DRAFT 2026 assessment for Big Creek Lake

Big Creek Lake IA 04-UDM-1242

Polk County S22T81NR25W near Polk City

This is a DRAFT assessment

Draft assessments may change without notice

2026
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

Overall IR

5 - Water is impaired or threatened and a TMDL is needed.

Trophic

Eutrophic

Trend

Stable

Current Phase

Public Comment

Created

10/27/2025 3:10:04 PM by

Updated

11/24/2025 2:28:39 PM by

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

Moderate

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Moderate

Cycle Added

2006

Impairment Rationale

Single-sample maximum criterion exceeded in significantly > 10% of samples

Data Source

Beach monitoring: Iowa DNR WQMA

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

303(d) Listing Status

Continuing

Source

Agriculture

Source Confidence

Moderate

Cycle Added

2020

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier I

Class BWW1

Aquatic Life - Warm Water Type 1

Not Supported

Organic Enrichment: Low Dissolved Oxygen

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Slight

303(d) Listing Status

New

Source

Unknown: Source Unknown

Source Confidence

Moderate

Cycle Added

2026

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier I

Class HH

Human Health -

Fully Supported

303(d) Delistings for this Cycle

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed (monitored) as “not supported” due to violations of the state water quality criteria for indicator bacteria and secondarily by the presence of aesthetically objectionable conditions caused by algal blooms. The Class BWW1 use was assessed (monitored) as “not supported” based on frequent violations of the BWW1 dissolved oxygen criteria. The Class HH (fish consumption) use was assessed (monitored) as “fully supported” based on fish tissue sampling data. Note: The DNR TMDL for the impairment due to indicator bacteria at this lake was approved by U.S. EPA in August 2011. Because this TMDL does not address all of the impairments identified for this lake (Algal Growth: Chlorophyll a, Dissolved Oxygen), this lake remains in IR Category 5a (impaired; TMDL required).

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	21770001	Big Creek Beach
Iowa DNR	6	WQ	CY 2020-2024	22770004	Big Creek	Lake Deep Point
Iowa DNR	6	WQ	CY 2021-2024	26770002	Big Creek 319 (BCL4)
Iowa DNR	6	WQ	CY 2021-2024	26770004	Big Creek 319 (BCL5)
Iowa DNR	6	WQ	CY 2021-2024	14000414	Big Creek TMDL 1
Iowa DNR	6	WQ	CY 2021-2024	14000415	Big Creek TMDL 2
Iowa DNR	6	WQ	CY 2021-2024	14000416	Big Creek TMDL 3
Iowa DNR	6	FT	See Below	1052	Big Creek

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

21770001

15/14/15

182

116

YES

6 / 4

4 / 4

5 / 4

YES

MONITORED

NOT

22770004

5/0/0

0 / NA

EVALUATED

FULL

26770002

5/0/0

0 / NA

EVALUATED

FULL

26770004

5/0/0

0 / NA

EVALUATED

FULL

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
22770004	6	47 / 5	7.67	8.62	8.2	0	8	NO	MONITORED	FULL
14000414	6	32 / 3	7.7	8.7	8.3	0	6	NO	EVALUATED	FULL
14000415	6	32 / 3	7.9	8.8	8.4	0	6	NO	EVALUATED	FULL
14000416	6	31 / 3	7.8	8.9	8.4	0	6	NO	EVALUATED	FULL
21770001	6	45 / 3	7.75	9.18	8.57	2	8	NO	EVALUATED	FULL
26770002	6	29 / 3	7.8	9	8.4	0	6	NO	EVALUATED	FULL
26770004	6	29 / 3	7.7	9	8.5	0	6	NO	EVALUATED	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
22770004	6	Chlorophyll a TSI	47 / 5	30	74	65	YES	MONITORED	NOT
22770004	6	Secchi TSI	47 / 5	38	80	62	NO	MONITORED	FULL
22770004	6	Total Phosphorus TSI	47 / 5	37	85	63	NA	NA	NA
22770004	6	Fixed suspended solids	15 / 5	0.7	12	2	NA	NA	NA

