Red Rock Reservoir IA 04-LDM-1017

Marion County S19T76NR18W near Pella.

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 3:09:48 PM
Updated: 1/12/2026 11:04:59 AM

Use Support

Class A1

Recreation - Primary contact

Not Supported

Turbidity

Support Level

Not Supported

Impairment Code

5a - Pollutant-caused impairment. TMDL needed.

Cause Magnitude

Moderate

Status

Continuing

Source

Other: Sediment Resuspension

Source Confidence

Moderate

Cycle Added

2010

Impairment Rationale

Narrative criteria violation: aesthetically objectionable conditions

Data Source

Ambient monitoring: Corps of Engineers/ISU/UI

TMDL Priority

Tier II

Class BWW1

Aquatic Life - Warm Water Type 1

WINOFI

Fish Kill: Cause Unknown

Support Level

Further Investigation Needed

Impairment Code

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

Cause Magnitude

Slight

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

N/A

Cycle Added

2020

Impairment Rationale

Non Pollutant-caused fish kill

Data Source

Fish kill investigation: Iowa DNR

Class HH

Human Health -

Fully Supported

Impairment Delistings

Cycle Added	Class	Cause	Data Source	Rationale
2014	Class A1	Bacteria: Indicator Bacteria- E. coli	Ambient monitoring: Corps of Engineers	New data: WQ improvement (chemical / physical / bacterial)

Documentation

Assessment Summary

The Class A1 use of the Red Rock Reservoir was assessed (monitored) as "not supported” due to low water clarity caused by non-algal turbidity. The Class BWW1 use was assessed (evaluated) as “WINOFI” due to fish kills that occurred in 2020, 2023, and 2024. 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
USACE	12	WQ	CY 2020-2024	27630001	Red Rock Reservoir	Deep Point
USACE	12	WQ	CY 2022-2024	99990029	North Overlook Beach (Red Rock Reservoir)	Beach
USACE	12	WQ	CY 2022-2024	99990032	Whitebreast Beach (Red Rock Reservoir)	Beach
DNR	6	FT	See Below	1322	Red Rock Reservoir

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

27630001

8/10/10

0 / 3

MONITORED

FULL

99990029

17/19/18

0 / 4

4 / 5

3 / 4

MONITORED

FULL

99990032

17/19/17

0 / 4

4 / 5

0 / 4

MONITORED

FULL

Red Rock Reservoir was assessed as "not supported" for the 2014 assessment/listing cycle. Based on DNR’s assessment methodology the current assessment/listing cycle must be without violations of the geometric mean criterion and without significantly greater than 10% of the samples violating the single-sample maximum criterion are necessary to propose delisting based on bacteria violations. There were no violations of the geometric mean criterion and there were less than 10% of the samples exceeding the single-sample maximum criterion for the current 2026assessment listing cycle. Therefore, the impairment for indicator bacteria is suggested for delisting for the 2026 assessment/listing cycle.

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
27630001	12	110 / 5	7.1	11.7	8.5	5	16	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
27630001	12	Chlorophyll a TSI	42 / 5	39	82	64	NO	MONITORED	FULL
27630001	12	Secchi TSI	62 / 5	41	76	66	YES	MONITORED	NOT
27630001	12	Total Phosphorus TSI	20 / 2	67	83	78	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
27630001	12	Dissolved Oxygen	110 / 5	4.1	19.3	10.4	1	NA / NA	16	NO	MONITORED	FULL
27630001	12	pH	110 / 5	7.1	11.7	8.5	5	NA / NA	16	NO	MONITORED	FULL
27630001	12	Temperature	110 / 5	4.3	31.7	20.3	0	NA / NA	16	NO	MONITORED	FULL

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
27630001	12	Ammonia	51 / 4	0.43	0.05	0.03	0 / 0	NO	MONITORED	FULL

Class BWW1 - Fish Kill Assessment:

The Class BWW1 aquatic life use assessment based on the 2020, 2023, and 2024 natural fish kill placed this segment in Category 3b. This assessment was “evaluated” because all the fish kill(s) were attributed to a non-pollutant cause (e.g., winterkill, natural, etc). Segments assessed using “evaluated” assessments are placed in Category 3b and are added to the state list of Waters In Need Of Further Investigation (WINOFI). If no additional pollutant caused fish kills are reported in this segment for a 5 year period, the Category will be changed from 3b to 3a (not assessed).

Details about the 2020 fish kill can be found here: https://programs.iowadnr.gov/fishkill/Events/1002

Details about the 2023 fish kill can be found here: https://programs.iowadnr.gov/fishkill/events/1041

Details about the 2024 fish kill can be found here: https://programs.iowadnr.gov/fishkill/events/1058

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)
1322	tissue plug	2021	flathead catfish	13	46.8	0.089
1322	tissue plug	2020	flathead catfish	2	46.8	0.084
1322	tissue plug	2019	flathead catfish	13	45.6	0.09
Support Level	Full

Monitoring and Methods

Assessment Key Dates

5/19/2020

Monitoring Start

11/4/2024

Monitoring End

6/30/2021

Fish Tissue Monitoring

7/1/2019

Fish Tissue Monitoring

7/1/2020

Fish Tissue Monitoring

8/26/2020

Fish Kill

8/12/2024

Fish Kill

8/23/2023

Fish Kill

Methods

120

Surveys of fish and game biologists/other professionals

140

Incidence of spills and/or fish kills

200

Physical/Chemical Monitoring

260

Fish tissue analysis

420

Indicator bacteria monitoring