Diamond Lake IA 03-NSK-861

Poweshiek County S2T78NR15W approx 2.5 mi SW of Montezuma.

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 3:09:41 PM
Updated: 12/29/2025 2:53:16 PM

Use Support

Class BLW

Aquatic Life - Lakes and wetlands

Not Supported

Algal Growth: Chlorophyll a

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Agriculture

Source Confidence

Moderate

Cycle Added

2020

Impairment Rationale

Adverse impacts on plant/animal communities

Data Source

Ambient monitoring: Iowa DNR-shallow lakes & wetlands

TMDL Priority

Tier I

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 BLW use was assessed (monitored) as “not supported” due to high levels of algae at this lake suggested impairment due to adverse impacts on plant/animal communities. Additionally, the Class BLW use was assessed as “WINOFI” due to poor water quality caused by an exotic fish population (common carp). 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. All other designated uses are “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 2020-2024	22790005	Diamond Lake

Class BLW - 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
22790005	6	Chlorophyll a TSI	15 / 5	45	80	67	YES	MONITORED	NOT

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
22790005	6	Dissolved Oxygen	14 / 5	5.4	12	7.9	0	NA / NA	4	NO	MONITORED	FULL
22790005	6	pH	15 / 5	7.5	8.8	8.2	0	NA / NA	4	NO	MONITORED	FULL
22790005	6	Temperature	15 / 5	18.8	28.9	23.9	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
22790005	6	8	22.9689	100.00%	2.5825	15.49%	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
22790005	6	Ammonia	15 / 5	0.657	0.14	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
22790005	6	Inorganic nitrogen (nitrate and nitrite)	14 / 5	0.03	2.07	0.46	0.17	0	NO	MONITORED	FULL
22790005	6	pH	15 / 5	7.5	8.8	8.2	8.3	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 Montezuma Muni 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 - Previous fish tissue results were removed based on the age of the data.

Monitoring and Methods

Assessment Key Dates

6/17/2020

Monitoring Start

8/29/2024

Monitoring End

Methods

150

Monitoring data more than 5 years old

200

Physical/Chemical Monitoring

260

Fish tissue analysis

275

Drinking water compliance reports

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

860

Other Agencies/Organizations provided monitoring data