Class BWW1 - 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
22770004	6	Dissolved Oxygen	46 / 5	2.8	16.2	8.3	8	NA / NA	8	YES	MONITORED	NOT
22770004	6	pH	47 / 5	7.67	8.62	8.2	0	NA / NA	8	NO	MONITORED	FULL
22770004	6	Temperature	47 / 5	7.27	29.06	22.91	0	NA / NA	8	NO	MONITORED	FULL
14000414	6	Dissolved Oxygen	32 / 3	5.1	17	10	0	NA / NA	6	NO	EVALUATED	FULL
14000414	6	pH	32 / 3	7.7	8.7	8.3	0	NA / NA	6	NO	EVALUATED	FULL
14000414	6	Temperature	32 / 3	7.313	29	22.721	0	NA / NA	6	NO	EVALUATED	FULL
14000415	6	Dissolved Oxygen	32 / 3	6.2	18.1	10.3	0	NA / NA	6	NO	EVALUATED	FULL
14000415	6	pH	32 / 3	7.9	8.8	8.4	0	NA / NA	6	NO	EVALUATED	FULL
14000415	6	Temperature	32 / 3	7.2	28.9	22.7	0	NA / NA	6	NO	EVALUATED	FULL
14000416	6	Dissolved Oxygen	31 / 3	6.79	21.08	11.13	0	NA / NA	6	NO	EVALUATED	FULL
14000416	6	pH	31 / 3	7.8	8.9	8.4	0	NA / NA	6	NO	EVALUATED	FULL
14000416	6	Temperature	31 / 3	7.338	28.929	22.88	0	NA / NA	6	NO	EVALUATED	FULL
21770001	6	Dissolved Oxygen	45 / 3	7	23.29	11.42	0	NA / NA	8	NO	EVALUATED	FULL
21770001	6	pH	45 / 3	7.75	9.18	8.57	2	NA / NA	8	NO	EVALUATED	FULL
21770001	6	Temperature	45 / 3	17.028	31.032	26.747	0	NA / NA	8	NO	EVALUATED	FULL
26770002	6	Dissolved Oxygen	29 / 3	7.3	15.8	10.7	0	NA / NA	6	NO	EVALUATED	FULL
26770002	6	pH	29 / 3	7.8	9	8.4	0	NA / NA	6	NO	EVALUATED	FULL
26770002	6	Temperature	29 / 3	7	29.8	23.3	0	NA / NA	6	NO	EVALUATED	FULL
26770004	6	Dissolved Oxygen	29 / 3	7.1	18.9	11.4	0	NA / NA	6	NO	EVALUATED	FULL
26770004	6	pH	29 / 3	7.7	9	8.5	0	NA / NA	6	NO	EVALUATED	FULL
26770004	6	Temperature	29 / 3	6.727	28.948	23.351	0	NA / NA	6	NO	EVALUATED	FULL

Class BWW1 - 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
22770004	6	5	10.5457	90.60%	0.7524	5.01%	NO	EVALUATED	NA

Class BWW1 - 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
22770004	6	Ammonia	26 / 5	0.479	0.08	0.02	0 / 0	NO	MONITORED	FULL
14000414	6	Ammonia	11 / 1	0.15	0.04	0.02	0 / 0	NO	EVALUATED	FULL
14000415	6	Ammonia	11 / 1	0.16	0.04	0.02	0 / 0	NO	EVALUATED	FULL
14000416	6	Ammonia	11 / 1	0.12	0.04	0.02	0 / 0	NO	EVALUATED	FULL
26770002	6	Ammonia	11 / 1	0.08	0.04	0.02	0 / 0	NO	EVALUATED	FULL
26770004	6	Ammonia	11 / 1	0.08	0.03	0.02	0 / 0	NO	EVALUATED	FULL

Class BWW1 - Fish Kill:According to Iowa DNR’s assessment/listing methodology, any IR 3b fish kills that occurred more than 5 years ago will be moved to IR Category 3a (not assessed) in not other fish kills have ocured within that 5 year period.

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)
1052	tissue plug	2019	largemouth bass	10	34.2	0.063
1052	tissue plug	2017	largemouth bass	3	41.1	0.097
1052	tissue plug	2021	muskie	3		0.215
1052	tissue plug	2019	walleye	20	47.9	0.057
Support Level	Full

Monitoring and Methods

Assessment Key Dates

5/28/2020

Monitoring Start

9/30/2024

Monitoring End

8/24/2011

TMDL Completed

6/14/2017

Fish Tissue Monitoring

7/1/2019

Fish Tissue Monitoring

10/4/2022

Fish Tissue Monitoring

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

420

Indicator bacteria monitoring