Big Sioux River IA 06-BSR-1523

from confluence with Indian Cr. (S9 T93N R48W Plymouth Co.) to confluence with Rock R. in S1 T95N R48W Sioux 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: 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
Trend: Stable
Created: 10/27/2025 3:10:02 PM
Updated: 1/8/2026 11:19:42 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

2000

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: adjacent state

Class BWW1

Aquatic Life - Warm Water Type 1

Fully Supported

Class HH

Human Health -

Fully Supported

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). The Class BWW1 use was assessed as "fully supported" based on the results of ambient chemical/physical water quality monitoring. The Class HH use was assessed as "fully supported" based on fish tissue sampling data. Note: A TMDL for pathogen indicators in this segment of the Big Sioux River was prepared by DNR and the South Dakota Dept. of Environment & Natural Resources; this TMDL was approved by EPA in January 2008. Because all Section 303(d) impairments identified for the current assessment/listing cycle were addressed by the TMDL, this waterbody remains in IR Category 4a (impaired; TMDL approved).

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	FT	See Below	1065	Big Sioux River	N of Hawarden
SDAK	16	WQ	CY 2022-2024	460667	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

460667

5/5/5

158

246

YES

2 / NA

0 / NA

YES

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
460667	16	36 / 3	7.69	9.53	8.6	6	7	NO	MONITORED	FULL

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
460667	16	Dissolved Oxygen	36 / 3	4.34	20.52	12.82	1	NA / NA	7	NO	MONITORED	FULL
460667	16	pH	36 / 3	7.69	9.53	8.6	6	NA / NA	7	NO	MONITORED	FULL
460667	16	Temperature	36 / 3	0.2	26.9	12.8	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
460667	16	Ammonia	36 / 3	1	0.08	0.02	0 / 0	NO	MONITORED	FULL

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)
1065	fillet	2024	common carp	3	52.3	0.075	<0.05	<0.0246
1065	whole	2024	common carp	3	53.3	0.039	<0.05	0.0365
Support Level	Full

Monitoring and Methods

Assessment Key Dates

1/4/2022

Monitoring Start

12/3/2024

Monitoring End

1/23/2008

TMDL Completed

9/25/2024

Fish Tissue Monitoring

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

420

Indicator bacteria monitoring

860

Other Agencies/Organizations provided monitoring data