Little Sioux River IA 06-LSR-1578

from confluence with Milford Cr. (NW 1/4 S14 T98N R37W Dickinson Co.) to confluence with West Fork Little Sioux R. in S36 T100N R38W Dickinson 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: Declining
Trend Comment: The aquatic life impairment is now for both benthic macroinvertebrates and fish.
Created: 10/27/2025 3:09:49 PM
Updated: 1/7/2026 10:03:54 AM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2008

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: Iowa DNR-rivers

TMDL Priority

Tier III

Class BWW1

Aquatic Life - Warm Water Type 1

Not Supported

Biological: low fish IBI

Support Level

Not Supported

Impairment Code

5b-t - Biological - tentative

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2022

Impairment Rationale

Low Biotic Index

Data Source

Biological monitoring: Iowa DNR WQMA

TMDL Priority

Tier IV

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 levels of indicator bacteria that violated state water quality criteria. The Class BWW1 was assessed as "not supported” based on biological sampling data. The Class HH use was assessed as "fully supported" based on water quality monitoring data.

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	10300001	Little Sioux River near Milford
Iowa DNR	6	BIO	See Below	63	Little Sioux River	Horseshoe Bend County Park - Milford
Iowa DNR	6	BIO	See Below	1148	Little Sioux River	Gravel Road Bridge W of Milford

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

10300001

7/7/7

558

446

369

YES

6 / 3

4 / 3

6 / 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
10300001	6	36 / 3	7.5	8.7	8	0	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
10300001	6	Chloride	36 / 3	14	34	22	NA	0 / 0	7	NO	MONITORED	FULL
10300001	6	Dissolved Oxygen	36 / 3	5.7	14.5	9.6	0	NA / NA	7	NO	MONITORED	FULL
10300001	6	pH	36 / 3	7.5	8.7	8	0	NA / NA	7	NO	MONITORED	FULL
10300001	6	Sulfate	36 / 3	45	180	93	0	NA / NA	7	NO	MONITORED	FULL
10300001	6	Temperature	36 / 3	0	26.5	10.9	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
10300001	6	Ammonia	36 / 3	0.37	0.07	0.02	0 / 0	NO	MONITORED	FULL
10300001	6	Cadmium	18 / 2	0.5	0.5	0.5	0 / 0	NO	MONITORED	FULL
10300001	6	Chromium	18 / 2	4	1.3	0.96	0 / 0	NO	MONITORED	FULL
10300001	6	Copper	18 / 2	35	3.7	2	0 / 1	NO	MONITORED	FULL
10300001	6	Lead	18 / 2	1	1	1	0 / 0	NO	MONITORED	FULL
10300001	6	Nickel	18 / 2	2.5	2	2	0 / 0	NO	MONITORED	FULL
10300001	6	Selenium	36 / 3	5	2	1.9	0 / 0	NO	MONITORED	FULL
10300001	6	Silver	18 / 2	1	1	1	0 / 0	NO	MONITORED	FULL
10300001	6	Zinc	18 / 2	13	1.3	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	47b BMIBI BIT	BMIBI Pass/Fail
63	8/7/23	333.4	47b	MO	Surber	54	Fair	62	Fail
1148	7/19/21	484.0	47b	MO	Art Sub	53	Fair	62	Fail
63	8/4/20	333.4	47b	MO	Surber	65	Good	62	Pass

Fish:

Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage	Riffle Stream?	FIBI score	FIBI Class	47b FIBI BIT	FIBI Pass/Fail
63	8/7/23	333.4	47b	MO	Yes	38	Fair	53	Fail
63	8/4/20	333.4	47b	MO	Yes	51	Good	53	Fail

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Monitored	Fail (1/3)	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.

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
10300001	6	Copper	18 / 2	35	3.7	2	1	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
10300001	6	Selenium	12/12/12	2.125	1.667	2.267	3/2/2	FS/FS/FS	MONITORED	FULL

Monitoring and Methods

Assessment Key Dates

7/19/2021

Biological Monitoring

8/4/2020

Biological Monitoring

8/7/2023

Biological Monitoring

1/12/2022

Monitoring Start

12/5/2024

Monitoring End

Methods

200

Physical/Chemical Monitoring

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

330

Fish surveys

420

Indicator bacteria monitoring