Iowa DNR
Iowa DNR

ADBNet

Water Quality Assessments
Impaired Waters List

Little Rock River IA 06-BSR-1800

from confluence with Argo Slough (S17 T99N R43W Lyon Co.) to the Iowa/Minnesota state line

This is a DRAFT assessment
Draft assessments may change without notice
  • 2026
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Overall IR
5 - Water is impaired or threatened and a TMDL is needed.
Trend
Unknown
Current Phase
Public Comment
Created
10/27/2025 3:11:29 PM by
Updated
1/8/2026 12:55:32 PM by
Use Support
Class A1
Recreation - Primary contact
Not Supported
Support Level
Not Supported
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Slight
303(d) Listing Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
Low
Cycle Added
2008
Impairment Rationale
Geometric mean criterion exceeded
Data Source
TMDL monitoring: Iowa DNR
TMDL Priority
Tier III
Support Level
Not Supported
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Moderate
303(d) Listing Status
New
Source
Unknown: Source Unknown
Source Confidence
Moderate
Cycle Added
2026
Impairment Rationale
Significantly > 10% of samples fail to meet criterion
Data Source
Ambient monitoring: adjacent state
TMDL Priority
Tier III
Class BWW2
Aquatic Life - Warm Water Type 2
Not Supported
Support Level
Not Supported
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Moderate
303(d) Listing Status
New
Source
Unknown: Source Unknown
Source Confidence
Moderate
Cycle Added
2026
Impairment Rationale
Significantly > 10% of samples fail to meet criterion
Data Source
Ambient monitoring: adjacent state
TMDL Priority
Tier III
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 BWW2 use was assessed as "not supported" due to violations of the pH criterion.

Assessment Explanation

Data Sources:

Data Source

Data Source ID

Data Type

Data Age

Site ID

Site Name

Site Description

MNPCA

15

WQ

CY 2022-2023

99990312

Little Rock River


Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples per Year (2022/ 2023)

2022 Geometric Mean

2023 Geometric Mean

Violates Annual Geometric Mean?

# 2022 SSM violations / SSM Violations needed for impairment

# 2023 SSM violations / SSM Violations needed for impairment

Violates Annual SSM Significantly >10% Rule?

Assessment Type

Support Level

99990312

15

8/6

2284

2293

YES

8 / 3

6 / NA

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

99990312

15

9 / 2

8.07

9.41

8.62

3

3

YES

MONITORED

NOT


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

99990312

15

Dissolved Oxygen

9 / 2

7.24

14.19

10.41

0

NA / NA

3

NO

MONITORED

FULL

99990312

15

pH

9 / 2

8.07

9.41

8.62

3

NA / NA

3

YES

MONITORED

NOT

99990312

15

Temperature

8 / 2

20.4

30.5

25

0

NA / NA

3

NO

MONITORED

FULL

Monitoring and Methods
Assessment Key Dates
5/10/2022
Monitoring Start
8/30/2023
Monitoring End
Methods
200
Physical/Chemical Monitoring
420
Indicator bacteria monitoring
860
Other Agencies/Organizations provided monitoring data