DRAFT 2026 assessment for Black Hawk Lake

Black Hawk Lake IA 04-RAC-1134

Sac County S35T87NR36W at Lake View.

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

Hypereutrophic

Trend

Stable

Current Phase

Public Comment

Created

10/27/2025 3:09:54 PM by

Updated

12/3/2025 10:46:42 AM by

Use Support

Class A1

Recreation - Primary contact

Not Supported

Turbidity

Support Level

Not Supported

Impairment Code

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

Cause Magnitude

High

303(d) Listing Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

High

Cycle Added

2004

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Iowa DNR-lakes

Bacteria: Indicator Bacteria- E. coli

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

303(d) Listing Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2016

Impairment Rationale

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

Data Source

Beach monitoring: Iowa DNR WQMA

TMDL Priority

Tier II

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

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

Cause Magnitude

High

303(d) Listing Status

New

Source

Natural: Internal Nutrient Recycling (Primarily Lakes)

Source Confidence

High

Cycle Added

2026

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 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 poor water clarity caused by algal and non-algal turbidity. The Class BLW use was assessed (monitored) as “fully supported.” The Class HH (fish consumption) use was assessed as “fully supported” based on fish tissue sampling data. Note: The TMDL prepared by Iowa DNR for impairments due to algae and turbidity was approved by U.S.EPA in February 2012. Not all Section 303(d) impairments identified for the current assessment/listing cycle (indicator bacteria), however, are addressed in the TMDL. Thus, this waterbody remains in IR Category 5a (impaired; TMDL required) for this assessment/listing cycle.

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	21810001	Denison Beach
Iowa DNR	6	WQ	CY 2021-2024	21810002	Black Hawk Beach
Iowa DNR	6	WQ	CY 2020-2024	22810002	Black Hawk Lake	Historical Lake Deep Point
Iowa DNR	6	WQ	CY 2020-2024	22810003	Black Hawk Lake West Arm
Iowa DNR	6	FT	See Below	1137	Black Hawk Lake	At Lake View

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

21810001

15/14/15

2 / 4

1 / 4

4 / 4

YES

MONITORED

NOT

21810002

15/14/15

102

3 / 4

2 / 4

3 / 4

MONITORED

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
22810002	6	40 / 5	8.04	9.29	8.43	1	7	NO	MONITORED	FULL
22810003	6	19 / 3	7.96	9.73	8.47	1	5	NA	NA	NA

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
22810002	6	Chlorophyll a TSI	47 / 5	49	75	70	YES	MONITORED	NOT
22810002	6	Secchi TSI	40 / 5	60	87	77	YES	MONITORED	NOT
22810002	6	Total Phosphorus TSI	47 / 5	61	83	74	NA	NA	NA
22810002	6	Fixed suspended solids	15 / 5	0.7	31	9	NA	NA	NA
22810003	6	Chlorophyll a TSI	32 / 5	64	75	71	NA	NA	NA
22810003	6	Secchi TSI	25 / 4	69	84	79	NA	NA	NA
22810003	6	Total Phosphorus TSI	32 / 5	61	85	74	NA	NA	NA

Class BLW - Conventional 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
22810002	6	Dissolved Oxygen	39 / 5	4.89	13.31	8.48	1	NA / NA	7	NO	MONITORED	FULL
22810002	6	pH	40 / 5	8.04	9.29	8.43	1	NA / NA	7	NO	MONITORED	FULL
22810002	6	Temperature	40 / 5	5.5	28.2	21	0	NA / NA	7	NO	MONITORED	FULL
22810003	6	Dissolved Oxygen	19 / 3	6.46	11.17	8.76	0	NA / NA	5	NA	NA	NA
22810003	6	pH	19 / 3	7.96	9.73	8.47	1	NA / NA	5	NA	NA	NA
22810003	6	Temperature	19 / 3	6.08	28.48	19.17	0	NA / NA	5	NA	NA	NA

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
22810002	6	2	3.2415	34.69%	1.6216	17.37%	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
22810002	6	Ammonia	15 / 5	0.4065	0.07	0.04	0 / 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)
1137	fillet	2015	channel catfish	5	48.1	0.04	0.02	0.06
1137	tissue plug	2020	yellow perch	10	19	0.028
Support Level	Full

Monitoring and Methods

Assessment Key Dates

4/15/2020

Monitoring Start

10/23/2024

Monitoring End

8/26/2015

Fish Tissue Monitoring

2/1/2012

TMDL Completed

7/1/2020

Fish Tissue Monitoring

Methods

150

Monitoring data more than 5 years old

200

Physical/Chemical Monitoring

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

420

Indicator bacteria monitoring