Rathbun Reservoir - Main Basin IA 05-CHA-1309

Appanoose County approx 6 miles N of Centerville.

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 3:10:41 PM
Updated: 1/5/2026 11:10:25 AM

Use Support

Class A1

Recreation - Primary contact

Fully Supported

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

Moderate

Status

New

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2026

Impairment Rationale

Significantly > 10% of samples fail to meet criterion

Data Source

Ambient monitoring: Iowa DNR-lakes

TMDL Priority

Tier III

Class HH

Human Health -

WINOFI

Metals: Mercury

Support Level

Further Investigation Needed

Impairment Code

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

Cause Magnitude

Slight

Status

Continuing

Source

Atmospheric Deposition

Source Confidence

Low

Cycle Added

2024

Impairment Rationale

Potential Impairment

Data Source

Fish contaminant monitoring: Iowa DNR

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class A1 use was assessed (monitored) as "fully supported.” The Class BWW1 use was assessed as “not supported” due to violations of the Class BLW criteria for Dissolved Oxygen. The Class HH (fish consumption) use was assessed (evaluated) as “WINOFI” based on fish tissue sampling data. Note: A TMDL for turbidity at four segments of Rathbun Lake was prepared by DNR and approved by EPA in 2019.

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	22040001	Rathbun Lake
USACE	12	WQ	CY 2020-2024	99990023	Rathbun Reservoir
PWS	27	WQ	CY 2020-2024	99990179	Caisson - Rathbun Lake Dedication	Rathbun Regional Water Association
Iowa DNR	6	FT	See Below	1053	Rathbun Reservoir

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
22040001	6	12 / 4	7.58	8.55	7.96	0	4	NO	MONITORED	FULL
99990023	12	12 / 2	7.41	8.67	8.12	0	4	NO	MONITORED	FULL
99990179	27	1826 / 5	7.3	8.4	7.8	0	200	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
22040001	6	Chlorophyll a TSI	12 / 4	42	70	55	NO	MONITORED	FULL
22040001	6	Secchi TSI	12 / 4	48	69	62	NO	MONITORED	FULL
99990023	12	Secchi TSI	30 / 5	56	69	62	NO	MONITORED	FULL
22040001	6	Total Phosphorus TSI	12 / 4	51	66	59	NA	NA	NA
99990023	12	Total Phosphorus TSI	16 / 3	62	74	70	NA	NA	NA
22040001	6	Fixed suspended solids	12 / 4	1.9	6	3	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
22040001	6	Dissolved Oxygen	12 / 4	3.4	8.9	6.4	4	NA / NA	4	YES	MONITORED	NOT
99990023	12	Dissolved Oxygen	27 / 5	2.3	11.4	8.2	3	NA / NA	6	NO	MONITORED	FULL
22040001	6	pH	12 / 4	7.58	8.55	7.96	0	NA / NA	4	NO	MONITORED	FULL
99990023	12	pH	12 / 2	7.41	8.67	8.12	0	NA / NA	4	NO	MONITORED	FULL
99990179	27	pH	1826 / 5	7.3	8.4	7.8	0	NA / NA	200	NO	MONITORED	FULL
22040001	6	Temperature	12 / 4	17.5	27.4	23.9	0	NA / NA	4	NO	MONITORED	FULL
99990023	12	Temperature	27 / 5	7.9	27.5	21.4	0	NA / NA	6	NO	MONITORED	FULL
99990179	27	Temperature	1824 / 5	5	26.2	15.1	0	NA / NA	200	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
22040001	6	3	5.995990247	62.15%	1.74509012	56.54%	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
22040001	6	Ammonia	12 / 4	0.093	0.04	0.03	0 / 0	NO	MONITORED	FULL
99990023	12	Ammonia	12 / 2	0.181	0.09	0.08	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)
1053	tissue plug	2021	flathead catfish	14		0.357
1053	tissue plug	2020	flathead catfish	6	49.0	0.242
1053	tissue plug	2019	flathead catfish	8	45.0	0.174
1053	tissue plug	2019	largemouth bass	10	38.8	0.102
1053	tissue plug	2017	largemouth bass	3	42.1	0.223
1053	tissue plug	2019	walleye	20	47.3	0.062
Support Level	WINOFI

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 Rathbun Regional Water Association 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.

Monitoring and Methods

Assessment Key Dates

1/1/2020

Monitoring Start

12/31/2024

Monitoring End

9/17/2017

Fish Tissue Monitoring

7/1/2019

Fish Tissue Monitoring

7/1/2020

Fish Tissue Monitoring

7/7/2021

Fish Tissue Monitoring

6/14/2019

TMDL Completed

Methods

200

Physical/Chemical Monitoring

260

Fish tissue analysis

340

Primary producer surveys (phytoplankton/periphyton/macrophyton)

420

Indicator bacteria monitoring