South Skunk River IA 03-SSK-927

from the Highway 63 bridge north of Oskaloosa (S25 T76N R16W Mahaska Co.) to confluence with Elk Cr. in NE 1/4 S19 T77N R17W Mahaska 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: Stable
Created: 10/27/2025 3:09:44 PM
Updated: 2/11/2026 10:37:34 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

N/A

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

WINOFI

Biological: low aquatic macroinvertebrate IBI

Support Level

Further Investigation Needed

Impairment Code

3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2014

Impairment Rationale

Potential Impairment

Data Source

Biological monitoring: Iowa DNR WQMA

Class C

Drinking Water -

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 levels of indicator bacteria (E. coli) that violated state water quality criteria. The Class BWW1 use was assessed as "WINOFI" based on biological sampling data. The Class C use was assessed as "fully supported" based on water quality monitoring data and drinking water compliance monitoring required by the DNR. 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	10620001	South Skunk River near Oskaloosa
Iowa DNR	6	BIO	See Below	945	South Skunk River	Oskaloosa

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

10620001

8/8/8

194

148

479

YES

4 / 3

3 / 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
10620001	6	35 / 3	7.5	8.9	8.3	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
10620001	6	Chloride	36 / 3	23	50	35	NA	0 / 0	7	NO	MONITORED	FULL
10620001	6	Dissolved Oxygen	36 / 3	7.3	17.2	10.8	0	NA / NA	7	NO	MONITORED	FULL
10620001	6	pH	35 / 3	7.5	8.9	8.3	0	NA / NA	7	NO	MONITORED	FULL
10620001	6	Sulfate	36 / 3	22	66	41	0	NA / NA	7	NO	MONITORED	FULL
10620001	6	Temperature	36 / 3	0	26.9	12.1	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
10620001	6	Ammonia	35 / 3	0.21	0.04	0.02	0 / 0	NO	MONITORED	FULL
10620001	6	Cadmium	18 / 2	0.5	0.5	0.5	0 / 0	NO	MONITORED	FULL
10620001	6	Chromium	18 / 2	3	1.04	1	0 / 0	NO	MONITORED	FULL
10620001	6	Copper	18 / 2	3.4	1.7	1.7	0 / 0	NO	MONITORED	FULL
10620001	6	Lead	18 / 2	1	1	1	0 / 0	NO	MONITORED	FULL
10620001	6	Nickel	18 / 2	2.1	1.6	1.6	0 / 0	NO	MONITORED	FULL
10620001	6	Selenium	36 / 3	2.7	1.2	1	0 / 0	NO	MONITORED	FULL
10620001	6	Silver	18 / 2	1	1	1	0 / 0	NO	MONITORED	FULL
10620001	6	Zinc	18 / 2	4.7	1.2	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	47f BMIBI BIT	BMIBI Pass/Fail
945	8/8/22	1635.0	47f	MS	Floating Art Sub	36	Fair	51	Fail

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Fail (0/1)	WINOFI		Assessment was evaluated because the drainage area of the site
Overall	Evaluated	Fail	WINOFI	3bu	used for the assessment was greater than the calibrated range.

Class C - Binomial Parameters:

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Mean Value	Median Value	# Violations	Violates Significantly >10% Rule?	Assessment Type	Support Level
10620001	6	Chloride	36 / 3	23	50	35	35	0	NO	MONITORED	FULL
10620001	6	Inorganic nitrogen (nitrate and nitrite)	36 / 3	0.05	14	4.94	4.1	4	NO	MONITORED	FULL
10620001	6	pH	35 / 3	7.5	8.9	8.3	8.3	0	NO	MONITORED	FULL

Class C - Iowa Drinking Water Compliance Report:

Compliance monitoring required by the Iowa DNR Water Supply Operations and Engineering Sections did not show violations of finished water caused by source water at the Oskaloosa Municipal Water Dept facility during the 2022-24 monitoring period. The results of compliance monitoring as reported in annual compliance reports are available at the following location at the Iowa DNR’s home page: https://www.iowadnr.gov/Environmental-Protection/Water-Quality/Water-Supply-Engineering/Annual-Compliance-Report.

Class C - Toxic Parameters: No Data

Class HH-FW - 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
10620001	6	Copper	18 / 2	3.4	1.7	1.7	3	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
10620001	6	Selenium	12/12/12	1.292	1.042	1.292	5/7/5	FS/FS/FS	MONITORED	FULL

Monitoring and Methods

Assessment Key Dates

1/4/2022

Monitoring Start

12/4/2024

Monitoring End

8/8/2022

Biological Monitoring

Methods

200

Physical/Chemical Monitoring

275

Drinking water compliance reports

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

420

Indicator bacteria monitoring