North Skunk River IA 03-NSK-854

from confluence with Cedar Cr. (S15 T75N R12W Keokuk Co.) to confluence with Middle Cr. in S35 T76N R14W Mahaska Co.

Assessment Cycle

2024

Release Status

Pending

Data Collection Period

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 2020 - 2022
Stream/River Water Quality Data	CY 2020 - 2022
Lake Water Quality Data	CY 2018 - 2022
Biological Sampling Data	CY 2018 - 2022*
Fish Kill Data	CY 2018 - 2022
Fish Tissue Sampling Data	CY 2018 - 2022
Continuous Dissolved Oxygen Sampling Data	CY 2018 - 2022
*Biological sites sampled in both 2017 and 2021 were also assessed.

Overall IR Category: 5 - Water is impaired or threatened and a TMDL is needed.
Trend: Unknown

Created: 11/1/2023 8:12:57 AM
Updated: 12/29/2023 2:07:56 PM

Assessment conducted in accordance with Iowa's 2024 IR methodology.

Use Support

Class A1

Recreation

Primary contact

Not Supported

Impairments

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2008

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: Iowa DNR-rivers

TMDL Priority

Tier IV

Class BWW2

Aquatic Life

Warm Water Type 2

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 (E. coli) that exceeded state water quality criteria. The Class BWW2 use was assessed as "fully supported" based on biological sampling data and 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 2020-2022	10540001	North Skunk River
Iowa DNR	6	BIO	See Below	944	North Skunk River	Oskaloosa

Class A1 - Indicator Bacteria:

Site ID

Data Source ID

# Samples per Year (2020/ 2021/2022)

2020 Geometric Mean

2021 Geometric Mean

2022 Geometric Mean

Violates Annual Geometric Mean?

# 2020 SSM violations / SSM Violations needed for impairment

# 2021 SSM violations / SSM Violations needed for impairment

# 2022 SSM violations / SSM Violations needed for impairment

Violates Annual SSM Significantly >10% Rule?

Assessment Type

Support Level

10540001

8/8/8

169

109

YES

2 / 3

3 / 3

1 / 3

YES

Mon

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
10540001	6	36 / 3	7.6	8.5	8.1	0	7	NO	Mon	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
10540001	6	Chloride	36 / 3	15	47	25	NA	0 / 0	7	NO	Mon	Full
10540001	6	Dissolved Oxygen	36 / 3	7	14.8	10.3	0	NA / NA	7	NO	Mon	Full
10540001	6	pH	36 / 3	7.6	8.5	8.1	0	NA / NA	7	NO	Mon	Full
10540001	6	Sulfate	36 / 3	19	180	42	0	NA / NA	7	NO	Mon	Full
10540001	6	Temperature	36 / 3	0	26.9	12.5	0	NA / NA	7	NO	Mon	Full

Class BWW2 - 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
10540001	6	Ammonia	36 / 3	0.4	0.04	0.02	0 / 0	NO	Mon	Full
10540001	6	Cadmium	20 / 3	0.5	0.5	0.5	0 / 0	NO	Mon	Full
10540001	6	Chromium	20 / 3	2	0.57	0.5	0 / 0	NO	Mon	Full
10540001	6	Lead	20 / 3	1	0.91	1	0 / 0	NO	Mon	Full
10540001	6	Nickel	20 / 3	3	1.8	1.7	0 / 0	NO	Mon	Full
10540001	6	Selenium	18 / 2	3	1	0.5	0 / 0	NO	Mon	Full
10540001	6	Silver	20 / 3	1	0.902	1	0 / 0	NO	Mon	Full
10540001	6	Zinc	20 / 3	4.3	1	0.5	0 / 0	NO	Mon	Full

Class BWW2 - Biological Sampling:

Benthic Macroinvertebrates:

Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage	BM Sample Gear	BMIBI score	BMIBI Class	47f BMIBI BIT	BMIBI Pass/Fail
944	8/29/19	631	47f	MS	Floating Art Sub	72	Good	51	Pass

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Pass (1/1)	Full		Assessment was evaluated because the drainage area of the site used for
Overall	Evaluated	Pass	Full	1	the assessment was greater than the calibrated range.

Monitoring and Methods

Assessment Key Dates

1/8/2020

Monitoring Start

8/29/2019

Biological Monitoring

12/6/2022

Monitoring End

Methods

200

Physical/Chemical Monitoring

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

420

Indicator bacteria monitoring