West Okoboji Lake - Main Basin South IA 06-LSR-1653

portion of West Okoboji Lake south of Manhattan Beach on the western shore and Omaha Beach on the eastern shore; approximate center of basin is in SW 1/4 S24T99NR37W 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.
Trophic: Mesotrophic
Trend: Stable
Created: 10/27/2025 3:10:42 PM
Updated: 1/2/2026 10:51:11 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

New

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2026

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Beach monitoring: Iowa DNR WQMA

TMDL Priority

Tier I

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Exotic/Invasive Species: Exotic Species

Support Level

Further Investigation Needed

Impairment Code

3b - Use potentially impaired based on an evaluated assessment.

Cause Magnitude

Slight

Status

Continuing

Source

Other: Exotic Species

Source Confidence

High

Cycle Added

2016

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Biological monitoring: Iowa DNR Fisheries

Class C

Drinking Water -

Fully Supported

Class HH

Human Health -

Not Assessed

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed as "not supported" due to high levels of indicator bacteria. The Class BLW use was assessed (evaluated) as “WINOFI” due to an invasive species introduction (Zebra Mussels). The Class C use was assessed (monitored) as “fully supported” based on water quality monitoring data and drinking water compliance monitoring required by the DNR. The Class HH (fish consumption) use was assessed (monitored) as “fully supported” based on fish tissue sampling 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	21300001	Gull Point Beach
Iowa DNR	6	WQ	CY 2020-2024	22300009	West Okoboji Lake	Deep Point
Iowa DNR	6	FT	See Below	1090	West Okoboji Lake

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

21300001

15/14/12

0 / 4

4 / 4

3 / 4

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
22300009	6	12 / 4	8.07	8.56	8.28	0	4	NO	MONITORED	FULL

Class A1 - Lake TSI's

Site ID	Data Source ID	Parameter Name	# Samples / # Years	Minimum Value	Maximum Value	Median Value	Median TSI Violation?	Assessment Type	Support Level
22300009	6	Chlorophyll a TSI	15 / 5	27	50	37	NO	MONITORED	FULL
22300009	6	Secchi TSI	14 / 5	25	56	34	NO	MONITORED	FULL
22300009	6	Total Phosphorus TSI	15 / 5	34	57	40	NA	NA	NA
22300009	6	Fixed suspended solids	15 / 5	0.7	7	1.9	NA	NA	NA

Class BLW - 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
22300009	6	Dissolved Oxygen	13 / 5	6.7	9.1	8.1	0	NA / NA	4	NO	MONITORED	FULL
22300009	6	pH	12 / 4	8.07	8.56	8.28	0	NA / NA	4	NO	MONITORED	FULL
22300009	6	Temperature	13 / 5	13.3	26.3	21.3	0	NA / NA	4	NO	MONITORED	FULL

Class BLW - Lake Planktonic Cyanobacteria:

Site ID	Data Source ID	# Samples	Maximum Cyanobacteria Wet Mass	Maximum Cyanobacteria % Composition	Median Cyanobacteria Wet Mass	Median Cyanobacteria % Composition	Median Cyanobacteria Concentration = 75th Percentile	Assessment Type	Support Level
22300009	6	2	0.075010011	31.01%	0.050381388	20.05%	NO	EVALUATED	NA

Class BLW - 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
22300009	6	Ammonia	12 / 4	0.028	0.02	0.02	0 / 0	NO	MONITORED	FULL

Class BLW - Additional Information

The Class BLW use was assessed (evaluated) as “WINOFI” based on information from the DNR Fisheries Bureau and results from the ISU lake surveys. Based on information from the DNR Fisheries Bureau, this lake was infested with Zebra mussels in 2012 and the population exploded in 2015. This invasive species will likely cause water quality to degrade in the future. This potential impairment is not appropriate for Iowa’s Section 303(d) list of impaired waters (Integrated Report Category 5) but is appropriate for IR Category 3b of Iowa’s Integrated Report and Iowa’s list of waters in need of further investigation.

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
22300009	6	Inorganic nitrogen (nitrate and nitrite)	14 / 5	0.03	2.32	0.24	0.06	0	NO	MONITORED	FULL
22300009	6	pH	12 / 4	8.07	8.56	8.28	8.27	0	NO	MONITORED	FULL

Class C - Iowa Drinking Water Compliance Report:

Compliance monitoring required by the DNR Water Supply Section did not show violations of finished water caused by source water at the Central Water System, Milford Municipal Utilities, and Wahpeton Water Supply facilities during the 2022-24 monitoring period. The results of compliance monitoring required by the DNR Water Supply Operations and Engineering Sections 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 HH - Fish Tissue Sampling:

Site ID	Biopart	Year	Species	# in sample / # of samples	Avg Length (cm)	(Avg) Hg (ppm)	Chlordane (ppm)	Sum PCBs (ppm)
1090	tissue plug	2021	black crappie	4		0.086
1090	tissue plug	2021	yellow bass	1		0.237
Support Level	Full

Monitoring and Methods

Assessment Key Dates

6/22/2020

Monitoring Start

9/10/2024

Monitoring End

4/19/2021

Fish Tissue Monitoring

4/13/2021

Fish Tissue Monitoring

Methods

120

Surveys of fish and game biologists/other professionals

200

Physical/Chemical Monitoring

260

Fish tissue analysis

275

Drinking water compliance reports

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

420

Indicator bacteria monitoring