Big Sioux River IA 06-BSR-1524

from confluence with Rock R. (S1 T95N R48W Sioux Co.) to confluence with Beaver Cr near Canton SD in S36 T98N R49W Lyon 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: Unknown
Created: 10/27/2025 3:10:02 PM
Updated: 1/8/2026 1:01:15 PM

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

Low

Cycle Added

2012

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: adjacent state

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Moderate

Cycle Added

2024

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: adjacent state

TMDL Priority

Tier III

Class BWW1

Aquatic Life - Warm Water Type 1

Not Supported

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Moderate

Cycle Added

2024

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: adjacent state

TMDL Priority

Tier III

Class HH

Human Health -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed as “not supported” due to violations of Iowa’s water quality criteria for indicator bacteria (E. coli) and violations of the Class A1 criterion for pH. The Class BWW1 use was assessed as "not supported" based on violations of the Class BWW1 criterion for pH. All other designated uses were “not assessed.” Note: A TMDL for pathogen indicators in this segment of the Big Sioux River was prepared by DNR and the South Dakota Department of Environment & Natural Resources; this TMDL was approved by EPA in January 2008. Because not all of the Section 303(d) impairments identified for the current assessment/listing cycle were addressed by the TMDL, this waterbody was moved to IR Category 5a (pollutant-caused impairment; TMDL needed).

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
SDAK	16	WQ	CY 2022-2024	460666	Big Sioux River

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

460666

5/5/5

177

YES

1 / NA

0 / NA

EVALUATED

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
460666	16	36 / 3	7.7	9.5	8.7	9	7	YES	MONITORED	NOT

Class BWW1 - 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
460666	16	Dissolved Oxygen	36 / 3	3.85	20.35	13.1	1	NA / NA	7	NO	MONITORED	FULL
460666	16	pH	36 / 3	7.7	9.5	8.7	9	NA / NA	7	YES	MONITORED	NOT
460666	16	Temperature	36 / 3	0	29.6	13	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
460666	16	Ammonia	36 / 3	0.69	0.07	0.02	0 / 0	NO	MONITORED	FULL

Class HH - No Data

Monitoring and Methods

Assessment Key Dates

1/4/2022

Monitoring Start

12/3/2024

Monitoring End

1/23/2008

TMDL Completed

Methods

200

Physical/Chemical Monitoring

420

Indicator bacteria monitoring

860

Other Agencies/Organizations provided monitoring data