DRAFT 2026 assessment for South Fork Iowa River

South Fork Iowa River IA 02-IOW-748

from confluence with unnamed tributary in SE 1/4 NE 1/4 S2 T87N R21W (SE of Logsdon Park) Hardin Co. confluence with unnamed tributary in W 1/2 S19 T88N R21W Hardin Co.

This is a DRAFT assessment

Draft assessments may change without notice

2026
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

Overall IR

5 - Water is impaired or threatened and a TMDL is needed.

Trend

Stable

Current Phase

Public Comment

Created

10/27/2025 3:09:43 PM by

Updated

1/5/2026 1:57:34 PM by

Use Support

Class A1

Presumptive Use

Recreation - Primary contact

Fully Supported

Class BWW1

Aquatic Life - Warm Water Type 1

Not Supported

Biological: low aquatic macroinvertebrate IBI

Support Level

Not Supported

Impairment Code

5b-v - Biological - verified

Cause Magnitude

Slight

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2016

Impairment Rationale

Low Biotic Index

Data Source

Biological monitoring: Iowa DNR WQMA

TMDL Priority

Tier IV

Organic Enrichment: Low Dissolved Oxygen

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2022

Impairment Rationale

Significantly > 10% of days fail to meet 24-hour criteria

Data Source

Ambient monitoring: Iowa DNR-continuous

TMDL Priority

Tier III

Class HH

Human Health -

Not Assessed

303(d) Delistings for this Cycle

No delistings for this assessment cycle.

Documentation

Assessment Summary

The presumptive Class A1 use was assessed as "fully supported" based on water quality monitoring data. The Class BWW1 use was assessed as “not supported” based on biological sampling data and continuous dissolved oxygen monitoring data. All other designated uses were "not assessed."

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	12420001	South Fork Iowa River Logsdon Co Park- Iowa Falls
Iowa DNR	6	WQ	CY 2022-2024	14000242	South Fork Iowa River 3
Iowa DNR	6	WQ	CY 2022-2024	14000297	South Fork Iowa River 5
Iowa DNR	6	BIO & WQ	See Below	21	South Fork Iowa River	Logsdon County Park – Iowa Falls
Iowa DNR	6	BIO & WQ	See Below	380	South Fork Iowa River	Buckeye – REMAP #79
Iowa DNR	6	BIO & WQ	See Below	1194	South Fork Iowa River	E of Buckeye – SFI-3
Iowa DNR	6	BIO & WQ	See Below	1229	South Fork Iowa River	4 miles E of Buckeye – SFI-5

Class A1 - Indicator Bacteria: No Data

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
12420001	6	17 / 3	7.8	8.5	8.2	0	4	NO	MONITORED	FULL
14000242	6	17 / 3	7.9	8.3	8.1	0	4	NO	MONITORED	FULL
14000297	6	17 / 3	7.7	8.2	8	0	4	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
12420001	6	Dissolved Oxygen	17 / 3	5.56	12.77	9.01	0	NA / NA	4	NO	MONITORED	FULL
14000242	6	Dissolved Oxygen	17 / 3	5.21	12.03	8.38	0	NA / NA	4	NO	MONITORED	FULL
14000297	6	Dissolved Oxygen	17 / 3	5.6	12.4	8.6	0	NA / NA	4	NO	MONITORED	FULL
12420001	6	pH	17 / 3	7.8	8.5	8.2	0	NA / NA	4	NO	MONITORED	FULL
14000242	6	pH	17 / 3	7.9	8.3	8.1	0	NA / NA	4	NO	MONITORED	FULL
14000297	6	pH	17 / 3	7.7	8.2	8	0	NA / NA	4	NO	MONITORED	FULL
12420001	6	Temperature	17 / 3	4.4	26.1	18	0	NA / NA	4	NO	MONITORED	FULL
14000242	6	Temperature	17 / 3	6.528	27.323	18.251	0	NA / NA	4	NO	MONITORED	FULL
14000297	6	Temperature	17 / 3	4.252	23.736	17.766	0	NA / NA	4	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
12420001	6	Ammonia	16 / 2	0.19	0.05	0.02	0 / 0	NO	MONITORED	FULL
14000242	6	Ammonia	16 / 2	0.07	0.03	0.02	0 / 0	NO	MONITORED	FULL
14000297	6	Ammonia	16 / 2	0.11	0.03	0.02	0 / 0	NO	MONITORED	FULL

Class BWW1 - Biological Sampling:

Benthic Macroinvertebrates:

# sites	# samples	Date Range	Ecoregion	Major Drainage	Low BMIBI Score	High BMIBI Score	Avg. BMIBI Score	47b BMIBI BIT	# BMIBI Scores pass/fail
3	34	2020-2024	47b	MS	34	66	53.6	62	6/34 Fail

Fish:

# sites	# samples	Date Range	Ecoregion	Major Drainage	Low FIBI Score	High FIBI Score	Avg. FIBI Score	47b FIBI BIT	# FIBI Scores pass/fail
4	10	2020-2022	47b	MS	50	76	65	32/53	9/10 Pass

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Monitored	Fail (6/34)	Not
Fish	Monitored	Pass (9/10)	Full		Assessment was monitored because the samples used to complete
Overall	Monitored	Fail	Not	5bv	the assessment were collected in a recent five year period.

Class BWW1 - Continuous DO Monitoring:

Site ID	Data Source ID	# Days Monitoring (2020/2021/2022)	DO Criteria Type	# 2020 violations / Violations needed for impairment	# 2021 violations / Violations needed for impairment	# 2022 violations / Violations needed for impairment	Violates Annual Significantly >10% Rule?	Assessment Type	Support Level
			24 hr	6/4			Yes
380	6	16/0/0	16 hr	0/4			No	Evaluated	Not
			24 hr		69/24	40/24	Yes
21	6	0/175/181	16 hr		44/24	14/24	Yes	Monitored	Not
			24 hr		55/27	7/26	Yes
1194	6	0/201/195	16 hr		39/27	0/26	Yes	Monitored	WINOFI
			24 hr		35/27	20/24	Yes
1229	6	0/203/181	16 hr		10/27	2/24	No	Monitored	Not

Class HH – No Data

Monitoring and Methods

Assessment Key Dates

4/10/2020

Monitoring Start

10/11/2022

Monitoring End

7/20/2020

Biological Monitoring

8/7/2024

Biological Monitoring

4/6/2021

Monitoring Start

10/25/2021

Monitoring End

4/13/2022

Monitoring Start

10/25/2022

Monitoring End

4/12/2022

Monitoring Start

8/7/2024

Monitoring End

Methods

200

Physical/Chemical Monitoring

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

330

Fish surveys