DRAFT 2026 assessment for Winnebago River

Winnebago River IA 02-WIN-831

from confluence with Pike Run (S25 T99N R24W Winnebago Co.) to the Iowa/Minnesota state line

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 3:09:17 PM by

Updated

2/11/2026 8:21:02 AM by

Use Support

Class A1

Presumptive Use

Recreation - Primary contact

WINOFI

Bacteria: Indicator Bacteria- E. coli

Support Level

Further Investigation Needed

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Slight

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2022

Impairment Rationale

Potential Impairment

Data Source

Ambient monitoring: adjacent state

Class BWW2

Aquatic Life - Warm Water Type 2

Not Supported

Biological: low fish & invert IBIs- cause unknown

Support Level

Not Supported

Impairment Code

5b-v - Biological - verified

Cause Magnitude

Moderate

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2004

Impairment Rationale

Low Biotic Index

Data Source

Biological monitoring: Iowa DNR WQMA

TMDL Priority

Tier IV

303(d) Delistings for this Cycle

No delistings for this assessment cycle.

Documentation

Assessment Summary

The presumptive Class A1 use was assessed as “WINOFI” due to violations of Iowa water quality criteria for indicator bacteria (E. coli). The Class BWW2 use was assessed as "not supported" based on biological sampling data.

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
MNPCA	15	WQ	CY 2019-2020	99990106	Lime Creek
Iowa DNR	6	BIO	See Below	30	Winnebago River	Lande Access – Lake Mills

Presumptive Class A1 - Indicator Bacteria:

Site ID	Data Source ID	# Samples per Year (2019/2020)	2019 Geometric Mean	2020 Geometric Mean	Violates Annual Geometric Mean?	# 2019 SSM violations / SSM Violations needed for impairment	# 2020 SSM violations / SSM Violations needed for impairment	Violates Annual SSM Significantly >10% Rule?	Assessment Type	Support Level
99990106	15	5/6	193	176	YES	1 / NA	2 / NA	NA	Evaluated	WINOFI

Presumptive 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
99990106	15	23 / 2	7	8.5	7.9	0	5	NO	Evaluated	Full

Class BWW2 - Binomial 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
99990106	15	Dissolved Oxygen	23 / 2	4.7	13.4	8.4	2	NA / NA	5	NO	Evaluated	Full
99990106	15	pH	23 / 2	7	8.5	7.9	0	NA / NA	5	NO	Evaluated	Full
99990106	15	Temperature	23 / 2	9.924	27	20.834	0	NA / NA	5	NO	Evaluated	Full

Class BWW2 - Biological Sampling:

Benthic Macroinvertebrates:

Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage	BM Sample Gear	BMIBI score	BMIBI Class	47b BMIBI BIT	BMIBI Pass/Fail
30	9/28/23	112.5	47b	MS	Art Sub	31	Fair	62	Fail
30	9/2/20	112.5	47b	MS	Art Sub	51	Fair	62	Fail

Fish:

Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage	Riffle Stream?	FIBI score	FIBI Class	47b FIBI BIT	FIBI Pass/Fail
30	9/28/23	112.5	47b	MS	No	31	Fair	32	Fail
30	9/2/20	112.5	47b	MS	No	27	Fair	32	Fail

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Monitored	Fail (0/2)	Not
Fish	Monitored	Fail (0/2)	Not		Assessment was monitored because the samples used to complete
Overall	Monitored	Fail	Not	5bv	the assessment were collected in a recent five-year period.

Monitoring and Methods

Assessment Key Dates

9/2/2020

Biological Monitoring

9/28/2023

Biological Monitoring

5/23/2019

Monitoring Start

9/24/2020

Monitoring End

Methods

200

Physical/Chemical Monitoring

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

330

Fish surveys

860

Other Agencies/Organizations provided monitoring data