment	Violates Significantly >10% Rule?	Assessment Type	Support Level
10640002	6	36 / 3	7.5	9.1	8.2	1	7	NO	MONITORED	FULL

Class BWW1 - Conventional Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Mean Value	# Violations	# Acute / Chronic Violations	Violations needed for impairment	Violates Significantly >10% Rule?	Assessment Type	Support Level
10640002	6	Chloride	36 / 3	11	70	33	NA	0 / 0	7	NO	MONITORED	FULL
10640002	6	Dissolved Oxygen	36 / 3	7.6	15.1	10.8	0	NA / NA	7	NO	MONITORED	FULL
10640002	6	pH	36 / 3	7.5	9.1	8.2	1	NA / NA	7	NO	MONITORED	FULL
10640002	6	Sulfate	36 / 3	15	58	35	0	NA / NA	7	NO	MONITORED	FULL
10640002	6	Temperature	36 / 3	0	25.3	11.9	0	NA / NA	7	NO	MONITORED	FULL

Class BWW1 - Toxic Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Acute / Chronic Violations	>1 Acute / Chronic Violation	Assessment Type	Support Level
10640002	6	Ammonia	36 / 3	0.59	0.1	0.02	0 / 0	NO	MONITORED	FULL
10640002	6	Cadmium	18 / 2	0.5	0.5	0.5	0 / 0	NO	MONITORED	FULL
10640002	6	Chromium	18 / 2	4	1.2	1	0 / 0	NO	MONITORED	FULL
10640002	6	Copper	18 / 2	2.6	1.5	1.2	0 / 0	NO	MONITORED	FULL
10640002	6	Lead	18 / 2	1	1	1	0 / 0	NO	MONITORED	FULL
10640002	6	Nickel	18 / 2	2.3	1.4	1.3	0 / 0	NO	MONITORED	FULL
10640002	6	Selenium	36 / 3	2.7	1.2	1	0 / 0	NO	MONITORED	FULL
10640002	6	Silver	18 / 2	1	1	1	0 / 0	NO	MONITORED	FULL
10640002	6	Zinc	18 / 2	2.8	0.9	0.5	0 / 0	NO	MONITORED	FULL

Class BWW1 - Biological Sampling:

Benthic Macroinvertebrates:

Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage	BM Sample Gear	BMIBI score	BMIBI Class	47f BMIBI BIT	BMIBI Pass/Fail
921	8/9/23	1635	47f	MS	Floating Art Sub	24	Poor	51	Fail
921	8/2/19	1635	47f	MS	Floating Art Sub	80	Excellent	51	Pass

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Fail (1/2)	WINOFI		Assessment was evaluated because the drainage area of
Overall	Evaluated	Fail	WINOFI	3bu	the sample site was greater than the calibrated range.

Class HH - Fish Tissue Sampling:

Site ID	Biopart	Year	Species	# in sample / # of samples	Avg Length (cm)	(Avg) Hg (ppm)	Chlordane (ppm)	Sum PCBs (ppm)
1371	tissue plug	2021	smallmouth bass	5	25.5	0.131
1371	tissue plug	2020	channel catfish	15	49.5	0.136
1003	tissue plug	2012	walleye	3	41.1	0.308
1003	tissue plug	2011	walleye	4	44.4	0.323
Support Level	Not	A historic one meal/week mercury consumption advisory existed on this segment. Based on IR Methodology, the Class HH (fish consumption) use was assessed as “not supported.”

Class HH-F - Toxic Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	Number of Non - Detects	Mean Value > Criterion or MCL	Assessment Type	Support Level
10640002	6	Copper	18 / 2	2.6	1.5	1.2	3	NO	MONITORED	FULL
Site ID	Data Source ID	Parameter Name	# Samples per Year (2022/ 2023/2024)	2022 Mean	2023 Mean	2024 Mean	# Non-Detects per Year (2022/ 2023/2024)	Support Level per Year (2022/ 2023/2024)	Assessment Type	Support Level
10640002	6	Selenium	12/12/12	1.083	1.167	1.283	6/4/5	FS/FS/FS	MONITORED	FULL

Monitoring and Methods

Assessment Key Dates

1/6/2022

Monitoring Start

12/3/2024

Monitoring End

9/14/2011

Fish Tissue Monitoring

9/26/2012

Fish Tissue Monitoring

8/2/2019

Biological Monitoring

3/9/2018

TMDL Completed

9/17/2020

Fish Tissue Monitoring

7/8/2021

Fish Tissue Monitoring

8/9/2023

Biological Monitoring

Methods

150

Monitoring data more than 5 years old

200

Physical/Chemical Monitoring

260

Fish tissue analysis

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

420

Indicator bacteria monitoring