Twelve Mile Creek Lake IA 05-GRA-1367

Union County S12T72NR30W approx 4 mi E of Creston.

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: Eutrophic
Trend: Stable
Created: 10/27/2025 3:10:12 PM
Updated: 1/8/2026 11:55:22 AM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Turbidity: Secchi Disk Transparency

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

2022

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier I

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

Status

Continuing

Source

Agriculture

Source Confidence

Moderate

Cycle Added

2024

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier II

Class BLW

Aquatic Life - Lakes and wetlands

WINOFI

Algal Growth: Cyanobacteria

Support Level

Further Investigation Needed

Impairment Code

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

Cause Magnitude

Slight

Status

New

Source

Natural: Internal Nutrient Recycling (Primarily Lakes)

Source Confidence

Moderate

Cycle Added

2026

Impairment Rationale

Potential Impairment

Data Source

Ambient monitoring: Iowa DNR-lakes

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 (monitored) as "not supported" due to aesthetically objectionable conditions caused by both algae blooms and non-algal turbidity. The Class BLW use was assessed (evaluated) as “WINOFI” due to cyanobacteria blooms. 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 2020-2024	22880004	Twelve Mile Lake
Iowa DNR	6	FT	See Below	1097	Twelve Mile Lake	E of Creston
Creston Water Supply	27	WQ	CY 2023-2024	99990212	Twelve Mile Creek Lake	Public Water Supply

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
22880004	6	15 / 5	7.47	8.96	8.29	0	4	NO	MONITORED	FULL
99990212	27	486 / 2	7.6	8.7	8.2	0	58	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
22880004	6	Chlorophyll a TSI	15 / 5	50	79	68	YES	MONITORED	NOT
22880004	6	Secchi TSI	15 / 5	49	77	65	YES	MONITORED	NOT
22880004	6	Total Phosphorus TSI	15 / 5	56	71	65	NA	NA	NA
22880004	6	Fixed suspended solids	15 / 5	0.7	9	2	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
22880004	6	Dissolved Oxygen	15 / 5	5	14.7956	7.7622	0	NA / NA	4	NO	MONITORED	FULL
22880004	6	pH	15 / 5	7.47	8.96	8.29	0	NA / NA	4	NO	MONITORED	FULL
99990212	27	pH	486 / 2	7.6	8.7	8.2	0	NA / NA	58	NO	MONITORED	FULL
22880004	6	Temperature	15 / 5	18.4	30.4	23.7	0	NA / NA	4	NO	MONITORED	FULL
99990212	27	Temperature	487 / 2	0.88889	28.8889	15.292	0	NA / NA	58	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
22880004	6	7	37.28139744	92.22%	17.27504885	62.90%	YES	EVALUATED	WINOFI

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
22880004	6	Ammonia	15 / 5	0.548	0.09	0.04	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
99990212	27	Fluoride	70 / 2	29	570	311	310	0	NO	MONITORED	FULL
22880004	6	Inorganic nitrogen (nitrate and nitrite)	14 / 5	0.03	0.95	0.21	0.16	0	NO	MONITORED	FULL
22880004	6	pH	15 / 5	7.47	8.96	8.29	8.29	0	NO	MONITORED	FULL
99990212	27	pH	486 / 2	7.6	8.7	8.2	8.2	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 Creston Water Supply 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 HH - Fish Tissue Sampling:

Site ID	Biopart	Year	Species	# in sample / # of samples	Avg Length (cm)	(Avg) Hg (ppm)	Chlordane (ppm)	Sum PCBs (ppm)
1097	tissue plug	2019	largemouth bass	10	33.5	0.191
1097	tissue plug	2020	walleye	9	46.2	0.208
1097	tissue plug	2019	walleye	2	42.7	0.188
1097	tissue plug	2020	yellow perch	10	26.0	0.063
Support Level	Full

Monitoring and Methods

Assessment Key Dates

6/2/2020

Monitoring Start

9/22/2020

Monitoring End

5/21/2018

Fish Tissue Monitoring

12/31/2024

Fish Tissue Monitoring

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

275

Drinking water compliance reports

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)