2024 Pending assessment for South Skunk River

South Skunk River IA 03-SSK-934

from the Ames Water Works dam in River Valley Park at Ames (S36 T84N R24W Story Co.) to the Co. Rd. at north line of S6 T85 R23W Story Co (approximately 1 mile NNE of Story City)

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 8:12:53 AM
Updated: 12/20/2023 9:44:36 AM

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

Use Support

Class A1

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

2004

Impairment Rationale

Geometric mean criterion exceeded

Data Source

Ambient monitoring: Iowa DNR-rivers

TMDL Priority

Tier III

Class BWW1

Not Supported

Biological: low fish & invert IBIs- cause unknown

Support Level

Not Supported

Impairment Code

5b-v - Biological - verified

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2016

Impairment Rationale

Low Biotic Index

Data Source

Biological monitoring: Iowa DNR WQMA

TMDL Priority

Tier IV

Class HH

Not Assessed

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 use was assessed as "not supported" based on biological sampling data. All other designated uses were "not assessed."

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	WQ	CY 2012-2014	10850003	South Skunk River Upstream of Ames (US1)
Prairie Rivers/City of Ames	26	WQ	CY 2020-2022	99990168	South Skunk @ W Riverside Rd (Ames)
Prairie Rivers/City of Ames	26	WQ	CY 2020-2022	99990169	South Skunk River @ Broad St (Story City)
Iowa DNR	6	BIO	See Below	38	South Skunk River	Ames

Class A1 - Indicator Bacteria: 2012-2014

Site ID	Data Source ID	# Samples / # Years	2012 Geometric Mean	2013 Geometric Mean	2014 Geometric Mean	Annual Geometric Mean Violation	# Violations	% Violations	Significantly >10% Violations	Assessment Type	Support Level
10850003	6	22 / 3	46	94	223	Yes	7	32%	Yes	Evaluated	Not

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

99990168

7/8/8

103.2

97.9

131.0

YES

2/3

1/3

Evaluated

WINOFI

99990169

7/8/8

425

133.8

175.9

YES

6/3

3/3

4/3

YES

Evaluated

WINOFI

The bacteria samples were collected by the Prairie Rivers Watershed Group and Story County Conservation with the analyses conducted by the City of Ames. Since the data were not collected under a DNR-approved quality assurance plan, the assessment was considered "evaluated" instead of "monitored". According to Iowa DNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). The Iowa DNR does, however, consider these impairments as appropriate for listing under Category 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Class A1 - pH: 2012-2014

Site ID	Data Source ID	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Violations	% Violations	Significantly >10% Violations	Assessment Type	Support Level
10850003	6	33 / 3	8.6	8.1	8.1	0	0%	No	Evaluated	Full

Class BWW1 - Binomial Parameters: 2012-2014

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Maximum Value	Mean Value	Median Value	# Acute / Chronic Violations	% Acute/ Chronic Violations	Significantly >10% Violations	Assessment Type	Support Level
10850003	6	Ammonia	33 / 3	1.7	0.15	0.05	0 / 0	0% / 0%	No	Evaluated	Full
10850003	6	Chloride	33 / 3	100	43	35	0 / 0	0% / 0%	No	Evaluated	Full
10850003	6	Dissolved oxygen	33 / 3	16.2	10.7	10.2	NA / 0	NA / 0%	No	Evaluated	Full
10850003	6	pH	33 / 3	8.6	8.1	8.1	0 / NA	0% / NA	No	Evaluated	Full
10850003	6	Sulfate	33 / 3	110	46	41	0 / NA	0% / NA	No	Evaluated	Full
10850003	6	Temperature	33 / 3	27.5	12	12.4	0 / 0	0% / 0%	No	Evaluated	Full

Class BWW1 - Biological Sampling:

Benthic Macroinvertebrates:

BioNet Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage/Thermal	BM Sample Gear	BMIBI score	BMIBI Class	47b BMIBI BIT	BMIBI Pass/Fail
38	9/19/22	254.3	47b	MS/WW	Surber	55	Good	62	Fail
38	8/31/17	254.3	47b	MS/WW	Surber	58	Good	62	Fail

Fish:

BioNet Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage/Thermal	Riffle Stream?	FIBI score	FIBI Class	47b FIBI BIT	FIBI Pass/Fail
38	9/19/22	254.3	47b	MS/WW	No	39	Fair	32	Pass
38	8/31/17	254.3	47b	MS/WW	No	70	Good	32	Pass

Overall:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Fail (0/2)	Not
Fish	Evaluated	Pass (2/2)	Full		Assessment was evaluated because the samples used for the
Overall	Evaluated	Fail	Not	5bv	the assessment were not collected in a recent five year period.

Class HH - No Data

Monitoring and Methods

Assessment Key Dates

9/19/2022	Biological Monitoring
1/10/2012	Monitoring Start
9/4/2014	Monitoring End
8/31/2017	Biological Monitoring
4/15/2020	Monitoring Start
10/19/2022	Monitoring End

Methods

150	Monitoring data more than 5 years old
200	Physical/Chemical Monitoring
315	Regional reference site approach
320	Benthic macroinvertebrate surveys
330	Fish surveys
420	Indicator bacteria monitoring

Water Quality Assessments

Impaired Waters List

South Skunk River IA 03-SSK-934

Assessment Summary

Assessment Explanation

Assessment Key Dates

Methods