West Okoboji Lake - Millers Bay IA 06-LSR-2068

western basin of West Okoboji Lake in Sections 23 and 24 T99N R37W 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: 2 - Some of the designated uses are met but there is insufficient data to determine if remaining designated uses are met.
Trophic: Mesotrophic
Trend: Stable
Created: 10/27/2025 3:10:43 PM
Updated: 11/24/2025 10:51:14 AM

Use Support

Class A1

Recreation - Primary contact

Fully Supported

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 (monitored) as “fully supported.” 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.” 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 2021-2024	14000411	West Lake Okoboji - Millers Bay

· Beginning in 2021, a central point was added to this segment for water quality monitoring. Previous assessment used water quality data from the main deep point of this lake.

Class A - Indicator Bacteria: No Data

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
14000411	6	12 / 4	7.64	8.57	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
14000411	6	Chlorophyll a TSI	12 / 4	26	51	38	NO	MONITORED	FULL
14000411	6	Secchi TSI	12 / 4	35	46	40	NO	MONITORED	FULL
14000411	6	Total Phosphorus TSI	12 / 4	37	83	46	NA	NA	NA
14000411	6	Fixed suspended solids	12 / 4	1.7	3	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
14000411	6	Dissolved Oxygen	12 / 4	7.3	9.1	8.4	0	NA / NA	4	NO	MONITORED	FULL
14000411	6	pH	12 / 4	7.64	8.57	8.28	0	NA / NA	4	NO	MONITORED	FULL
14000411	6	Temperature	12 / 4	17.2	26.3	22.3	0	NA / NA	4	NO	MONITORED	FULL

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
14000411	6	Ammonia	12 / 4	0.031	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
14000411	6	Inorganic nitrogen (nitrate and nitrite)	11 / 4	0.05	0.39	0.11	0.06	0	NO	MONITORED	FULL
14000411	6	pH	12 / 4	7.64	8.57	8.28	8.33	0	NO	MONITORED	FULL

Class HH - No Data

Monitoring and Methods

Assessment Key Dates

9/10/2024

Monitoring Start

6/8/2021

Monitoring End

Methods

120

Surveys of fish and game biologists/other professionals

200

Physical/Chemical Monitoring

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)