Clear Lake IA 02-WIN-841

Cerro Gordo County S13T96NR22W at Clear Lake.

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: 4 - Water is impaired or threatened and a TMDL has been completed or is not needed.
Trophic: Eutrophic
Trend: Stable
Created: 10/27/2025 3:09:41 PM
Updated: 11/24/2025 2:24:26 PM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

High

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

High

Cycle Added

2004

Impairment Rationale

Geometric mean criterion exceeded

Data Source

TMDL monitoring: Iowa DNR

Turbidity

Support Level

Not Supported

Impairment Code

4a - Pollutant-caused impairment. TMDL has been completed.

Cause Magnitude

Moderate

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

Moderate

Cycle Added

2004

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

Class BLW

Aquatic Life - Lakes and wetlands

Fully Supported

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 violations of the state water quality criteria for indicator bacteria and due to poor water transparency primarily due to high levels of inorganic suspended solids. The Class BLW use was assessed as “fully supported.” The Class C (drinking water) use was assessed as “fully supported” based on water quality monitoring data. The Class HH (fish consumption) use was assessed as “fully supported” based on fish tissue sampling data. Note: A TMDL for indicator bacteria E. coli at Clear Lake was prepared by DNR and approved by EPA in June 2020. Additionally, a TMDL for turbidity and algae at Clear Lake was prepared by DNR and approved by EPA in 2005. Because all of the Section 303(d) impairments were addressed in these TMDL's, this waterbody is placed in IR Category 4a (impaired; TMDL approved).

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	21170001	Clear Lake State Park Beach
Iowa DNR	6	WQ	CY 2022-2024	21170002	McIntosh Woods Beach
Iowa DNR	6	WQ	CY 2020-2024	22170001	Clear Lake
Iowa DNR	6	FT	See Below	1121	Clear Lake	North Shore, near Clear 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

21170001

15/14/15

133

YES

1 / 4

3 / 4

5 / 4

YES

MONITORED

NOT

21170002

15/14/15

122

3 / 4

4 / 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
22170001	6	18 / 5	7.96	8.64	8.36	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
22170001	6	Chlorophyll a TSI	18 / 5	53	74	62	NO	MONITORED	FULL
22170001	6	Secchi TSI	18 / 5	51	73	66	YES	MONITORED	NOT
22170001	6	Total Phosphorus TSI	18 / 5	49	71	60	NA	NA	NA
22170001	6	Fixed suspended solids	18 / 5	0.7	21	5	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
22170001	6	Dissolved Oxygen	17 / 5	7.1	10.9	8.6	0	NA / NA	4	NO	MONITORED	FULL
22170001	6	pH	18 / 5	7.96	8.64	8.36	0	NA / NA	4	NO	MONITORED	FULL
22170001	6	Temperature	18 / 5	12.7	28.2	21.1	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
22170001	6	5	570.0396	96.08%	11.2285	68.04%	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
22170001	6	Ammonia	18 / 5	0.047	0.02	0.02	0 / 0	NO	MONITORED	FULL

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
22170001	6	Inorganic nitrogen (nitrate and nitrite)	17 / 5	0.03	0.4	0.1	0.05	0	NO	MONITORED	FULL
22170001	6	pH	18 / 5	7.96	8.64	8.36	8.36	0	NO	MONITORED	FULL

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)
1121	tissue plug	2019	muskie	2	90.5	0.154
1121	tissue plug	2019	walleye	20	47.2	0.152
1121	tissue plug	2021	muskie	1		0.24
1121	tissue plug	2021	white Bass	8		0.213
1121	tissue plug	2021	yellow bass	1		0.07
Support Level	Full

Monitoring and Methods

Assessment Key Dates

5/18/2020

Monitoring Start

10/22/2024

Monitoring End

5/5/2021

Fish Tissue Monitoring

7/1/2019

Fish Tissue Monitoring

3/10/2005

TMDL Completed

6/11/2020

TMDL Completed

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

420

Indicator bacteria monitoring